Publication Search Results

CMDL Publication Search Results:

Header

Abstract

Arends, B.G., G.P.A. Kos, R. Maser, D. Schell, W. Wobrock, P. Winkler, J.A. OGREN, K.J. Noone, A. Hallberg, I.B. Svenningsson, A. Wiedensohler, H.-C. Hansson, A. Berner, I. Solly, and C. Kruisz. Microphysics of clouds at Kleiner Feldberg. The Kleiner Feldberg Cloud Experiment 1990 S. Fuzzi (ed.). Kluwer Academic Publishers, Dordrecht, 153-171 (1994).

During a field measuring campaign at Kleiner Feldberg (Taunus) in 1990, microphysical characteristics of clouds have been measured by Forward Scattering Spectrometer Probes (FSSP). The aim was to study the influence of aerosol and meteorological factors on droplet size and number. The results are: More mass in the accumulation size range of the aerosol leads to more droplets in stratocumulus clouds and to higher soluble masses in droplets of stratus clouds. However, the aerosol distribution was coarser in the stratus clouds compared to the stratocumulus clouds. Within the first 200 m from cloud base, the droplets grow while their number decreases. The growth results in a stable size of about 14 m diameter over a large distance from cloud base in many stratocumulus clouds. Two types of mixing processes were observed: processes with reductions in the number of droplets (inhomogeneous mixing) and with reductions in the size of the droplets (homogeneous mixing).

BAKWIN, P.S., P.P. TANS, and P. NOVELLI. Carbon monoxide budget in the Northern Hemisphere. Geophysical Research Letters 21(6):433-436 (1994).

To improve urban air quality, the major industrialized nations of the West took steps during the 1970s and 1980s to reduce carbon monoxide (CO) emissions from automobiles and other industrial sources. Overall, CO/CO₂ emission ratios from the mix of fossil fuel combustion sources have been reduced by about half during 1976-1990. Also, the tropospheric abundance of hydroxyl radical (OH), which is the main sink for CO, is proposed to have increased globally by about 1.0 0.8% yr^-1 [Prinn et al., 1992]. We use a simple two-box model to examine the impact of shrinking emissions and increasing OH on the global abundance of CO. We find that these factors contribute about equally in reducing CO levels in the Northern Hemisphere troposphere by about 1.8 0.8 ppb yr^-1 on average.

Bridgman, H.A., and B.A. BODHAINE. On the frequency of long-range transport events at Point Barrow, Alaska, 1983-1992. Atmospheric Environment 28(21):3537-3549 (1994).

The Point Barrow, Alaska pollutant and meteorological data bases from the Climate Monitoring and Diagnostics Laboratory (CMDL) baseline station are evaluated for the first 120 days of each year between 1983 and 1992. The purpose of this study is to investigate whether a relationship between gaseous and aerosol pollutants during clean sector winds could be used to indicate periods of long-range transport of pollutants. Representative pollutant parameters used include carbon dioxide and aerosol light scattering (s_sp), with methane, condensation nuclei, and black carbon used in support. Several interesting relationships emerge between the gases and s_sp during long-range transport events: (1) the frequency of clean sector winds for most months is greater than 70%, with the range of 31.6-92.8%; (2) high correlations between CO₂ and s_sp do not necessarily occur during long-range transport events, and the relationship between the two parameters is weak and erratic; (3) a considerable majority of transport periods occur in January and March, with the highest frequency in the AGASP measurement years 1983 and 1986, in some contrast to previous analysis using only haze as the pollution indicator; (4) pollution reaching surface monitors at Point Barrow most often originates from pooled air in the Arctic Basin, with no clear definition of more distant source regions.

BUTLER, J.H. The potential role of the ocean in regulating atmospheric CH3Br. Geophysical Research Letters 21(3):185-188 (1994).

An increase or decrease in anthropogenic emissions of methyl bromide (CH₃Br) will induce a compensating flux of this gas from the ocean to the atmosphere. This, in turn, will tend to lessen the expected change in atmospheric mixing ratio from that calculated in models that incorporate a constant oceanic source. To a first degree, this buffering effect by the ocean is independent of in situ oceanic production and land based sources. The partial lifetime of atmospheric CH₃Br with respect to the ocean could reasonably range from 1.3 to 14 y, with a current best estimate of 3.7 y. This yields an effective atmospheric lifetime, which incorporates oceanic losses and thus can be used in simple, mass balance calculations of atmospheric CH₃Br, of 1.2 y, with a probably range of 0.7 to 1.8 y.

Colvile, R.N., R. Sander, T.W. Choularton, K.N. Bower, D.W.F. Inglis, W. Wobrock, D. Schell, I.B. Svenningsson, A. Wiedensohler, H.-C. Hansson, A. Hallberg, J.A. OGREN, K.J. Noone, M.C. Facchini, S. Fuzzi, G. Orsi, B.G. Arends, W. Winiwarter, T. Schneider, and A. Berner. Computer modeling of clouds at Kleiner Feldberg. The Kleiner Feldberg Cloud Experiment 1990, S. Fuzzi (ed.). Kluwer Academic Publishers, Dordrecht, 153-171 (1994).

The airflow, cloud microphysics and gas- and aqueous-phase chemistry on Kleiner Feldberg have been modeled for the case study of the evening of 1 November 1990, in order to calculate parameters that are not easily measured in the cloud and thus to aid the interpretation of the GCE experimental data-set. An airflow model has been used to produce the updraught over complex terrain for the cloud model, with some care required to ensure realistic modeling of the strong stable stratification of the atmosphere. An extensive set of measurements has been made self-consistent and used to calculate gas and aerosol input parameters for the model. A typical run of the cloud model has calculated a peak supersaturation of 0.55% which occurs about 20 s after entering cloud where the updraught is 0.6 m s-1. This figure has been used to calculate the efficiency with which aerosol particles were scavenged; it is higher than that calculated by other methods, and produces a cloud with slightly too many droplets. A broad cloud droplet size spectrum has been produced by varying the model inputs to simulate turbulent mixing and fluctuations in cloud parameters in space and time, and the ability of mixing processes near cloud-base to produce a lower peak supersaturation is discussed. The scavenging of soluble gases by cloud droplets has been observed and departures from Henry's Law in bulk cloud-water samples seen to be caused by variation of pH across the droplet spectrum and the inability of diffusion to adjust initial distributions of highly soluble substances across the spectrum in the time available. Aqueous-phase chemistry has been found to play a minor role in the cloud as modeled, but circumstances in which these processes would be more important are identified.

CONWAY, T.J., P.P. TANS, L.W. WATERMAN, K.W. THONING, D.R. KITZIS, K.A. MASARIE, and N. ZHANG. Evidence for interannual variability of the carbon cycle from the National Oceanic and Atmospheric Administration/Climate Monitoring and Diagnostics Laboratory Global Air Sampling Network. Journal of Geophysical Research 99(D11):22,831-22,855 (1994).

The distribution and variations of atmospheric CO₂ from 1981 to 1992 were determined by measuring CO₂ mixing ratios in samples collected weekly at a cooperative global air sampling network. The results constitute the most geographically extensive, carefully calibrated, internally consistent CO₂ data set available. Analysis of the data reveals that the global CO₂ growth rate has declined from a peak of ~2.5 ppm yr^-1 in 1987-1988 to ~0.6 ppm yr^-1 in 1992. In 1992 we find no increase in atmospheric CO₂ from 30° to 90° N. Variations in fossil fuel CO₂ emissions cannot explain this result. The north pole-south pole CO₂ difference increased from ~3 ppm during 1981-1987 to ~4 ppm during 1988-1991. In 1992 the difference was again ~3 ppm. A two-dimensional model analysis of the data indicates that the low CO₂ growth rate in 1992 is mainly due to an increase in the northern hemisphere CO₂ sink from 3.9 Gt C yr^-1 in 1991 to 5.0 Gt C yr^-1 in 1992. The increase in the north pole-south pole CO₂ difference appears to result from an increase in the southern hemisphere CO₂ sink from ~0.5 to ~1.5 Gt C yr^-1.

Dessler, A.E., E.M. Weinstock, E.J. Hintsa, J.G. Anderson, C.R. Webster, R.D. May, J.W. ELKINS, and G.S. Dutton. An examination of the total hydrogen budget of the lower stratosphere. Geophysical Research Letters 21(23):2563-2566 (1994).

We analyze the hydrogen budget of the lower stratosphere using simultaneous in situ measurements of northern hemispheric water vapor (H₂O) and methane (CH₄) obtained during the spring Stratospheric Photochemistry, Aerosols, and Dynamics Expedition (SPADE), as well as previously published in situ H₂ data. Based on this data, we conclude that approximately two H₂O molecules are produced for each CH₄ molecule destroyed. This implies that H₂ production from CH₄ oxidation is balanced by H₂ oxidation. The uncertainty in this analysis is greatly reduced by the use of multiple data sets. Additionally, we infer that, on an annual and global average, H₂O enters the stratosphere with a mixing ratio of 4.2 ± 0.5 ppmv, and that the quasi-conserved quantity 2´ [CH₄] + [H₂O] has a value of 7.6 ± 0.6 ppmv in these northern hemisphere air parcels (where [x ] denotes the mixing ratio of the constituent x ).

DLUGOKENCKY, E.J., K.A. MASARIE, P.M. LANG, P.P. TANS, L.P. Steele, and E.G. Nisbet. A dramatic decrease in the growth rate of atmospheric methane in the northern hemisphere during 1992. Geophysical Research Letters 20(1):45-48 (1994).

Global measurements of atmospheric methane have revealed a sharp decrease in the growth rate in the Northern Hemisphere during 1992. The average trend for the Northern Hemisphere during 1983-1991 was (11.6 0.2) ppbv yr^-1, but the increase in 1992 was only (1.8 1.6) ppbv. In the Southern Hemisphere, the average increase (1983-1991) was (11.1 0.2) ppbv yr^-1, and the 1992 increase was (7.7 1.0) ppbv. Various possibilities for a change in methane sources or sinks are discussed, but the most likely explanation is a change in an anthropogenic source such as fossil fuel exploitation, which can be rapidly and easily affected by man's activities.

DLUGOKENCKY, E.J., K.A. MASARIE, P.M. LANG, P.P. TANS, L.P. Steele, and E.G Nisbit. Reply to "Comments on 'A dramatic decrease in the growth rate of atmospheric methane in the northern hemisphere during 1992'." Geophysical Research Letters 21(22):2447-2448 (1994).

No abstract.

DUTTON, E.G., P. Reddy, S. RYAN, and J.J. DELUISI. Features and effects of aerosol optical depth observed at Mauna Loa, Hawaii: 1982-1992. Journal of Geophysical Research 99(34):8295-8306 (1994).

Spectral aerosol optical depth, t_a, observed at Mauna Loa, Hawaii, for the past 11 years is analyzed for background variations and the effects of two major volcanic eruptions: El Chichón in 1982 and Mount Pinatubo in 1991. A previously known annual variation and near-background levels are present in the record. The data are of high accuracy, being primarily obtained from an automatic precision sunphotometer and reduced using the Langley-plot slope method. The t_a values over Mauna Loa were greater immediately after the eruption of El Chichón than after Mount Pinatubo due to more direct transport from El Chichón. However, Pinatubo had a greater temporally integrated impact because of greater erupted sulfur mass. A mean solar irradiance decrease of 6.5 (± 2.5) W m^-2 per 0.1 t_a (500 nm) averaged over 24 hours is observed for both volcanic eruptions. Slight differences are suggested between the eruptions, but the differences are not statistically significant. Small differences between the two eruptions in the aerosol size distributions derived from t_a observations are also indicated and are consistent with the suggested difference in total solar irradiance aerosol sensitivity. The near-background t_a values compare well with in situ surface-based aerosol light-scattering measurements extrapolated through the upper troposphere.

Hallberg, A., K.J. Noone, J.A. OGREN, I.B. Svenningsson, A. Flossmann, A. Wiedensohler, H.-C. Hansson, J. Heintzenberg, T.L. Anderson, B.G. Arends, and R. Maser. Phase partitioning of aerosol particles in clouds at Kleiner Feldberg. The Kleiner Feldberg Cloud Experiment 1990 S. Fuzzi (ed.). Kluwer Academic Publishers, Dordrecht, 107-127 (1994).

The partitioning of aerosol particles between cloud droplets and interstitial air by number and volume was determined both in terms of an integral value and as a function of size for clouds on Mt. Kleiner Feldberg (825 m asl), in the Taunus Mountains north-west of Frankfurt am Main, Germany. Differences in the integral values and the size dependent partitioning between two periods during the campaign were observed. Higher number and volume concentrations of aerosol particles in the accumulation mode were observed during Period II compared to Period I. In Period I on average 87 11% (one standard deviation) and 73 7% of the accumulation mode volume and number were incorporated into cloud droplets. For Period II the corresponding fractions were 42 6% and 12 2% in one cloud event and 64 4% and 18 2% in another cloud event. The size dependent partitioning as a function of time was studied in Period II and found to have little variation. The major processes influencing the partitioning were found to be nucleation scavenging and entrainment.

Hallberg, A., J.A. OGREN, K.J. Noone, K. Okada, J. Heintzenberg, and I.B. Svenningsson. The influence of aerosol particle composition on cloud droplet formation. The Kleiner Feldberg Cloud Experiment 1990 S. Fuzzi (ed.). Kluwer Academic Publishers, Dordrecht, 153-171 (1994).

A difference in partitioning between cloud droplets and interstitial air for two chemical species (elemental carbon and sulfur) with different expected behavior in nucleation scavenging was observed in clouds at Mt. Kleiner Feldberg (825 m asl), near Frankfurt, Germany. The fraction of sulfur incorporated in cloud droplets was always higher than the fraction of elemental carbon. This difference in partitioning has also been observed in fog but under different pollution conditions in the Po Valley, Italy. Both of these studies were based on bulk samples. In the present study at Kleiner Feldberg, impactor samples of the particles in the interstitial air and the cloud droplet residuals were taken and a single particle analysis was done on the samples. It was found that, for a given particle size, the majority of particles forming cloud droplets were soluble and that insoluble particles preferentially remained in the interstitial air.

HARRIS, J.M., and J.D.W. Kahl. Analysis of 10-day isentropic flow patterns for Barrow, Alaska: 1985-1992. Journal of Geophysical Research 99(D12):25,845-25,855 (1994).

Atmospheric transport patterns to Barrow, Alaska, during 1985-1992 were investigated using a newly developed isentropic air trajectory model. The new model features a layer-averaged mode that is activated whenever an air parcel traveling isentropically approaches the Earth's surface. A dynamic preprocessing program ensures that trajectories always arrive at a constant, predetermined altitude. Ten-day back trajectories arriving twice daily at 500, 1500, and 3000 m above sea level revealed no long-term trends in flow patterns during the 8-year period. Frequency of transport type was fairly stable from year to year, except in the anomalously warm year of 1989 when increased numbers of trajectories from the Aleutian region were observed. During the Arctic haze season, trajectories suggest that transport of pollution from north central Russia occurs near the surface (about 20% frequency), whereas that from northern Europe occurs at higher elevations (about 10% frequency).

HOFMANN, D.J., S.J. OLTMANS, J.M. HARRIS, J.A. LATHROP, G.L. KOENIG, W.D. KOMHYR, R.D. EVANS, D.M. QUINCY, T. Deshler, and B.J. JOHNSON. Recovery of stratospheric ozone over the United States in the winter of 1993-1994. Geophysical Research Letters 21(17):1779-1782 (1994).

Total ozone levels, which were 10-15% below normal over the U.S. during the winter of 1992-1993, returned to levels slightly above normal during the winter of 1993-1994. Investigation of ozone vertical profiles indicates that in the region where severe depletion occurred in 1992-1993 (25% reductions at 12-22 km), ozone had returned to normal, while above this region, ozone was abnormally high. Thus total ozone was also high. Low ozone values in 1992-1993 were believed to be related to heterogeneous chemistry on the Pinatubo volcanic aerosol. This interpretation is strengthened by these observations since the particle surface area available for heterogeneous processes in the stratosphere diminished substantially at midlatitudes during 1993 and was not replenished by transport from the equatorial reservoir during the winter as had occurred during the previous winter. However, the observation of continued unusually high ozone above 24 km in winter suggests that this phenomenon, thought to also have been at least partially due to heterogeneous chemistry, is mainly related to dynamics. Unusually high total ozone levels in high northerly latitudes during the winter of 1993-1994 and especially in early February, associated with warm stratospheric temperatures during December and January, are probably the source of high ozone above 42 km in midlatitudes at this time.

HOFMANN, D.J., S.J. OLTMANS, W.D. KOMHYR, J.M. HARRIS, J.A. LATHROP, A.O. Langford, T. Deshler, B.J. Johnson, A. Torres, and W.A Matthews. Ozone loss in the lower stratosphere over the United States in 1992-1993: Evidence for heterogeneous chemistry on the Pinatubo aerosol. Geophysical Research Letters 98(D1):65-68 (1994).

Ozone profiles obtained at Boulder, Colorado, and Wallops Island, Virginia, indicate that ozone was about 25% below normal during the winter and spring of 1992-93 in the 12-22 km region. This large ozone reduction in the lower stratosphere, though sometimes partially compensated by higher than normal ozone above 24 km, was responsible for the low total column ozone values observed across the United States during this period. Normal temperatures throughout the low ozone region suggest that transport-related effects are probably not the most important cause of the ozone deficits. The region of low ozone at Boulder corresponds closely with the location of the enhanced H2SO4/H2O aerosol from the Pinatubo eruption of 1991 as measured near Boulder and at Laramie, Wyoming. Trajectory analyses suggest that except at low altitudes in spring, air parcels on the days of the ozone measurements generally arrived at Boulder from higher latitude, although seldom higher than 60N, and hence may have been subjected to heterogeneous chemical processing on the surface on Pinatubo aerosol droplets resulting in chlorine-catalyzed ozone destruction, a process which is believed to be more effective under the lower winter temperatures and sunlight levels of higher latitudes.

HOFMANN, D.J., S.J. OLTMANS, J.A. LATHROP, J.M. HARRIS, and H. VÖMEL. Record low ozone at the South Pole in the spring of 1993. Geophysical Research Letters 21(6): 421-424 (1994).

On October 12, 1993, a balloon-borne ozone detector recorded a total ozone value of 91 5 Dobson Units (DU) at the U.S. Amundsen-Scott Station at the South Pole. This is the lowest value of total ozone ever recorded anywhere, 13% below the previous low of 105 DU at the South Pole in October of 1992 [Hofmann and Oltmans, 1993]. A region with a thickness of 5 km, from 14 to 19 km, was totally devoid of ozone as compared to only about half this thickness for the ozone void in 1992. Sub-100 DU total ozone values were observed on several soundings during 1993 whereas the 105 DU value was observed on only one occasion in 1992. The vertical profile of ozone indicates that the main reason for the record low ozone values in 1993 was an approximately 1 km upward extension of the ozone hole caused by unusual ozone loss in the 18-23 km region. Temperatures in this region were unusually low in September and October. Thus, the extension of the ozone hole may have been the result of the prolonged presence of polar stratospheric clouds at 18-23 km combined with the continued presence of sulfate aerosol from the Pinatubo eruption and, finally, increased chlorine levels. This scenario resulted in elevated ozone loss in a region where the ozone loss process is normally not saturated.

Kerr, J.B., H. Fast, C.T. McElroy, S.J. OLTMANS, J.A. LATHROP, E. Kyro, A. Paukkunen, H. Claude, U. K�hler, C.R. Sreedharan, T. Takao, and Y. Tsukagoshi. The 1991 WMO international ozonesonde intercomparison at Vanscoy, Canada. Atmosphere Ocean 32(4):685-716 (1994).

An intercomparison of ozonesondes was held at Vanscoy, Saskatchewan, from 13 to 24 May 1991. The intercomparison, which was sponsored by the WMO and hosted by the Atmospheric Environment Service (AES) of Canada, was attended by scientists from six countries: Canada, Finland, Germany, India, Japan, and the United States. Four different makes of ozonesondes were used: the ECC sonde, The Brewer-Mast sonde, the Indian ozonesonde, and the Japanese RSII-KC79 ozonesonde. These represent most of the sonde types that are in routine operation in the Global Ozone Observing System. A balloon payload and telemetry system was developed to accommodate up to eight ozonesondes that could operate independently and transmit data simultaneously to a ground receiver. Ten flights were launched, each carrying 7 or 8 sondes, and a total of 65 successful profile measurements were made. The payloads were carried to altitudes between 35 and 40 km. The measured profiles are used to determine statistically meaningful evaluations of the different sonde types. The results compared with those from previous intercomparisons indicate that there has been a general improvement in performance for most of the types. In addition there appears to have been changes with time in the relative sensitivity to tropospheric ozone for different sonde types. This result should be considered when drawing conclusions regarding trends in tropospheric ozone.

KOMHYR, W.D., R.D. GRASS, R.D. EVANS, R.K. LEONARD, D.M. QUINCY, D.J. HOFMANN, and G.L. KOENIG. Unprecedented 1993 ozone decrease over the United States from Dobson spectrophotometer observations. Geophysical Research Letters 21(3):201-204 (1994).

Dobson spectrophotometer observations conducted since the early-to-mid 1960s at Bismarck, North Dakota; Caribou, Maine; Boulder, Colorado; Wallops Island, Virginia; Nashville, Tennessee; and at Fresno, California, since 1983, have revealed record low total ozone values during 1993. The tendency toward the low ozone values began in May 1992, but accelerated in early 1993. During January-August 1993, ozone monthly means at the stations were more than 2 standard deviations below long-term normal monthly means 72% of the time and more than 3 standard deviations below normals 42% of the time. On average, the January-April 1993 ozone values were 12.6% below normal, with ozone deficits as large as 18% observed at Caribou and Wallops Island in January. Of particular concern are unusually low ozone values that occur in summertime when solar ultraviolet insulation is high. Such record lows occurred at four of the six stations (Caribou, Wallops Island, Fresno, and Nashville). During May-August 1993, ozone was on average 8.5% below normal at these sites. Monthly means at these stations were, furthermore, lower on average by 3.7% than corresponding lowest values observed there in the past. The ozone decrease of 12.6% below normal at the six continental Dobson instrument stations during the winter and spring months of 1993 implies a possible average increase in UV erythemal radiation at that time of 16-25% above normal. The 8.5% decrease in ozone at Caribou, Wallops Island, Fresno, and Nashville, implies that on average, UV erythemal radiation may have been higher than normal at these stations during the summer of 1993 by 11-17%.

MONTZKA, S.A., R.C. MYERS, J.H. BUTLER, and J.W. ELKINS. Early trends in the global tropospheric abundance of hydrochlorofluorocarbon-141b and 142b. Geophysical Research Letters 21(23):2483-2486 (1994).

Hydrochlorofluorocarbons (HCFCs) are rapidly replacing chlorofluorocarbons (CFCs) as foam-blowing agents, solvents, and refrigerants. These substitutes, however, still contain chlorine and have the potential to deplete stratospheric ozone. Although the ozone destruction capacity of HCFCs is believed to be significantly less than that for CFCs, concern over unrestricted chlorine input to the atmosphere from HCFC use has prompted proposals for future limits on production. Here we report the first global time-series for mixing ratios of HCFC-141b (1,1-dichloro-1-fluoroethane) and HCFC-142 (1-chloro-1, 1-difluoroethan) in air collected at seven remote sampling stations. Global mean mixing ratios in mid-1993 were 0.7 parts per 1012 (ppt) for HCFC-141b and 4.3 ppt for HCFC-142b. The global atmospheric abundance of HCFC-141b increased by 0.9 ppt, or a factor of ~3, in 1993. Global abundance of HCFC-142b increased by 1.1 ppt yr^-1 during 1992-1993. The results suggest that HCFCs are currently used extensively for replacing CFCs in selected applications. Furthermore, measured levels are significantly higher than expected based on available emission estimates and consumption predictions.

Nemesure, S., R.D. Cess, E.G. DUTTON, J.J. DELUISI, Z. Li, and H.G. Leighton. Impact of clouds on the shortwave radiation budget of the surface atmosphere system for snow-covered surfaces. Journal of Climate 7(4):579-585 (1994).

Recent data from the Earth Radiation Budget Experiment (ERBE) have raised the question as to whether or not the addition of clouds to the atmospheric column can decrease the top-of-the atmosphere (TOA) albedo over bright snow-covered surfaces. To address this issue, ERBE shortwave pixel measurements have been collocated with surface insolation measurements made at two snow-covered locations: the South Pole and Saskatoon, Saskatchewan. Both collocated datasets show a negative correlation (with solar zenith angle variability removed) between TOA albedo and surface insolation. Because increased cloudiness acts to reduce surface insolation, these negative correlations demonstrate that clouds increase the TOA albedo at both snow-covered locations.

NOVELLI, P. C., K. A. MASARIE, P. P. TANS, and P. M. LANG. Recent changes in atmospheric carbon monoxide. Science 263:1587-1590 (1994).

Measurements of carbon monoxide (CO) in air samples collected from 27 locations between 71°N and 41°S show that atmospheric levels of this gas have decreased worldwide over the past 2 to 5 years. During this period, CO decreased at nearly a constant rate in the high northern latitudes. In contrast, in the tropics an abrupt decrease occurred beginning at the end of 1991. In the Northern Hemisphere, CO decreased at a spatially and temporally averaged rate of 7.3 (± 0.9) parts per billion per year (6.1 percent per year) from June 1990 to June 1993, whereas in the Southern Hemisphere, CO decreased 4.2 (± 0.5) parts per billion per year (7.0 percent per year). This recent change is opposite a long-term trend of a 1 to 2 percent per year increase inferred from measurements made in the Northern Hemisphere during the past 30 years.

NOVELLI, P. C., J. E. Collins, Jr., R. C. MYERS, G. W. Sachse, and H. E. Scheel. Reevaluation of the NOAA/CMDL carbon monoxide reference scale and comparisons with CO reference gases at NASA-Langley and the Fraunhofer Institut. Journal of Geophysical Research 99(D6), 12,833-12,839 (1994).

The carbon monoxide (CO) reference scale created by the National Oceanic and Atmospheric Administration/Climate Monitoring and Diagnostics Laboratory (NOAA/CMDL) is used to quantify measurements of CO in the atmosphere, calibrate standards of other laboratories and to otherwise provide reference gases to the community measuring atmospheric CO. This reference scale was created based upon a set of primary standards prepared by gravimetric methods at CMDL and has been propagated to a set of working standards. In this paper we compare CO mixing ratios assigned to the working standards by three approaches: (1) calibration against the original gravimetric standards, (2) calibration using only working standards as the reference gas, and (3) calibration against three new gravimetric standards prepared by CMDL. The agreement between these values was typically better than 1%. The calibration histories of CMDL working standards are reviewed with respect to expected rates of CO change in the atmosphere. Using a Monte Carlo approach to simulate the effect of drifting standards on calculated mixing ratios, we conclude that the error solely associated with the maintenance of standards will limit the ability to detect small CO changes in the atmosphere. We also report results of intercalibraion experiments conducted between CMDL and the Diode Laser Sensor Group (DACOM) at the NASA Langley Research Center (Hampton, Virginia), and CMDL and the Fraunhofer-Institut (Garmisch-Partenkirchen, Germany). Each laboratory calibrated several working standards for CO using their reference gases, and these results were compared to calibrations conducted by CMDL. The intercomparison of eight standards (CO concentrations between ~100 and ~165 ppb) by CMDL and NASA agreed to better than 2%. The calibration of six standards (CO concentrations between ~50 and ~210 ppb) by CMDL and the Fraunhofer-Institut agreed to within 5% for all six standards.

NOVELLI, P.C., and R.M. ROSSON (eds.). Report of the WMO meeting of experts on global carbon monoxide measurements. World Meteorological Organization, Global Atmosphere Watch No. 98, WMO/TD-NO. 645, Geneva, Switzerland, 82 pp. (1994).

No abstract.

OLTMANS, S.J., and H. Levy, II. Surface ozone measurements from a global network. Atmospheric Environment 28(1):9-24 (1994).

From a network of sites, primarily in the Atlantic and Pacific Ocean regions, measurements of the surface ozone concentration yield information on the seasonal, synoptic, and diurnal patterns. These sites, generally removed from the effects of local pollution sources, show characteristics that typify broad geographical regions. At Barrow, AK; Mauna Loa, HI; American Samoa; and South Pole, data records of 15-20 years show trends that in all cases are a function of season. This dependence on season is important in understanding the causes of the long-term changes. At Barrow, the summer (July, August, September) increase of 1.7% per year is probably indicative of photochemical production. At South Pole, on the other hand, the summer (December, January, February) decrease is related to photochemical losses and enhanced transport from the coast of Antarctica. At all the sites there is a pronounced seasonal variation. In the Southern Hemisphere (SH), all locations which run from 14 to 90°S show a winter (July-August) maximum and summer minimum. In the Northern Hemisphere (NH) most of the site show a spring maximum and autumn minimum. At Barrow (70°N) and Barbados (14), however, the maxima occur during the winter, but for every different reasons. At many of the sites, the transport changes associated with synoptic scale weather patterns dominate the day-to-day variability. This is particularly pronounced at Bermuda and the more tropical sites. In the tropics, there is a very regular diurnal surface ozone cycle with minimum values in the afternoon and maxima early in the morning. This appears to result from photochemical destruction during the day in regions with very low concentrations of nitrogen oxides. At Niwot Ridge, CO, and Mace Head, Ireland, there is clear evidence of photochemical ozone production in the summer during transport from known regional pollution sources.

Parungo, F., C. Nagamoto, M.-Y. Zhou, A.D.A. Hansen, and J. HARRIS. Aeolian transport of aerosol black carbon from China to the Ocean. Atmospheric Environment 28(20): 3251-3260 (1994).

To investigate long-range transport of aerosol black carbon (BC) from China to the downwind seas and ocean, BC concentrations were measured in Bejing city, at a rural station near Bejing city, and at a rural station near Shanghai, for information about source strengths. Aerosol samples were also collected on board research ships in four cruises over the East China Sea and Western Pacific Ocean to determine BC distribution. The data were used to verify a simple one-dimensional transport model. In the marine boundary layer with prevailing westerlies of 5 m s^-1, the BC total deposition rate, including both dry deposition and wet deposition, was computed as -1 ´ 10^-5s^-1, and the half-life was estimated to be 19 h. The residence time was approximately 5 d, concentrations subsequently diminishing to the background level. Longer lifetime and farther transport would be probable in the free troposphere. Because BC particles have great surface areas and are very absorbent of electromagnetic waves, their concentration and distribution in the atmosphere may have profound effects on radiation budgets and climate change.

Parungo, F., C. Nagamoto, M.-Y. Zhou, A.D.A. Hansen, and J. HARRIS. Aeolian transport of aerosol black carbon from China to the ocean. Atmospheric Environment 28(20):3251-3260 (1994).

To investigate long-range transport of aerosol black carbon (BC) from China to the downwind seas and ocean, BC concentrations were measured in Beijing city, at a rural station near Beijing city, and at a rural station near Shanghai, for information about source strengths. Aerosol samples were also collected on board research ships in four cruises over the East China Sea and Western Pacific Ocean to determine BC distribution. The data were used to verify a simple one-dimensional transport model. In the marine boundary layer with prevailing westerlies of 5 ms^-1, the BC total deposition rate, including both dry deposition and wet deposition, was computed as �1 x 10^-5 s^-1, and the half-life was estimated to be 19h. The residence time was approximately 5 d, concentrations subsequently diminishing to the background level. Longer lifetime and farther transport would be probable in the free troposphere. Because BC particles have great surface areas and are very absorbent of electromagnetic waves, their concentration and distribution in the atmosphere may have profound effects on radiation budgets and climate change.

Parungo, F., Z. Li, X. Li, D. Yang, and J. HARRIS. Gobi dust storms and The Great Green Wall. Geophysical Research Letters 21(11):999-1002 (1994).

Vast belts of forest planted across the northern arid lands of China, called "The Great Green Wall," are probably one of the most aggressive weather modification programs in the twentieth century. The purpose is to reduce eolian transport of dust from the Gobi Desert. Preliminary data indicate a negative trend in dust-storm frequency and duration since the 1960s. Effects on atmospheric radiation and cloud microphysics appear to be statistically insignificant in the studied period. However, only time can show any long-term impact on our environment.

Penner, J.E., R.J. Charlson, J.M. Hales, N.S. 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. Bulletin of the American Meteorological Society 75(3):375-400 (1994).

Anthropogenic aerosols are composed of a variety of aerosol types and components including water-soluble inorganic species (e.g., sulfate, nitrate, ammonium), condensed organic species, elemental or black carbon, and mineral dust. Previous estimates of the clear sky forcing by anthropogenic sulfate aerosols and by organic biomass-burning aerosols indicate that this forcing is of sufficient magnitude to mask the effects of anthropogenic greenhouse gases over large regions. Here, the uncertainty in the forcing by these aerosol types is estimated. The clear-sky forcing by other anthropogenic aerosol components cannot be estimated with confidence, although the forcing by these aerosol types appears 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 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 is not possible to quantify the uncertainty of the estimates. In view of present concerns over greenhouse gas-induced climate change, this situation dictates the need to quantify the forcing by anthropogenic aerosols and to define and minimize uncertainties in the calculated forcings. In this article, a research strategy for improving the estimates of the clear sky forcing is defined. The strategy encompasses five major, and necessarily coordinated, activities: surface-based observations of aerosol chemical and physical properties and their influence on the radiation field; aircraft-based observations of the same properties; process studies to refine model treatments; satellite observations of aerosol abundance and size distribution; and modeling studies to demonstrate consistency between the observations, to provide guidance for determination of the most important parameters, and to allow extension of the limited set of observations to the global scale. Such a strategy, if aggressively implemented, should allow these effects to be incorporated into climate models in the next several years. A similar strategy for defining the magnitude of the cloudy sky forcing should also be possible, but the less firm understanding of this forcing suggests that research of a more exploratory nature be carried out before undertaking a research strategy of the magnitude recommended for the clear sky forcing.

PETERSON, J.T., and R.M. ROSSON (eds.). Climate Monitoring and Diagnostics Laboratory No. 21 Summary Report 1993. Environmental Research Laboratories, Boulder, CO. 152 pp. (1994).

No abstract.

Salawitch, R.J., S.C. Wofsy, P.O. Wennberg, R.C. Cohen, J.G. Anderson, D.W. Fahey, R.S. Gao, E.R. Keim, E.L. Woodbridge, R.M. Stimpfle, J.P. Koplow, D.W. Kohn, C.R. Webster, R.D. May, L. Pfister, E.W. Gottlieb, H.A. Michelsen, G.K. Yue, J.C. Wilson, C.A. Brock, H.H. Jonsson, J.E. Dye, D. Baumgardner, M.H Proffitt, M. Loewenstein, J.R. Podolske, J.W. ELKINS, G.S. DUTTON, E.J. Hintsa, A.E. Dessler, E.M. Weinstock, K.K. Kelly, K.A. Boering, B.C. Daube, K.R. Chan, and S.W. Bowen. The distribution of hydrogen, nitrogen, and chlorine radicals in the lower stratosphere: Implications for changes in O₃ due to emission of NO_y from supersonic aircraft. Geophysical Research Letters 21(23):2547-2550 (1994).

In situ measurements of hydrogen, nitrogen, and chlorine radicals obtained in the lower stratosphere during SPADE are compared to results from a photochemical model that assimilates measurements of radical precursors and environmental conditions. Models allowing for heterogeneous hydrolysis of N₂O₅ agree well with measured concentrations of NO and ClO, but concentrations of HO₂ and OH are underestimated by 10 to 25%, concentrations of NO₂ are overestimated by 10 to 30%, and concentrations of HC1 are overestimated by a factor of 2. Discrepancies for [OH] and [HO₂] are reduced if we allow for higher yields of O(¹D) from O₃ photolysis and for heterogeneous production of HNO₂. The data suggest more efficient catalytic removal of O₃ by hydrogen and halogen radicals relative to nitrogen oxide radicals than predicted by models using recommended rates and cross sections. Increases in [O₃] in the lower stratosphere may be larger in response to inputs of NO_y from supersonic aircraft than estimated by current assessment models.

Svenningsson, B., H.-C. Hansson, A. Wiedensohler, K. Noone, J. OGREN, A. Hallberg, and R. Colville. Hygroscopic growth of aerosol particles and its influence on nucleation scavenging in cloud: Experimental results from Kleiner Feldberg. The Kleiner Feldberg Cloud Experiment 1990, S. Fuzzi (ed.). Kluwer Academic Publishers, Dordrecht, 129-152 (1994).

The hygroscopic growth of individual aerosol particles has been measured with a Tandem Differential Mobility Analyzer. The hygroscopic growth spectra were analyzed in terms of diameter change with increasing RH from 20% to 85%. The measurements were carried out during the GCE cloud experiment at Kleiner Feldberg, Taunus, Germany in October and November 1990. Two groups of particles with different hygroscopic growth were observed. The less-hygroscopic group had average growth factors of 1.11, 1.04, and 1.02 for particle diameters of 50, 150, and 300 nm, respectively. The more-hygroscopic group had average growth factors of 1.34, 1.34, and 1.37 for the same particle diameters. The average fraction of less-hygroscopic particles was about 50%. Estimates of the soluble fractions of the particles belonging to the two groups are reported. Hygroscopic growth spectra for total aerosol, interstitial aerosol, and cloud drop residuals were measured. A comparison of these hygroscopic growths of individual aerosol particles provides clear evidence for the importance of hygroscopic growth in nucleation scavenging. The measured scavenged fraction of particles as a function of diameter can be explained by the hygroscopic growth spectra.

Tuck, A.F., D.W. Fahey, M. Lowenstein, J.R. Podolske, K.K. Kelly, S.J. Hovde, D.M. Murphy, and J.W. ELKINS. Spread of denitrification from 1987 Antarctic and 1988-1989 Arctic stratospheric vortices. Journal of Geophysical Research 99(D10):20,573-20,583 (1994).

Vertical profiles of N₂O and NO_y taken by the ER-2 outside the vortex are used to construct average vertical profiles of F(NO_y) = NO_y/(A-N₂O), where A is the tropospheric content of N₂O three years prior to the measurements. The southern hemisphere has less nitrous oxide in the range 400 < q < 470 K, by up to 25% relative to the northern hemisphere. F(NO_y) is the ratio of NO_y produced to N₂O lost in a stratospheric air mass since entry from the troposphere. The profiles of F(NO_y) have the following characteristics: (1) Relative to 1991-1992, a year without denitrification inside or outside the vortex, the northern hemisphere in 1988-1989 showed denitrification outside the vortex ranging up to 25% and averaging 17% above q = 425 K. (2) Relative to the northern hemisphere in 1991-1992, the southern hemisphere in 1987 showed denitrification outside the vortex ranging up to 32% and averaging 20% above q = 400 K. (3) Below q = 400 K the southern hemisphere showed enhancements of F(NO_y) relative to the northern hemisphere in 1991-1992 ranging up to 200% at q = 375 K, outside the vortex. Corresponding profiles of residual water, R(H₂O) = H₂O - 2 [1 · 6 - CH₄], are considered and shown to be consistent with those of F(NO_y) in the sense that they show deficits outside the Antarctic vortex, which was both dehydrated and denitrified, but not outside the 1988-1989 Arctic vortex, which was denitrified but not dehydrated. R(H₂O) is the water content of stratospheric air with the contribution from methane oxidation subtracted. Comparison of F(NO_y) and R(H₂O) below 400 K outside the Antarctic vortex leads to the suggestion that dehydration in the Antarctic vortex occurs by the sedimentation of ice crystals large enough to fall out of the stratosphere, whereas denitrification occurs mainly on mixed nitric acid-water crystals which evaporate below the base of the vortex at q = 400 K but above the tropopause.

Waugh, D.W., R.A. Plumb, P.A. Newman, M.R. Schoeberl, L.R. Lait, M. Loewenstein, J.R. Podolske, J.W. ELKINS, and K.R. Chan. Fine-scale, poleward transport of tropical air during AASE 2. Geophysical Research Letters 21(23):2603-2606 (1994).

The poleward transport of tropical air in the lower stratosphere during the winter period of the second Airborne Arctic Stratospheric Expedition (December 1991-March 1992) is examined using contour advection calculations. These calculations show that filaments of tropical air extend into midlatitudes and are wrapped around the equatorward edge of the polar jet. Simultaneously, filaments are drawn from the polar vortex and are intermingled with the filaments of tropical air. The tropical filaments are consistent with measurements of chemical tracers taken aboard the ER-2 and DC-8 aircraft which show localized regions, in midlatitudes, of air with the characteristics of tropical air.

Wobrock, W., D. Schell, R. Maser, W. Jaeschke, H.-W. Georgii, W. Wieprecht, B.G. Arends, J.J. Mols, G.P.A. Kos, S. Fuzzi, M.C. Facchini, G. Orsi, A. Berner, I. Solly, C. Kruisz, I.B. Svenningsson, A. Wiedensohler, H.-C. Hansson, J.A. OGREN, K.J. Noone, A. Hallberg, S. Pahl, T. Schneider, P. Winkler, W. Winiwarter, R.N. Colvile, T.W. Chourlarton, A.I. Flossman, and S. Borrmann. The Kleiner Feldberg cloud experiment 1990. An Overview. The Kleiner Feldberg Cloud Experiment 1990, S. Fuzzi (ed.). Kluwer Academic Publishers, Dordrecht, 153-171 (1994).

An overview is given of the Kleiner Feldberg cloud experiment performed from 27 October until 13 November 1990. The experiment was carried out by numerous European research groups as a joint effort within the EUROTRAC-GCE project in order to study the interaction of cloud droplets with atmospheric trace constituents. After a description of the observational site and the measurements which were performed, the general cloud formation mechanisms encountered during the experiment are discussed. Special attention is given here to the process of moist adiabatic lifting. Furthermore, an overview is given regarding the pollutant levels in the gas phase, the particulate and the liquid phase, and some major findings are presented with respect to the experimental objectives. Finally, a first comparison attempts to put the results obtained during this campaign into perspective with the previous GCE field campaign in the Po Valley.

Wofsy, S.C., K.A. Boering, B.C. Daube, Jr., M.B. McElroy, M. Loewenstein, J.R. Podolske, J.W. ELKINS, G.S. DUTTON, and D.W. Fahey. Vertical transport rates in the stratosphere in 1993 from observations of CO₂, N₂O , and CH₄. Geophysical Research Letters 21(23):2571-2574 (1994).

Measurements of CO₂, N₂O, and CH₄ are analyzed to define hemispheric average vertical exchange rates in the lower stratosphere from November 1992 to October 1993. Effective vertical diffusion coefficients were small in summer, 1 m² s^-1 at altitudes below 25 km; values were similar near the tropopause in winter, but increased markedly with altitude. The analysis suggests possibly longer residence times for exhaust from stratospheric aircraft, and more efficient transport from 20 km to the middle stratosphere, than predicted by many current models. Seasonally-resolved measurements of stratospheric CO₂ and N₂O provide significant new constraints on rates for global-scale vertical transport.

Climate Monitoring & Diagnostics Laboratory
325 Broadway R/E/CG
Boulder, CO 80303
(303) 497-6074