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BODHAINE, B.A., B.I. Nazarov, and A. Kh. Shukurov. Comparison of aerosol concentrations at Fedchenko Glacier and Dushanbe. In Joint Soviet-American Experiment on Arid Aerosol, edited by G.S. Golitsyn, pp. 181-184, Hydrometeoizdat, St. Petersburg, Russia (1993).

No abstract.

BODHAINE, B.A., and E.G. DUTTON. A long-term decrease in Arctic Haze at Barrow, Alaska. Geophysical Research Letters 20:947-950 (1993).

Surface aerosol scattering measurements have been conducted at Barrow, Alaska, from May 1976 to the present using a four-wavelength nephelometer. Total column aerosol optical depth measurements have been obtained over the same time period. Both data sets show a maximum in 1982 and then a decreasing trend to the present. This decreasing trend is apparent, and statistically significant, only in March-April. Arctic haze, caused by long-range transport from midlatitude industrial regions, is most evident in the vicinity of Barrow during this time of year. We suggest that the decrease in Arctic haze at Barrow, as observed in the aerosol light scattering and optical depth records, is due to decreased anthropogenic pollution emissions in Europe and the former Soviet Union, the primary source regions for the springtime aerosol at Barrow. Volcanic effects in the stratosphere have been subtracted from the optical depth data, and are not believed to be significant in the surface-based data.

Bradley, R.S., F.T. Keimig, and H.F. DIAZ. Recent Changes in the North American Arctic Boundary Layer in Winter. Journal of Geophysical Research 98(D5):8851-8858 (1993).

Analysis of significant level radiosonde data from a network of arctic stations reveals a systematic reduction in midwinter surface-based inversion depths over the past few decades, accompanied by a rise in surface temperature. Similar trends are observed over a wide sector, from 62°W to 162°W and from 70°N to 83°N. Possible causes for these changes include increases in warm air advection, cloud cover, ice crystals, aerosols, and greenhouse gases, but the specific reasons are difficult to identify, due to strong interactions between many potentially important factors. Nevertheless, the changes are significant for studies of Arctic haze, since the midwinter stable boundary layer has been decreasing in depth over time.

Cess, R.D., S. Nemesure, E.G. DUTTON, J.J. DE LUISI, G.L. Potter, and J.-J. Morcrette. The impact of clouds on the shortwave radiation budget of the surface-atmosphere system: Intefacing measurements and models. Journal of Climate 6(2):308-316 (1993).

Two data sets have been combined to demonstrate how the availability of more comprehensive data sets could serve to elucidate the shortwave radiative impact of clouds on both the atmospheric column and the surface. These data sets consist of two measurements of net downward shortwave radiation: one of near-surface measurements made at the Boulder Atmospheric Observatory tower, and the other of collocated top-of-the atmosphere measurements from the Earth Radiation Budget Experiment. Output from the European Centre for Medium-Range Weather Forecasts General Circulation Model also has been used as an aid in interpreting the data, while the data have in turn been employed to validate the model's shortwave radiation code as it pertains to cloud radiation properties. Combined, the data sets and model demonstrate a strategy for determining under what conditions the shortwave radiative impact of clouds leads to a heating or cooling of the atmospheric column. The data sets also show, in terms of a linear slope-offset algorithm for retrieving the net downward shortwave radiation at the surface from satellite measurements, that the clouds present during this study produced a modest negative bias in the retrieved surface flux relative to that inferred from a clear-sky algorithm.

Chiou, E.W., M.P. McCormick, L.R. McMaster, W.P. Chu, J.C. Larsen, D. Rind, and S. OLTMANS. Intercomparison of stratospheric water vapor observed by satellite experiments: Stratospheric Aerosol and Gas Experiment II versus Limb Infrared Monitor of the Stratosphere and Atmospheric Trace Molecule Spectroscopy. Journal of Geophysical Research 98(D3):4875-4887 (1993).

The paper presents a comparison of the stratospheric water vapor measurements made by the satellite-borne sensors the Stratospheric Aerosol and Gas Experiment II (SAGE II), the Nimbus 7 Limb Infrared Monitor of the Stratosphere (LIMS), and the Spacelab 3 Atmospheric Trace Molecule Spectroscopy (ATMOS) experiment. LIMS obtained data for 7 months between November 1978 and May 1979; ATMOS was carried on Shuttle and observed eight profiles from April 30 to May 6, 1985 at approximately 30°N and 50°S; and, SAGE II continues to make measurements since its launch in October 1984. For both 30°N and 50°S in May, the comparisons between SAGE II and ATMOS show agreement within the estimated combined uncertainty of the two experiments. Several important features identified by LIMS observations have been confirmed by SAGE II: a well-developed hygropause in the lower stratosphere at low- to mid-latitudes, a poleward latitudinal gradient, increasing water vapor mixing ratios with altitude in the tropics, and the transport of dry lower stratospheric water vapor upward and southward in May, and upward and northward in November. A detailed comparative study also indicates that the two previously suggested corrections for LIMS, a correction in tropical lower stratosphere due to a positive temperature bias and the correction above 28 km based on improved emissivities will bring LIMS measurements much closer to those of SAGE II. The only significant difference occurs at high southern latitudes in May below 18 km, where LIMS measurements are 2-3 ppmv greater. It should be noted that LIMS observations are from 16 to 50 km, ATMOS from 14 to 86 km, and SAGE II from mid-troposphere to 40 km. With multiyear coverage, SAGE II observations should be useful for studying tropospheric-stratospheric exchange, for stratospheric transport, and for preparing water vapor climatologies for the stratosphere and the upper troposphere.

Chu, W.P., E.W. Chiou, J.C. Larsen, L.W. Thomason, D. Rind, J.J. Buglia, S. OLTMANS, M.P. McCormick, and L.M. McMaster. Algorithms and sensitivity analyses for Stratospheric Aerosol and Gas Experiment II water vapor retrieval. Journal of Geophysical Research 98(D3):4857-4866 (1993).

This paper provides a detailed description of the current operational inversion algorithm for the retrieval of water vapor vertical profiles from the Stratospheric Aerosol and Gas Experiment II (SAGE II) occultation data at the 0.94-m wavelength channel. This algorithm is different from the algorithm used for the retrieval of the other species such as aerosol, ozone, and nitrogen dioxide because of the nonlinear relationship between the concentration versus the broad band absorption characteristics of water vapor. Included in the discussion of the retrieval algorithm are problems related to the accuracy of the computational scheme, accuracy of the removal of other interfering species, and the expected uncertainty of the retrieved profile. A comparative analysis on the computational schemes used for the calculation of the water vapor transmission at the 0.94-m wavelength region is presented. Analyses also presented on the sensitivity of the retrievals to interferences from the other species which contribute to the total signature as observed at the 0.94-m wavelength channel on SAGE II instrument. Error analyses of the SAGE II water vapor retrieval will be shown, indicating that good quality water vapor data are being produced by the SAGE II measurements.

CONWAY, T.J., L.P. STEELE, and P.C. NOVELLI. Correlations among atmospheric CO₂, CH₄ and CO in the Arctic, March 1989. Atmospheric Environment 27A(17/18):2881-2894 (1993).

During six aircraft flights conducted as part of the third Arctic Gas and Aerosol Sampling Program (AGASP III, March 1989), 189 air samples were collected throughout the Arctic troposphere and lower stratosphere for analysis of CO₂, CH₄ and CO. The mixing ratios of the three gases varied significantly both horizontally and vertically. Elevated concentrations were found in layers with high anthropogenic aerosol concentrations (Arctic Haze). The mixing ratios of CO₂, CH₄ and CO were highly correlated on all flights. A linear regression of CH₄ vs. CO₂ for pooled data from all flights yielded a correlation coefficient (r2) of 0.88 and a slope of 13.5 ppb CH₄/ppm CO₂ (n=186). For CO vs. CO₂ a pooled linear regression gave r² - 0.91 and a slope of 15.8 ppb CO/ppm CO₂ (n=182). Carbon dioxide, CH₄ and CO also exhibited mean vertical gradients with slopes of 0.37, -4.4 and -4.2 ppb km^-1, respectively. Since the carbon dioxide variations observed in the Arctic atmosphere during winter are due primarily to variations in the emissions and transport of anthropogenic CO₂ from Europe and Asia, the strong correlations that we have found suggest that a similar interpretation applies to CH₄ and CO. Using reliable estimates of CO₂ emissions for the source regions and the measured CH₄ /CO₂ CO/CO₂ ratios, we estimate a regional European CH₄ source of 47 6 Tg CH₄ yr^-1 that may be associated with fossil fuel combustion. A similar calculation for CO results in an estimated regional CO source of 82 2 Tg CO yr^-1.

Davidson, C.I., J.-L. Jaffrezo, B.W. Mosher, J.E. Dibb, R.D. Borys, B.A. BODHAINE, R.A. Rasmussen, C.F. Boutron, U. Gorlach. H. Cachier, J. Ducret, J.-L. Colin, N.Z. Heidam, K. Kemp, and R. Hillamo. Chemical constiuents in the air and snow at Dye 3, Greenland-I. Seasonal variations. Atmospheric Environment 27A:2709-2722 (1993).

Chemical constituent concentrations in air and snow from the Dye 3 Gas and Aerosol Sampling Program show distinct seasonal patterns. These patterns are different from those observed at sea-level sites throughout the Arctic. Airborne SO₄^2-and several trace metals of crustal and anthropogenic origin show strong peaks in the spring, mostly in April. Some species also have secondary maxima in the fall. The spring peaks are attributed to transport over the Pole from Eurasian sources, as well as transport from eastern North America and western Europe. The fall peaks are attributed primarily to transport from North America, and less frequent transport from Europe. Airborne ⁷Be and ²¹⁰Pb show strong peaks in both spring and fall, suggesting that vertical atmospheric mixing is favored during these two seasons. Several other airborne constituents peak at other times. For example, Na peaks in winter due to transport of sea spray from storms in ice-free oceanic areas, while MSA peaks in summer due to biogenic production in the oceans nearby. Many trace gases such as freons and other chlorine-containing species show roughly uniform concentrations throughout the year. CO and CH₄ show weak peaks in February-March. Concentrations of chemical constituents in fresh snow at Dye 3 also show distinct seasonal patterns. SO₄^2- and several trace metals show springtime maxima, consistent with the aerosol data. Na shows a winter maximum and MSA shows a summer maximum in the snow, also consistent with the aerosols. ⁷Be and ²¹⁰Pb in the snow do not show any strong variation with season. Similarly, soot and total carbon in snow do not show strong variation. When used with dry deposition models, these air and snow concentration data suggest that dry deposition of submicron aerosol species has relatively minor influence on constituent levels in the snowpack at Dye 3 compared to wet deposition inputs (including scavenging by fog); crustal aerosol, on the other hand, may have a more significant input by dry deposition. Overall, the results suggest that gross seasonal patterns of some aerosol species are consistent in the air and in fresh snow, although individual episodes in the air are not always reflected in the snow. The differences in data reported here compared with data sets for sea-level arctic sites demonstrate the need for sampling programs on the Ice Sheet in order to properly interpret Greenland glacial record data.

Davidson, C.I., J.-L. Jaffrezo, B.W. Mosher, J.E. Dibb, R.D. Borys, B.A. BODHAINE, R.A. Rasmussen, C.F. Boutron, F.M. Ducroz,. H. Cachier, J. Ducret, J.-L. Colin, N.Z. Heidam, K. Kemp, and R. Hillamo. Chemical constiuents in the air and snow at Dye 3, Greenland-II. Analysis of episodes in April 1989. Atmospheric Environment 27A:2723-2737 (1993).

Detailed examination of a two-week period in April 1989 during the Dye 3 Gas and Aerosol Sampling Program shows that episodes of relatively high concentration of certain chemical constituents occur at this time of year. Airborne concentrations of crustal metals such as Al and Ca can exceed 100 ng m^-3, while concentrations of SO₄^2- can exceed 1000 ng m^-3. Elevated concentrations of MSA, ⁷Be and ²¹⁰Pb are also noted. Consideration of synoptic maps and backward air mass trajectories suggests that the episodes are due to transport from a variety of source regions, including Eurasia (transport over the Pole), North America and western Europe. In addition to elevated airborne concentrations, levels of these constituents in surface snow are high during April. However, it is difficult to develop quantitative relationships between concentrations in air and in snow due to the difficulty in measuring airborne concentrations at cloud-level; variations in scavenging by clouds may also be significant. It is concluded that the springtime maxima in airborne concentrations resulting from long-range transport from a variety of source regions are responsible for strong identifiable signals in ice cores and snowpits from this region.

DeLuisi, J.J., D.U. LONGENECKER, C.L. Mateer, and W.P. Chu. Estimation of solar backscatter ultraviolet albedo using ground-based Umkehr measurements. Journal of Geophysical Research 98(D2):2985-2993 (1993).

A retrieval method was developed to estimate the solar backscatter ultraviolet (SBUV) satellite albedo for the ozone profiler wavelengths using ground-based ultraviolet measurements. For the present investigation the Umkehr was used as the ground-based ultraviolet measurement. Simulated SBUV data and Umkehr data theoretically computed from a priori ozone profiles observed by the SAGE II satellite were used to develop the retrieval algorithm and to test its capability. The test indicated that albedos for the SBUV ozone profiler wavelengths should allow estimates to a precision of ±5% or better, depending on the accuracy of the ultraviolet measurement. Retrievals using actual Umkehr observations were also performed to provide a preliminary look at the magnitude and annual variation of retrieved albedos. A case study was performed, comparing retrieved albedos with SBUV-measured albedos. The SBUV albedo change was seen to be approximately twice as large as the albedo changes estimated by the Umkehr method. Results of the investigation suggest that the method of estimation may be useful for determining the drift rate of the SBUV calibration.

DLUGOKENCKY, E.J., J.M. HARRIS, Y.S. Chung, P.P. TANS, and I. FUNG. The relationship between the methane seasonal cycle and regional sources and sinks at Tae-ahn Peninsula, Korea. Atmospheric Environment 27A(14):2115-2120 (1993).

Methane measurements from weekly air samples collected at Tae-ahn Peninsula, Korea (TAP) present new constraints on the regional methane source strength of eastern Asia. Analysis of atmospheric trajectories shows that the lowest methane values observed at Tae-ahn are associated with southeasterly flow off the tropical Pacific Ocean and are similar to those observed at Cape Kumukahi, Hawaii. During June to August, northwesterly flow from the peat-rich wetlands located in the maritime provinces of the Far East former Soviet Union elevates methane at TAP by ~80 ppb above the annual mean. Analysis of the Tae-ahn observations using a 3-D atmospheric methane model suggests that methane emission rates from the Far East Soviet wetlands may be ~2 times those of Alaskan wetlands. Also, the relative maximum in May/June at Tae-ahn constrains global CH₄ emissions from rice cultivation to ~100 Tg yr^-1.

DLUGOKENCKY, E.J., J.M. HARRIS, Y.S. Chung, P.P. Tans, and I. Fung. The relationship between the methane seasonal cycle and regional sources and sinks at Tae-ahn Penninsula, Korea. Atmospheric Environment 27A(14): 2115-2120 (1993).

Methane measurements from weekly air samples collected at Tae-ahn Peninsula, Korea (TAP) present new constraints on the regional methane source strength of eastern Asia. Analysis of atmospheric trajectories shows that the lowest methane values observed at Tae-ahn are associated with southeasterly flow off the tropical Pacific Ocean and are similar to those observed at Cape Kumukahi. During June to August, northwesterly flow from the peat-rich wetlands located in the maritime provinces of the Far East former Soviet Union elevates methane at TAP by ~80 ppb above the annual mean. Analysis of the Tae-ahn observations using a 3-D atmospheric methane model suggests that methane emission rates from Far East Soviet wetlands may be ~2 times those of Alaskan wetlands. Also, the relative maximum in May/June at Tae-ahn constrains global CH₄ emissions from rice cultivation to ~100 Tg yr^-1.

DUTTON, E.G. An extended comparison between LOWTRAN7 computed and observed broadband thermal irradiances: Global extreme and intermediate surface conditions. Journal of Atmospheric and Oceanic Technology 10(3):326-336 (1993).

Differences between observed and LOWTRAN7-computed downward longwave irradiances were examined at each of four globally diverse locations for an entire year at each site. The final results are restricted to times determined to be completely or nearly cloud-free. The irradiances from 367 such times range from 60 to 435 W m^-2, and results indicate that the modeled irradiances and those measured directly using a pyrgeometer agree to within 5 W m^-2 at individual sites to within less than 0.2 W m^-2 when averaged over all four sites, neglecting any site-specific biases. The standard deviations and standard errors associated with these results are roughly 10 and 1 W m^-2, respectively. An unbiased estimate of the agreement between the model and observations results in a mean difference of 0.62 W m^-2 with standard deviation of 5 W m^-2 but an even larger 95% confidence interval because of the small sample size. The comparison variance can be logically ascribed to a number of different sources, including atmospheric variability and inhomogeneity, as well as to short-term instrument and LOWTRAN7 input variations. LOWTRAN7 and the observations agree better, in the mean, than the commonly accepted uncertainties for either would suggest. Maximum cloud radiative forcing at the surface for each site is quantified as a by-product of the comparison process.

ELKINS, J.W., T.M. THOMPSON, T.H. SWANSON, J.H. BUTLER, B.D. HALL, S.O. CUMMINGS, D.A. Fisher, and A.G. Raffo. Decrease in the growth rates of atmospheric chlorofluorocarbons 11 and 12. Nature 364:780-783 (1993).

The discovery of the Antarctic ozone hole in 1985 led to international efforts to reduce emissions of ozone-destroying chlorofluorocarbons. These efforts culminated in the Montreal Protocol and its subsequent amendments, which called for the elimination of CFC production by 1996. Here we focus on CFC-11 (CCl3F) and CFC-12 (CCl2F2), which are used for refrigeration, air conditioning and the production of aerosols and foams, and which together make up about half of the total abundance of stratospheric organic chlorine. We report on a significant recent decrease in the atmospheric growth rates of these two species, based on measurements spanning the past 15 years and latitudes ranging from 83°N to 90°S. This is consistent with CFC-producers' own estimates of reduced emissions. If the atmospheric growth rates of these two species continue to slow in line with predicted changes in industrial emission, global atmospheric mixing ratios will reach a maximum before the turn of the century, and then begin to decline.

FERGUSON, E.E. Proceses associated with stratospheric ozone decrease: homogeneous chemistry. The Role of the Stratosphere in Global Change M.-L. Chanin (ed.). NATO ASI Series, Vol. 18, Springer-Verlag, Berlin Heidelberg 47-64 (1993).

The role of homogeneous (gas-phase) chemical reactions in controlling the stratospheric ozone concentration is briefly reviewed. An outline of definitions, conventions, and data sources is followed by a specific discussion of the most important ozone destroying reactions. The catalytic HOx, NOx, and ClOx cycles are described.

Gillette, D.A., and B.A. BODHAINE. U.S. results from a joint US/USSR experiment for the study of desert dust and its impact on local meteorological conditions and climate. In Joint Soviet-American Experiment on Arid Aerosol, edited by G.S. Golitsyn, pp. 7-22, Hydrometeoizdat, St. Petersburg, Russia (1993).

No abstract.

GILLETTE, D.A., B.A. BODHAINE, and D.Mackinnon. Transport and deposition of desert dust in the Kafirnigan River Valley (Tadzhikistan) from Shaartuz to Esanbay: Measurements and a simple model. Atmospheric Environment 27A(16):2545-2552 (1993).

A model of deposition and transport was constructed for the Kafirnigan Valley, in Soviet Central Asia. Data, consisting of deposition measurements at Shaartuz, atmospheric columnar mass, aerosol concentrations, wind speed, optical scattering, and movement of soil, were collected for the dust storms of 16 and 20 September 1989. Results from the model were compared with measurements of total atmospheric columnar mass loading for the dust storm of 16 September. Although sensitivity of the model to dust layer height does not recommend the model for general use, the model has some merit in predicting transport and deposition for dust contained in a river valley.

Gleason, J.F., PK. Bhartia, J.R. Herman, R. McPeters, P. Newman, R.S. Stolarski, L. Flynn, G. Labow, D. Larko, C. Seftor, C. Wellemeyer, W.D. KOMHYR, A.J. Miller, and W. Planet. Record low global ozone in 1992. Science 260:523-526 (1993).

The 1992 global average total ozone, measured by the Total Ozone Mapping Spectrometer (TOMS) on the Nimbus-7 satellite, was 2 to 3 percent lower than any earlier year observed by TOMS (1979 to 1991). Ozone amounts were low in a wide range of latitudes in both the Northern and Southern hemispheres, and the largest decreases were in the regions from 10°S and 20°S and 10°N to 60°N. Global ozone in 1992 is at least 1.5 percent lower than would be predicted by a statistical model that includes a linear trend and accounts for solar cycle variation and the quasi-biennial oscillation. These results are confirmed by comparisons with data from other ozone monitoring instruments: the SBUV/2 instrument on the NOAA-11 satellite, the TOMS instrument on the Russian Meteor-3 satellite, the World Standard Dobson Instrument 83, and a collection of 22 ground-based Dobson instruments.

Goldan, P.D., W.C. Kuster, F.C. Fehsenfeld, and S.A. MONTZKA. The observation of a C5 alcohol emission in a North American pine forest. Geophysical Research Letters 20(11):1039-1042 (1993).

During a recent study carried out at an isolated site in the Colorado mountains, a C5 alcohol, 2-methyl-3-buten-2-ol, was found to be the most abundant volatile organic compound of biogenic origin present in the atmosphere. This finding, if generally characteristic of the natural chemical species present in the atmosphere in forested areas, has important implications. First, the presence in large quantities of a reactive chemical compound at these high levels can signifiantly influence the local atmospheric chemistry. Secondly, this compound, although previously identified as a pheromone for Ips typographus (spruce bark beetle), an insect predator responsible for major forest die-backs in this region, is strongly correlated with isoprene. Since isoprene is known to be emitted by the local vegetation, the observed 2-methyl-3-buten-2-ol appears also to have a vegetative rather than etomological source.

Gunter, R.L., A.D.A. Hansen, J.F. Boatman, B.A. BODHAINE, R.C. SCHNELL, and D.M. Garvey. Airborne measurements of aerosol optical properties over south-central New Mexico. Atmospheric Environment 27A(8):1362-1368 (1993).

We used a three-wavelength nephelometer (449, 536, and 690 nm) and an aethalometer on board the NOAA King Air research aircraft to assess the contributions of aerosol optical scattering and absorption to shortwave extinction. The measurements were made over south-central New Mexico in February and July 1989. The winter measurements revealed a shallow, polluted planetary boundary layer with cleaner air above. The summer measurements showed a uniformly mixed planetary boundary layer extending form ground level to the operational ceiling of 4.5 km above ground. In both cases the total optical thickness values for the column were similar (0.03) and the fractional contribution of aerosol absorption to the extinction was between 5 and 10%. These results suggest that the aerosol extinction in summer and winter is similar, even though the planetary boundary layer thickness is quite different during the two seasons. They also demonstrate that a suitable instrumented light aircraft can profile the optical properties of the troposphere with high sensitivity and good spatial resolution.

We used a three-wavelength nephelometer (449, 536, and 690 nm) and an aethalometer on board the NOAA King Air research aircraft to assess the contributions of aerosol optical scattering and absorption to shortwave extinction. The measurements were made over south-central New Mexico in February and July 1989. The winter measurements revealed a shallow, polluted planetary boundary layer with cleaner air above. The summer measurements showed a uniformly mixed planetary boundary layer extending from ground level to the operational ceiling of 4.5 km above ground. In both cases the total optical thickness values for the column were similar (0.03) and the fractional contribution of aerosol absorption to the extinction was between 5 and 10%. These results suggest that the aerosol extinction in summer and winter is similar, even though the planetary boundary layer thickness is quite different during the two seasons. They also demonstrate that a suitably instrumented light aircraft can profile the optical properties of the troposphere with high sensitivity and good spatial resolution.

Hansen, A.D.A., V.N. Kapustin, V.M. Kopeikin, D.A. Gillette, and B.A. BODHAINE. Optical absorption by aerosol black carbon and dust in a desert region of Soviet Central Asia. In Joint Soviet-American Experiment on Arid Aerosol, edited by G.S. Golitsyn, pp. 95-105, Hydrometeoizdat, St. Petersburg, Russia (1993).

No Abstract.

Hansen, A.D.A., V.N. Kapustin, V.M. Kopeikin, D.A. GILLETTE, and B.A. BODHAINE. Optical absorption by aerosol black carbon and dust in a desert region of Central Asia. Atmospheric Environment 27A(16):2527-2531 (1993).

In September 1989 a joint U.S.S.R.-U.S. study of the causes and effects of desert dust on the environment was conducted in the Tadzhik S.S.R. in Soviet Central Asia. The objectives of the study included measurements of optical absorption by suspended material, both windblown dust and aerosol "black" carbon. This latter material is a combustion effluent, prevalent in emissions from poorly controlled burning with a long atmospheric lifetime and a large cross-section for the absorption of visible radiation. The measurements obtained from the analysis of filter samples indicate that only during periods of active dust production was there a significant contribution of dust to total absorption. At other times, the presence of black carbon from local and regional sources accounted for approximately 90% of the total aerosol optical absorption. The conclusions are that fuel combustion may produce a greater optical impact on the atmosphere in less-developed areas of the world than that arising from the effects of desert dust production.

HERBERT, G.A., P.J. SHERIDAN, R.C. SCHNELL, M.Z. BIENIULIS, B.A. BODHAINE, and S.J. OLTMANS. Analysis of meteorological conditions during AGASP-IV: March 30-April 23, 1992. NOAA TM ERL CMDL-5, Environmental Research Laboratories, Boulder, CO, 118 pp. (1993).

The fourth Arctic Gas and Aerosol Sampling Program (AGASP-IV) was conducted over Alaska and the Beaufort Sea during March and April 1992. The NOAA WP-3D aircraft made nine flights. On the first eight flights special aerosol and gas sampling instrumentation was installed, and extensive time was spent over the pack ice. Measurements of wind, pressure, temperature, relative humidity, ozone, and condensation nucleus (CN) concentration were used to identify the air mass type, recent origin, and existence of pollution-derived aerosols, i.e., haze. While small patches of elevated CN concentrations and higher aerosol scattering coefficients were observed, significantly large regions, of the type found in previous AGASP missions, were not observed during this series. On most flights the CN concentrations in the troposphere were representative of the "clean" background conditions at this latitude. Significant concentrations of CN (CN>7000 cm^-3) found above the tropopause on three fights indicated the presence of volcanic aerosol probably from the Pinatubo volcanic plume at high latitudes. In four instances low ozone concentrations suggest the destruction of ozone in the surface layer.

HERBERT, G.A., R.C. SCHNELL, B.J.B. Stunder, B.A. BODHAINE, S.J. OLTMANS, and M.Z. BIENIULIS. Meteorology and aerosol distribution during AGASP-III: The "Haze" flights (March 16-30, 1989). NOAA TM ERL CMDL-6. Environmental Research Laboratories, Boulder, CO, 103 pp. (1993).

The third Arctic Gas and Aerosol Sampling Program (AGASP-III) was conducted over the Norwegian Sea and near Svalbard, in the Scandinavian Arctic, in March 1989. The NOAA WP-3D aircraft, with special aerosol and gas sampling instrumentation added, made six flights (out of a total of eleven) in which extensive time was spent over the pack ice. These six flights are analyzed in this study. Measurements of wind, pressure, temperature, relative humidity, ozone, and condensation nucleus (CN) concentration were used to identify the air mass type, recent origin, and existence of pollution-derived aerosols, i.e., haze. Significant regions of elevated CN concentrations that could be traced to known northern European source regions were encountered during three of the flights in the lowest 3 km. On the other three flights the CN concentrations were representative of "clean background" conditions at this latitude. Significant concentrations of CN (CN>10,000 cm^-3) found immediately below the tropopause on three flights indicated vigorous gas-to-particle conversion. In two instances significant ozone destruction was detected in the surface layer.

HOFMANN, D.J. Twenty years of balloon-borne tropospheric aerosol measurements at Laramie, Wyoming. Journal of Geophysical Research 98(D7):12,753-12,766 (1993).

Over the period 1971 to 1990, 265 high-altitude balloons with optical particle counters were launched at Laramie, Wyoming, in a long-term study of the stratospheric sulfate aerosol layer. The tropospheric aerosol record obtained at this background continental site is examined here. All aerosol particle size ranges display pronounced seasonal variations, with the condensation nuclei concentration and the optically active (r ³ 0.15 m m) component showing a summer maximum throughout the troposphere. The dominant source of this aerosol is believed to be photochemically driven gas to particle conversion. Larger particles (r > 1 m m) peak in spring and are probably the result of long range transport of Asian desert dust although there is evidence that sulfate aerosol is also present in these air masses. Mass estimates, assuming spherical sulfate particles, indicate an average column mass between altitudes of 2.5 km (33 m above the surface) and 10 km of about 4 and 16 mg m^-2 in winter and summer, respectively. The annual average of about 10 mg m^-1 is somewhat larger than estimated with model calculations for sulfate aerosol in this region but is within the uncertainties of model and measurements. Calculated optical depths vary between 0.01 and 0.04 from winter to summer; the estimated mass scattering cross section is about 3 m² g^-1 throughout the troposphere. A distinct anticorrelation exists between the optically active and the condensation nuclei components, resulting in a maximum in the mixing ratio of the latter just below the tropopause where the larger particles generally show a minimum. This relation is due to coagulation of the small, newly nucleated particles with the existing larger particles and to the competition for available condensable vapors presented by the larger particles, resulting in an effective new particle source and reservoir region occurring in the upper troposphere. There is evidence for a decreasing trend of 1.6-1.8% per year in the optically active tropospheric aerosol over the past 20 years which may be related to a similar reduction of SO₂ emissions in the United States over this period.

HOFMANN, D.J., S.J. OLTMANS, J.M. HARRIS, W.D. KOMHYR, J.A. LATHROP, T. DEFOOR, and D. KUNIYUKI. Ozonesonde measurements at Hilo, Hawaii following the eruption of Pinatubo. Geophysical Research Letters 20(15):1555-1558 (1993).

Ozonesonde measurements at Hilo, Hawaii (20°N), after the eruption of Pinatubo in June 1991, are compared to measurements made there from 1985 to 1990 in order to investigate possible volcanic effects. The general nature of the ozone anomalies in 1991-1992 can be summarized as lower than normal ozone below about 25 km and higher than normal ozone above. The net result was that total ozone was somewhat lower than average and, during late 1992, was as low as recorded in 1982, following the eruption of El Chichon. Elevated temperatures in the region of the volcanic aerosol layer and upward motion of the aerosol layer were observed at Hilo following the eruption. Although the nature of the perturbed ozone profile may be the result of enhanced upward motion associated with volcanic aerosol particle heating, and the coupling of quasi-biennial oscillation effects, the persistent nature of the perturbation, still present more than a year after the eruption, is not easily explained.

Kahl, J.D., D.J.Charlevoix, N.A. Zaitseva, R.C. SCHNELL, and M.C. Serreze. Absence of evidence for greenhouse warming over the Arctic Ocean in the past 40 years. Nature 361:335-337 (1993).

Atmospheric general circulation models predict enhanced greenhouse warming at high latitudes owing to positive feedbacks between air temperature, ice extent and surface albedo. Previous analyses of Arctic temperature trends have been restricted to land-based measurements on the periphery of the Arctic Ocean. Here we present temperatures measured in the lower troposphere over the Arctic Ocean during the period 1950-90. We have analyzed more than 27,000 temperature profiles, measured by radiosonde at more than 27,000 temperature profiles, measured by radiosonde at Russian drifting ice stations and by dropsonde from US "Ptarmigan" weather reconnaissance aircraft, for trends as a function of season and altitude. Most of the trends are not statistically significant. In particular, we do not observe the large surface warming trends predicted by models; indeed, we detect significant surface cooling trends over the western Arctic Ocean during winter and autumn. This discrepancy suggests that present climate models do not adequately incorporate the physical processes that affect the polar regions.

Kahl, J.D., M.C. Serreze, R.S. STONE, S. Shiotani, M. Kisley, and R.C. SCHNELL. Tropospheric temperature trends in the Arctic: 1958-1986. Journal of Geophysical Research 98(D7):12,825-12,838 (1993).

Arctic temperature trends in four tropospheric layers during the period 1958-1986 are examined through analysis of a comprehensive archive of Arctic upper air meteorological data. The goals of the study are to describe the trends in Arctic tropospheric temperatures and to provide verification data for model simulations of Arctic and global climate. This analysis extends the work of previous researchers by examining rawinsonde time series from a much denser distribution of stations than was previously available and by resolving the vertical distribution of tropospheric temperatures as well. Absolute trends of 3° C/30 yr or larger were found, with both cooling and warming tendencies observed in all layers. The majority of the trends, however, are not statistically significant. Considerable regional and seasonal variability is observed. Trends at many stations in Eurasia and Greenland are highly sensitive to large positive anomalies during the period 1958-1963, which may be artifacts of the data. On the basis of our analysis, we conclude that greenhouse-induced warming is not detectable in the Arctic troposphere for the 1958-1986 period.

Kaufman, Y.G., S.S. Khmelevtsov, and T.E. DE FOOR. Lidar measurements of stratospheric aerosols during the SAGA 3 expedition. Journal of Geophysical Research 98(D9):16,909-16,913 (1993).

Backscatter lidar measurements of stratospheric aerosols were carried out over the tropical Pacific aboard the Akademik Korolev during SAGA 3 of February to March 1990. Measurements were made using a Maket 1 lidar system from the Institute of Experimental Meteorology. Regions of enhanced stratospheric backscatter, apparently from the February 10, 1991, eruption of Kelut (7.9°S, 11.3°E), were detected on several occasions. Integrated non-Rayleigh backscatter at 532 nm between 16 km an 33 km ranged from a background value of 1.5 ´ 10^-4 sr^-1 to as high as 14.5 ´ 10^-4 sr^-1 in the volcanic plume. During the Akademik Korolev port of call at Hilo, Hawaii, at the start of SAGA 3, intercomparisons were attempted with the NOAA, ERL, CMDL Mauna Loa Observatory lidar located approximately 60 km SW at Mauna Loa Observatory (19.5°N, 155.6°W). The intercomparison results were encouraging but were inconclusive because of limited data and signal degradation resulting from poor observing conditions due to bad weather.

KOMHYR, W.D., C.L. Mateer, and R.D. Hudson. Effective Bass-Paur 1985 ozone absorption coefficients for use with Dobson ozone spectrophotometers. Geophysical Research Letters 98(D11):20,451-20,465 (1993).

Details are presented concerning the derivation of new, standard, effective ozone absorption coefficients for use with Dobson ozone spectrophotometers. The work was performed under auspices of the World Meteorological Organization (WMO) Global Ozone Research and Monitoring Program, with the goal of improving and standardizing ozone measurements throughout the world. The new coefficients, based on the laboratory measurements of Bass and Paur (1985), were sanctioned for use worldwide, beginning January 1, 1992, by the WMO Executive Panel on Environmental Pollution and Atmospheric Chemistry. The new coefficients, together with use of slightly improved Rayleigh molecular scattering coefficients also adopted as standard, yield total ozone amounts 2.6% smaller than values obtained during July 1, 1957 to December 31, 1991, derived from the use of Vigroux (1953, 1967) ozone absorption coefficients. Information is provided also on correcting ozone data obtained in the past to the new ozone absorption coefficient scale.

Larsen, J.C., E.W. Chiou, W.P. Chu, M.P. McCormick, L.R. McMaster, S. OLTMANS, and D. Rind. A comparison of the Stratospheric Aerosol and Gas Experiment II tropospheric water vapor to radiosonde measurements. Journal of Geophysical Research 98(D3):4897-4917 (1993).

Upper tropospheric Stratospheric Aerosol and Gas Experiment II (SAGEII) water vapor observations are compared to correlative radiosonde observations and radiosonde based climatologies. The SAGE II 1987 monthly zonal mean water vapor climatology is compared to both the Global Atmospheric Circulation Statistics (1963-1978) climatology and to the 1987 radiosonde climatology. The clear sky SAGE II climatology is found to be approximately half the level of both the clear/cloudy sky radiosonde climatologies. To determine whether this is realistic for these two different climatologies or includes additional observational and instrumental biases, we took the 1987 radiosonde data set and identified approximately 800 correlative profile pairs. The observational biases inherent to SAGE II and the radiosondes produce a set of profile pairs characteristic of clear sky, land conditions. A critical review of the radiosonde measurement capability was carried out to establish the operating range and accuracy in the upper troposphere. We show that even with tight coincidence criterion, the quality of the profile pair comparisons varies considerably because of strong water vapor variability occurring on small time and space scales. Annual zonal means calculated from the set of profile pairs again finds SAGE II significantly drier in many latitude bands. Resolving the radiosonde data base by hygrometer type shows this to be true for all hygrometers except for the thin film capacitive type (Vaisala Humicap). For this hygrometer, between 4.5 and 6.5 km SAGE II is drier by approximately 25% and from 8.5 to 11.5 km they are nearly equivalent when global annual means are compared. The good agreement with the Vaisala Humicap, currently the most accurate and responsive hygrometer in operational use, suggests existing radiosonde climatologies contain a significant moist bias in the upper troposphere.

LOBERT, J.M., and J. Warnatz. Emissions from the Combustion Process in Vegetation. Fire in the Environment: The Ecological Atmospheric, and Climatic Importance of Vegetation Fires P.J. Crutzen and J.G. Goldammer (eds.). Wiley, New York, 15-37 (1993).

In this chapter we present a detailed description of the processes involved in vegetation fires, emphasizing the flaming and smoldering stages of biomass burning. The major factors influencing the appearance of the single stages are discussed as well as their impact on emissions from fires. General production patterns, especially the importance of the ratio of flaming to smoldering combustion, are outlined for several classes of compounds, and some specific reaction paths that are likely to dominate certain emission formations are discussed. Finally, we summarize the global estimates of the most important emissions and propose that the complex nature of vegetation fires be simplified to enable reliable predictions of the emissions from biomass burning in a global scientific effort.

McCormick, E.W. Chiou, L.R. McMaster, W.P. Chu, J.C. Larsen, D. Rind, and S. OLTMANS. Annual variations of water vapor in the stratosphere and upper troposphere observed by the Stratospheric Aerosol and Gas Experiment II. Journal of Geophysical Research 98(D3):4867-4874 (1993).

This paper presents a description of the annual variations of water vapor in the stratosphere and the upper troposphere derived from observations of the Stratospheric Aerosol and Gas Experiment II (SAGE II). The altitude-time cross sections exhibit annually repeatable patterns in both hemispheres. The appearance of a yearly minimum in water vapor in both hemispheres at approximately the same time supports the idea of a common source(s) for stratospheric dry air. Annual patterns observed at northern mid-latitudes, like the appearance of a hygropause in winter and the weakening and upward shifting of the hygropause from January to May, agree with in situ balloon observations previously obtained over Boulder and Washington, D.C. An increase in water vapor with altitude in the tropics is consistent with methane oxidation in the upper stratosphere to lower mesosphere as a source of water vapor. A poleward gradient is also shown as expected based on a Lagrangian mean circulation. A linear regression analysis using SAGE II data from January 1986 to December 1988 shows that little annual variation occurs in the middle and upper stratosphere with the region of large annual variability near the tropopause. The semi-annual variability is relatively marked at altitudes of 24 and 40 km in the tropics.

Miller, J.M., J.L. Moody, J.M. HARRIS, and A. Gaudry. A 10-year trajectory flow climatology for Amsterdam Island, 1980-1989. Atmospheric Environment 27A(12):1909-1916 (1993).

This paper documents the meteorology and flow climatology of Amsterdam Island, a World Meteorological Organization baseline observatory located in the southern Indian Ocean. The island is strongly influenced by two meteorological systems: the subtropical anticyclone and the westerlies, known as the roaring forties. Westerly surface winds dominate ground-level flow on the island, which receives an annual average precipitation of 1100 mm. A 10-year back air trajectory climatology at 850 hPa to the island showed the same predominant westerly-southwesterly flow (55% of the period). Special emphasis was placed on trajectories that had their origin on the African continent because this was the predominant nonmarine source of trace materials that could be transported to Amsterdam Island. This transport, most frequent in the winter and early spring, coincided with the winter peak of continental radon. A comparison of trajectories and 12-h precipitation amounts on the island showed that most of the rain coincided with southwesterly flow, but many large individual events were frequently associated with northerly and westerly flow regimes. Meteorological information about local conditions and flow climatology is necessary to aid in the interpretation of atmospheric chemistry measurements made at global stations such as Amsterdam Island.

MONTZKA, S.A., R.C. MYERS, J.H. BUTLER, and J.W. ELKINS. Global tropospheric distribution and calibration scale of HCFC-22. Geophysical Research Letters 20(8):703-706 (1993).

Measurements of atmospheric chlorodifluoromethane (HCFC-22), based upon a new calibration scale developed in this laboratory, suggest a global tropospheric mean that is ~28% lower than determined previously from surface-based measurements. A global mean of 101.8 (± 1.2, 1 s.d.) ppt and interhemispheric difference of 13 (± 1) ppt were determined for HCFC-22 in 1992 from air collected in flasks from seven remote sites located between 82°N and 90°S. These results are consistent with mixing ratios predicted from recent emission estimates and a lifetime for HCFC-22 of 13.6 (+1.9, -1.5) yr. Based upon the analysis of flasks and archived air samples from mid-1987 through 1992, a mean growth rate for HCFC-22 of 7.3 (± 0.3)% yr^-1 was estimated for this period.

MONTZKA, S.A., M. Trainer, P.D. Goldan, W.C. Kuster, and F.C. Fehsenfeld. Isoprene and its oxidation products, methyl vinyl ketone and methacrolein, in the rural troposphere. Journal of Geophysical Research 98(D1):1101-1111 (1993).

The mixing ratios of methyl vinyl ketone (CH₂ = CHCOCH₃) and methacrolein (CH₂ = C(CH₃)COH) were measured at a site located in the Kinterbish Wildlife Management Area in western Alabama. The measurements were made between June 15 and July 20, 1990. Considering all the data over the whole measurement period, the concentrations of these two cabonyls were approximately equal to this isolated rural site. The average mixing ratios for methyl vinyl ketone and methacrolein were 0.98 ppbv and 0.66 ppbv, respectively, while the medians were 0.87 ppbv and 0.57 ppbv. The methyl vinyl ketone mixing ratio varied from 3.4 ppbv to the detection limit of the instrument, 0.01 ppbv, while the methacrolein mixing ratio varied from 2.6 ppbv to 0.027 ppbv. These cabonyls constituted a significant fraction of the volatile organic compounds observed at the site: their mixing ratios measured 2 m above the top of the forest canopy were less than that of the dominant compound, isoprene, but were considerably greater than the mixing ratios of anthropogenic compounds (e.g., benzene). The mixing ratios of methyl vinyl ketone and methacrolein were found to be highly correlated and exhibited a systematic variation with respect to each other. On average, during the day, methyl vinyl ketone was larger than methacrolein while methacrolein tended to be slightly larger during the night. The systematic behavior of these compounds with respect to each other and other compounds measured at the site were simulated using a one-dimensional photochemical model. These observations were consistent with the production and loss of isoprene, methyl vinyl ketone, and methacrolein by photochemical oxidation reactions.

Noone, K.B., K.J. Noone, J. Heintzenberg, J. Ström, and J.A. OGREN. In situ observations of cirrus cloud microphysical properties using the counterflow virtual impactor. Journal of Atmospheric and Oceanic Technology 10:294-303 (1993).

This study presents a new technique for making in situ measurements of cirrus cloud microphysical properties. Sampling of cirrus clouds was performed using a counterflow virtual impactor (CVI). The CVI was used to sample cloud elements larger than 4 m in aerodynamic diameter. In conjunction with a Lyman- hygrometer, this gave a direct measurement of the condensed water content. Sampling the cloud elements with the CVI also allowed the authors to examine the size distribution of the residual particles produced by evaporation of the cloud elements. This study discusses and evaluates the CVI technique for use in sampling cirrus clouds, especially for sampling small cloud elements. Measurements of condensed water content and cloud-element (crystal and droplet) concentrations for cirrus uncinus, floccus, and cirrostratus clouds made using the CVI during the International Cirrus Experiment experiment are presented. Examples of size distributions of the residual aerosol particles from cirrus cloud elements are also presented.

Pachenko, M.V., S.A. Terpugova, B.A. BODHAINE, A.A. Isakov, M.A. Sviridenkov, I.N. SOKOLIK, E.V. Romashova, B.I. Nazarov, A.K. Shukurov, E.I. Chistyakova, and T.C. JOHNSON. Optical investigations of dust storms during U.S.S.R.-U.S. experiments in Tadzhikistan, 1989. Atmospheric Environment 27A(16):2503-2508 (1993).

The results of studies on dust aerosol optical properties obtained during the U.S.S.R.-U.S. experiment are discussed. The ground-based and aircraft measurements carried out during the experiment allow the estimation of characteristic values of aerosol optical depth, aerosol light-scattering coefficients, the degree of linear polarization, and aureole phase functions for different atmospheric conditions in Central Soviet Asia. Two dust storms were observed for which the recorded aerosol optical depth at l = 0.55 mm {t_a (0.55)} reached 1.5 on 16 and 17 September 1989, and 3.5 on 20 and 21 September 1989. The optical characteristics (spectral dependence of the optical depth, degree of linear polarization) were similar for two dust episodes.

Parungo, F., C. Nagamoto, B. Kopcewicz, L. Xingsheng, Y. Congzeng, Y. Xiaolan, and J. HARRIS. Investigation of atmospheric aerosols and gases at an East China Station. NOAA Technical Memorandum ERL ARL-201, 37 pp., Air Resources Laboratory, Silver Spring, MD (1993).

From August to October of 1991, when the West Pacific Exploratory Mission (PEM-west) airborne expedition was conducted, ground-level measurements of gases and aerosols were carried out at Lin-an station near the east coast of China. Meteorological parameters such as temperature, pressure, humidity, solar radiation, wind direction, and wind speed were recorded continuously. Concentrations of SO₂, NO₂, O₃, and black carbon were monitored in situ intermittently. Aerosol samples were collected and later analyzed in laboratories. A transmission electron microscope was used to analyze particle concentration, morphology, and size distribution. Elemental compositions of aerosol samples, collected on filters, were determined with a neutron activation analyzer and with a proton induced x-ray energy spectrometer. The water soluble portions of the aerosols were analyzed by ion chromatography. All measurements were recorded in the PEM-west archive. The averages, or the medians, of all measured quantities as well as their standard variations or confidence intervals are documented in this report. The results show that the concentrations of both natural crustal dust and anthropogenic air pollution were very high at this rural station. Most crustal elements (e.g., A1, Si, Fe, Sc) were present in larger particles (d>1 mm), whereas most enriched elements (e.g., S, As, Sb) were in smaller particles. Concentrations of sea-salt ions Na⁺, Mg⁺⁺, and Cl^- were not high, and they were found in large particles. Concentrations of SO₄⁼ and NH₄⁺ correlated well with each other (r – 0.81), and they were found mostly in small particles. Concentrations of NO₃^-, which were < one-fifth of the concentrations of SO₄⁼, also correlated significantly with SO₄⁼ (r = 0.79). Most NO₃^- mass was present in large particles. The variability of air chemistry is mostly influenced by local wind direction and wind speed as well as long-distance air trajectories. Solar radiation also played a role in SO₂-gas-to-SO₄⁼-particle conversion. Rainfall appeared to be an effective way to wash out air contaminants and to improve air quality. However, the effect was generally short-lived. The result of this investigation provided a basic understanding of trace species in the regional surface air, including their sources, transport, transformations, and sinks. The data sets collected at the station can be used as a source inventory for long-range transport of air masses from China to the West Pacific Ocean. Such journeys are believed to have a profound impact on climatic changes.

Parungo, F., C. Nagamoto, G. HERBERT, J. HARRIS, R. SCHNELL, P. SHERIDAN, and N. ZHANG. Individual particle analyses of Arctic aerosol samples collected during AGASP-III. Atmospheric Environment 27(17/18):2825-2837 (1993).

Airborne aerosol samples were collected during the third experiment of the Arctic Gas and Aerosol Sample Program (AGASP-III), and the individual particles were analyzed with electron microscopes and an X-ray energy spectrometer. The temporal and spatial variations of arctic aerosol physicochemical characteristics were studied relevant to the source, transport, and transformation. Air trajectories arriving at the sampling sites generally provided useful information to interpret the aerosol chemistry. When the air masses passed over northern Russia, most of the aerosols were crustal dust, and approximately one-half of them were coated with sulfate. When the air masses were from northwestern Europe, solid particles, coated with sulfuric acid droplets and sulfate particles were the majority. These were probably formed by heterogeneous nucleation of H₂SO₄ followed by partial or complete neutralization. Over open water, numerous large drops containing solid particles and cubic NaCl crystals were observed. However, over the frozen ocean, the drops and seasalt crystals were diminished. Instead, small sulfuric acid droplets, which were probably formed by homogeneous nucleation, were the principal aerosol species. At high altitudes (>5 km), pure sulfuric acid droplets and sulfuric acid drops with foreign nuclei were the dominant aerosols; however, alumina particles occasionally appeared in large quantities. Sulfate aerosols were omnipresent in the arctic stratosphere, troposphere and planetary boundary layer, whereas few nitrate-containing particles were found and then only in the boundary layer.

Penner, J.E., R.J. Charlson, J.M. Hales, N. Laulainen, R. Leifer, T. Novakov, J. OGREN, L.F. Radke, S.E. Schwartz, and L. Travis. Quantifying and minimizing uncertainty of climate forcing by anthropogenic aerosols. DOE/NBB-0092T UC-402. U.S. Department of Energy, Washington, DC, 53 pp. (1993).

The clear-sky climate forcing by anthropogenic aerosols has been shown to be of sufficient magnitude to mask the effects of anthropogenic greenhouse gases over large regions. Anthropogenic aerosols are composed of a variety of aerosol types including water-soluble inorganic species (e.g., sulfate, nitrate, ammonium), organic condensed species, elemental or black carbon, and mineral dust. Estimates of the clear-sky forcing by anthropogenic sulfate aerosols and by organic biomass-burning aerosols have been published previously. Here we estimate the uncertainty in the forcing by these aerosol types. Estimates of the clear-sky forcing by other anthropogenic aerosol types do not even exist though the forcing by these aerosol types is thought to be smaller than that by sulfate and biomass burning aerosols. The cloudy-sky forcing by anthropogenic aerosols, wherein aerosol cloud condensation nuclei concentrations are increased, thereby increasing cloud droplet concentrations and cloud albedo and possibly influencing cloud persistence, may also be significant. In contrast to the situation with the clear-sky forcing, estimates of the cloudy-sky forcing by anthropogenic aerosols are little more than guesses, and it has not been possible to quantify the uncertainty of the estimates. This situation dictates that considerable importance must be attached to quantifying forcing by the anthropogenic aerosol and to defining and minimizing uncertainties in the calculated forcings. In this article, we define a research strategy for improving the estimates of the clear-sky forcing which, if implemented, should allow these effects to be incorporated into climate models within the next several years. The strategy encompasses five major, and necessarily coordinated, activities: surface-based observations of aerosol chemical and physical properties and the radiation field, aircraft-based observations of the same properties, process studies to refine model treatments, satellite observa- tions of aerosol abundance and size distribution, and modeling studies to demonstrate consistency between the observations and to allow extension of the limited set of observations to the global scale. A similar strategy for defining the magnitude of the cloud-sky forcing should also be possible, but our uncertain understanding of this forcing requires that research of a more exploratory nature be carried out prior to engaging in a large-scale experiment of the magnitude of that recommended for the clear-sky forcing.

Rind, D., E.-W. Chiou, W. Chu, S. OLTMANS, J. Lerner, J. Larsen, M.P. McCormick, and L. McMaster. Overview of the Stratospheric Aerosol and Gas Experiment II water vapor observations: method, validation, and data characteristics. Journal of Geophysical Research 98(D3):4835-4856 (1993).

Water vapor observations obtained from the Stratospheric Aerosol and Gas Experiment II (SAGE II) solar occultation instrument for the troposphere and stratosphere are presented and compared with correlative in situ measurement techniques and other satellite data. The SAGE II instrument produces water vapor values from cloud top to approximately 1 mbar, except in regions of high aerosol content such as occurs in the low to middle stratosphere after volcanic eruptions. Details of the analysis procedure, instrumental errors, and data characteristics are discussed. Various features of the data set for the first 5 years after launch (1985-1989) are identified. These include an increase in middle and upper tropospheric water vapor during northern hemisphere summer and autumn, thus at times of warmest sea surface temperature; minimum water vapor values of 2.5-3 ppmv in the tropical lower stratosphere, with lower values during northern hemisphere winter and spring; slowly increasing water vapor values with altitude in the stratosphere, reaching 5-6 ppmv or greater near the stratopause; extratropical values with minimum profile amounts occurring above the conventionally defined tropopause; and higher extratropical than tropical water vapor values throughout the stratosphere except in locations of possible polar stratospheric clouds. SAGE II data will be useful for studying individual water vapor profiles, tropospheric response to climate perturbations, tropospheric-stratospheric exchange (due to its inherent high vertical resolution), and stratospheric transports. It should also aid in the preparation, for the first time on a global scale, of climatologies of the stratosphere and the upper level cloud-free troposphere, for use in radiative, dynamical, and chemical studies.

SHERIDAN, P.J., R.C. SCHNELL, J.D. Kahl, J.F. BOATMAN, and D.M. Garvey. Microanalysis of the aerosol collected over south-central New Mexico during the ALIVE field experiment, May-December 1989. Atmospheric Environment 27A(8):1169-1183 (1993).

Thirty-eight size-segregated aerosol samples were collected in the lower troposphere over the high desert of south-central New Mexico, using cascade impactors mounted onboard two research aircraft. Four of these samples were collected in early May, sixteen in mid-July, and the remaining ones in December 1989, during three segments of the ALIVE field initiative. Analytical electron microscope analyses of aerosol deposits and individual particles from the these samples were performed to physically and chemically characterize the major particulate species present in the aerosol. Air-mass trajectories arriving at the sampling area in the May program were quite different from those calculated for the July period. In general, the May trajectories showed strong westerly winds, while the July winds were weaker and southerly, consistently passing over or very near the border cities of El Paso, Texas, and Ciudad Juarez, Mexico. Aerosol samples collected during the May period were predominantly fine (0.1-0.5 mm dia.), liquid H₂SO₂ droplets. Samples from the July experiment were comprised mostly of fine, solid (NH₂)₂SO₂ or mostly neutralized sulfate particles. In both sampling periods, numerous other particle classes were observed, including many types with probable terrestrial or anthropogenic sources. The numbers of these particles, however, were small when compared with sulfates. Composite particle types, including sulfate/crustal and sulfate/carbonaceous, were also found to be present. The major differences in aerosol composition between the May and July samples (i.e., the extensive neutralization of sulfates in the July samples) can be explained by considering the different aerosol transport pathways and the proximity of the July aerosol to the El Paso/Juarez urban plume. Winds during the December experiment were quite variable, and may have contributed to the widely varying aerosol compositions observed in these samples. When the aircraft sampled the El Paso/Juarez urban plume, high concentrations of carbonaceous particles were collected. These C-rich particles were of three distinct types, two of which showed combustion morphologies and the third an irregular morphology. Concurrent aethalometer measurements of aerosol black carbon concentration were well correlated (r = 0.83) with the total carbonaceous particle fraction in the aerosol samples. Carbonaceous particles were not observed in abundance in any of the May or July samples (even when the wind passed over El Paso), and we attribute the high concentrations in December to increased wintertime burning of wood, fossil fuels, and other combustibles in the urban area.

SHERIDAN, P.J., R.C. SCHNELL, W.H. Zoller, N.D. Carlson, R.A. Rasmussen, J.M. HARRIS, and H. SIEVERING. Composition of Br-containing aerosols and gases related to boundary layer ozone destruction in the Arctic. Atmospheric Environment 27A(17/18);2839-2849 (1993).

During the third Arctic Gas and Aerosol Sampling Program (March 1989), aircraft measurements of atmospheric gases and aerosols were performed in the European Arctic for the purpose of investigating the phenomenon of boundary layer O₃ destruction. Eight high-volume aerosol filter samples were collected in tropospheric air over the pack ice. In these sampling periods, continuous O₃ measurements were made and trace gases were collected in flasks. For all samples, total elemental bromine collected on the filters in excess of the estimated sea salt component (XSFBr) was found to anticorrelate strongly (r = -0.90) with the mean ozone concentration observed during the sampling period. These findings are similar to earlier observations at Alert and Barrow. Air samples collected during these periods were analyzed for Br-containing gases and hydrocarbons. None of these compounds were well correlated with either O₃ or XSFBr concentration over the course of the experiment. This is probably because variable conditions of local meteorology, atmospheric structure and geographic location influenced the degree to which O₃ was depleted, by affecting the size of the reaction reservoir and the source(s) of the reactants. Samples collected in the surface (~50 m deep) isothermal or slightly stable layer (SSL) over pack ice and with light winds from the direction of the central Arctic showed the highest O₃ depletions. When winds were from the direction of open water, significatnly higher O₃ and lower XSFBr values were observed. When the SSL was not present, samples collected below the strong inversion showed less O₃ destruction and lower XSFBr concentrations than similar low altitude samples collected within the SSL. This is consistent with the notion of a larger reservoir volume available for reaction. Gas and aerosol chemistry results were compared for two samples collected close spatially and temporally over ice north of Spitsbergen. Our data indicate that (1) CHBr₃ may be the key organobromine species supplying Br atoms and BrO radicals in a heterogeneous photochemical reaction cycle causing the photolytic destruction of O₃ in the springtime Arctic surface layers, and (2) ambient hydrocarbons (especially C₂H₂) are depleted during O₃ destruction, and may be important in the overall reaction mechanism. This catalytic O₃ depletion process was observed to occur to an extent causing near-total O₃ destruction in the SSL over a 1-2 d period. Thus, relatively rapid photochemical reactions between atmospheric Br, hydrocarbons and aerosols are suggested as driving the photolytic O₃ destruction process.

STONE, R.S., J.R. Key, and E.G. DUTTON. Properties and decay of stratospheric aerosols in the Arctic following the 1991 eruptions of Mount Pinatubo. Geophysical Research Letters 20(21):2359-2362 (1993).

Sunphotometer observations made from an aircraft several months after the June 1991 eruptions of Mount Pinatubo are used to quantify the spectral opacity of the Arctic stratrosphere. Ancillary surface-based measurements are presented in support of the aircraft data that show large increases in stratospheric optical depth attributed to the presence of volcanic aerosols. Visible optical depths greater than 0.2 were observed during flight segments flown above the tropopause. An inversion algorithm and the optical depth data are used to infer effective aerosol size distributions. The distributions tend to be bimodal, having a large-particle mode radius of about 0.50 mm and a small-particle mode of higher concentration with radii less than 0.18 mm. Surface measurements made during spring 1992 and 1993 are also used to estimate a time constant (e-folding time) of about 13.5 months assuming that the Arctic stratosphere's opacity decays exponentially; this estimate is larger than decay times observed following other major volcanic eruptions. Our results suggest that any climate perturbations in the Arctic caused by the eruptions of Pinatubo may be significant and will very likely persist longer than any volcanically-induced changes observed there during the past century.

STURGES, W.T., and J.W. ELKINS. Use of adsorbents to collect selected halocarbons and hydrohalocarbons of environmental interest from large air volumes. Journal of Chromatography 642:123-134 (1993).

Hydrohalocarbons are the proposed replacement compounds for the chlorofluorocarbons. They will initially have very low concentrations, on the order of a ppt (v/v), in the global atmosphere. Neither gas chromatography-electron-capture detection, nor gas chromatography-mass spectrometry, have adequate sensitivity to measure such concentrations directly from air samples; concentration techniques are required to achieve this. We have examined a range of commercially-available adsorbents, including activated charcoals, carbon molecular sieves, porous polymers, and graphitized carbons, for their suitability as ambient temperature concentrating traps for a range of man-made halocarbons and hydrohalocarbons (CFC-12, CFC-11, CFC-13, HCFC-22, HCFC-123 and HFC-134a). From our measurements of specific retention and desorption volumes it was found that no one adsorbent could both collect all of the target compounds with high efficiency, and also allow efficient recovery by thermal desorption. A sequence of adsorbents is required. We designed a 30 cm long ´ 0.64 cm O.D. trap containing HayeSep D_B (a porous polymer), Carboxen 1000 and Carbosieve S-11 (both carbon molecular sieves) to collect all of the target compounds from a 5-l air sample at 25° C and allow efficient recovery with 500 ml of nitrogen carrier gas at 200° C. Good comparability was demonstrated between the adsorbent trapping system and direct loop injection analysis for CFC-12 in ambient air. Precision for all of the compounds analyzed with the adsorbent trap was better than 4%, and improved to better than 1% when ratioed to CFC-12.

STURGES, W.T., J.F. Hopper, L.A. Barrie, and R.C. SCHNELL. Stable lead isotope ratios in Alaskan Arctic aerosols. Atmospheric Environment 27A(17/18):2865-2871 (1993).

Aerosol samples collected at Barrow, Alaska, during February and March 1990 were found to have uniform stable lead isotope compositions. The mean ²⁰⁸Pb/²⁰⁷Pb ratios was 2.423 0.009 and the mean ²⁰⁶Pb/s²⁰⁷Pb ratio was 1.161 0.014. The latter ratio is essentially the same as that obtained from an earlier study of aerosols at two Canadian stations in the High Arctic and is typical of, but not unique to, Eurasian sources of atmospheric lead. Further discriminating power was available in this study through the inclusion of ²⁰⁸Pb/²⁰⁷Pb ratios, which provided additional evidence that the former Soviet Union and eastern Europe are major contributors to atmospheric particulate lead in the Alaskan Arctic, accounting for around two-thirds of the particulate lead measured at Barrow. The remaining third of the lead is attributed to west European sources. There was no evidence for a substantial North American component, other than local contamination.

STURGES, W.T., R.C. SCHNELL, G.S. DUTTON, S.R. Garcia, and J.A. Lind. Spring measurements of tropospheric bromine at Barrow, Alaska. Geophysical Research Letters 20(2):201-204 (1993).

The partitioning of bromine in the lower troposphere between particulate, inorganic gaseous, and organic gaseous phases was measured during the arctic spring. Rapid photochemical production of particulate Br from organic gaseous Br was indicated by (1) an inverse correlation between particulate bromide and organic gaseous bromine (r² = -0.67), (2) a diurnal cycle in the ratio of these two phases, and (3) a diurnal cycle in the ratio of organic gaseous Br to Cl. Organic gaseous Br and Cl were correlated (r² = 0.67) indicating a common, possibly marine, source.

STURGES, W.T., R.C. SCHNELL, S. Landsberger, S.J. OLTMANS, J.M. HARRIS, and S.-M. Li. Chemical and meteorological influences on surface ozone destruction at Barrow, Alaska, during spring 1989. Atmospheric Environment 27A(17/18):2851-2863 (1993).

Surface ozone, particulate bromine and inorganic and organic gaseous bromine species were measured at Barrow, AK, during March and April 1989 to examine the causes of surface ozone destruction during the Arctic spring. Satellite images of the Alaskan Arctic taken during the same period were also studied in conjunction with calculated air mass trajectories to Barrow to investigate the possible origins of the ozone-depleted air. It was found that during major ozone depletion events (O3<25 ppbv) concentrations of particulate bromine and the organic brominated gases bromoform and dibromochloromethane were elevated. Air mass trajectories indicated that the air had crossed areas of the Arctic Ocean where leads had been observed by satellite. The transport time from the leads was typically a day or less, suggesting a fast loss mechanism for ozone. A similarly fast production of particulate bromine was shown by irradiating ambient nighttime air in a chamber with actinic radiation that approximated daylight conditions. Such rapid reactions are not in keeping with gas-phase photolysis of bromoform, but further studies showed evidence for a substantial fraction of organic bromine in the particulate phase; thus heterogeneous reactions may be important in ozone destruction.

STURGES, W.T., C.W. Sullivan, R.C. SCHNELL, L.E. Heidt, and W.H. Pollock. Bromoalkane production by Antarctic ice algae. Tellus 45B:120-126 (1993).

Ice microalgae, collected from the underside of annual sea ice in McMurdo Sound, Antarctica, were found to contain and release to seawater a number of brominated hydrocarbons. These included bromoform, dibromomethane, mixed bromochloromethanes, and methyl bromide. Atmospheric measurements in the McMurdo Sound vicinity revealed the presence of bromoform and methyl bromide in the lower atmosphere, with lowest concentrations inland, further indicating that biogenic activity in the Sound is a source of organic bromine gases to the Antarctic atmosphere. This may have important implications for boundary layer chemistry in Antarctica. in the Arctic, the presence of bromoform has been linked to loss of surface ozone in the spring. We report here preliminary evidence for similar surface ozone loss at McMurdo Station.

Zou, X., A. Barcilon, I.M. Navon, J. WHITAKER, and D.G. Cacuci. An adjoint sensitivity study of blocking in a two-layer isentropic model. Monthly Weather Review 121:2833-2857 (1993).

This paper presents a new methodology for adjoint sensitivity analysis, previously developed in general terms, by Cacuci, into a form directly applicable to meteorological problems. This technique is illustrated by examining the sensitivity of a blocking index in a two-layer isentropic model. The index represents a response function for the sensitivity analysis that, unlike previous meteorological applications, is an operator and not a functional, and thus, extends the scope of adjoint sensitivity to general operator-type responses depending on time and/or space. The sensitivity of the blocking index to perturbations introduced into a model atmosphere, as well as to model parameters, is discussed. The methodology of generalized adjoint sensitivity analysis described in this paper constitutes a prototype for further applications in the atmospheric and/or oceanic sciences.

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