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Anderson, T.L., and J.A. OGREN. Determining aerosol radiative properties using the TSI 3563 integrating nephelometer. Aerosol Science and Technology 29:57-69 (1998).

Methods for reducing and quantifying the uncertainties in aerosol optical properties measured with the TSI 3563 integrating nephelometer are presented. For nearly all applications, the recommended calibration gases are air and CO₂. By routinely characterizing the instrumental response to these gases, a diagnostic record of instrument performance can be created. This record can be used to improve measurement accuracy and quantify uncertainties due to instrumental noise and calibration drift. When measuring scattering by particles, size segregation upstream of the nephelometer at about 1 mm aerodynamic diameter greatly increases the information content of the data for two reasons: one stemming from the independence of coarse and fine particles in the atmosphere, and the second stemming from the size dependence of the nephelometer response. For many applications (e.g., extinction budget studies) it is important to correct nephelometer data for the effects of angular nonidealities. Correction factors appropriate to a broad range of sampling conditions are given herein and are shown to be constrained by the wavelength dependence of light scattering, as measured by the nephelometer. Finally, the nephelometer measurement is nondestructive, such that the sampled aerosol can be further analyzed downstream. Data from two nephelometers operated in series are used to evaluate this procedure. A small loss of super-mm particles (5-10%) is found, while the sub-mm data demonstrates measurement reproducibility with ±1%.

Angevine, W.M., P.S. BAKWIN, and K.J. Davis. Wind profiler and RASS measurements compared with measurements from a 450-m-tall tower. Journal of Atmospheric and Oceanic Technology 15:818-825 (1998).

A 915-MHz boundary layer wind profiler with radio acoustic sounding system (RASS) was sited 8 km from a very tall (450 m) television transmitting tower in north-central Wisconsin during the spring, summer, and autumn of 1995. The profiler measured wind means and variances, and the RASS attachment measured virtual temperature. These quantities are compared to measurements from cup and sonic anemometers and a thermometer/hygrometer at 396 m above ground level on the tower. The precision of hour-averaged profiler winds is better than 1 m s^-1, and the precision of the RASS virtual temperature is better than 0.9 K. Corrections to the virtual temperature measured by the RASS are discussed, and a new virtual temperature retrieval method is proposed. Vertical velocity variance correlation is similar to a previous study, and the fact that bias is small indicates that the calculation method used is reliable.

BAKWIN, P.S., P.P. TANS, D.F. HURST, and C. ZHAO.Measurements of carbon dioxide on very tall towers: results of the NOAA/CMDL program.Tellus 50B:401-415 (1998).

Measurements of carbon dioxide (CO₂) mixing ratios have been carried out since 1992 on a 610-m tall communications tower in North Carolina and since 1994 on a 447-m tall tower in Wisconsin. The data provide insights into the influence of pollution (fossil fuel combustion), biological exchange, boundary layer dynamics, and advective transport on CO₂ mixing ratios over the continents. In this paper we provide an overview of the data, describe access to the data, and suggest ways in which these results could be used to improve simulations of the global carbon cycle. In particular, the data will be very useful to constrain model estimates of covariance between terrestrial ecosystem fluxes of CO₂ and diurnal and seasonal variations of planetary boundary layer mixing.

BAKWIN, P.S., P.P. TANS, J.W.C. White, and R.J. Andres. Determination of the isotopic (¹³C/¹²C) discrimination by terrestrial biology from a global network of observations. Global Biogeochemical Cycles 12(3):555-562 (1998).

We analyze data from the National Oceanic and Atmospheric Administration/Climate Monitoring and Diagnostics Laboratory global air sampling network in order to extract the signatures of isotopic (¹³C/¹²C) discrimination by the terrestrial biota and of fossil fuel combustion for the regions surrounding the sampling sites. We utilize measurements of carbon monoxide (CO) to give an estimate of the contribution of fossil fuel combustion to the short-term variability of carbon dioxide (CO₂). In general, variations of CO₂ are more strongly dominated by biological exchange, so the isotopic signature of fossil fuel combustion, while consistent with inventory estimates, is not well constrained by the observations. Conversely, results for isotope discrimination by the terrestrial biosphere are not strongly dependent on assumptions about fossil fuel combustion. Our analysis appears valid primarily for stations fairly near continental source/sink regions, particularly for midlatitude regions of the northern hemisphere. For these stations we derive a mean discrimination of –16.8 per mil (‰), with site-to-site variability of 0.8‰ (1 standard deviation) and with little or no consistent latitudinal gradient.

BERGIN, M.H., E.A. Meyerson, J.E. Dibb, and P.A. Mayewski. Relationship between continuous aerosol measurements and firn core chemistry over a 10-year period at the South Pole. Geophysical Research Letters 25(8):1189-1192 (1998).

Before ice core chemistry can be used to estimate past atmospheric chemistry it is necessary to establish an unambiguous link between concentrations of chemical species in the air and snow. For the first time a continuous long-term record of aerosol properties (aerosol light scattering coefficient, s_sp, and Ångström exponent, å) at the South Pole are compared with the chemical record from a high resolution firn core (~10 samples per year) covering the period from 1981 to 1991. Seasonal signals in å, associated with winter minima due to coarse mode seasalt and summer maxima due to accumulation mode sulfate aerosol, are reflected in the firn core SO₄^2-/Na⁺concentration ratio. Summertime ratios of s_sp and aerosol optical depth, t, to corresponding firn core sulfur concentrations are determined and the "calibrations" are applied to sulfur concentrations in snowpits from a previous study. Results show that s_sp estimates from snowpit sulfur concentrations are in agreement with atmospheric measurements while t estimates are significantly different, which is likely due to the lack of understanding of the processes that mix surface air with air aloft.

BODHAINE, B.A., E.G. DUTTON, R.L. McKenzie and P.V. Johnston. Calibrating broadband UV instruments: Ozone and solar zenith angle dependence. Journal of Atmospheric and Oceanic Technology 15:916-926 (1998).

A UV spectroradiometer was installed at Mauna Loa Observatory (MLO), Hawaii, in July 1995. This instrument has been employed to characterize several broadband UV instruments of a type commonly used to estimate erythemal irradiance at many sites around the globe. One year of clear sky data from MLO has been analyzed for solar zenith angles (SZA) of 5°-85°, in steps of 5°, and for total ozone values in the range 220-310 DU measured with a Dobson spectrophotometer. Because the spectral responses of various broadband instruments can be quite different, and particularly because the erythemal response defined for human skin is significantly different than that of many broadband instruments, the calibration of a broadband instrument reporting in erythemal units is strongly dependent on total ozone and SZA. When a broadband instrument is placed in the field it is necessary to know the calibration as a function of ozone and SZA to determine accurate erythemal irradiance. However, the manufacturers of broadband instruments do not generally provide information on the ozone dependence of the calibration. A procedure is described here for determining the calibration of a broadband UV instrument by comparison with a calibrated spectroradiometer. This procedure does not require precise knowledge of the spectral response of the broadband instrument. This analysis shows that if, for example, total ozone concentration decreased from 300 DU to 200 DU, the calibration constant of a broadband instrument should be increased by almost 20%. Therefore, the broadband instrument would significantly underestimate the increase of erythema.

Borys, R.D., D.H. Lowenthal, M.A. Wetzel, F. Herrera, A. Gonzolez, and J. HARRIS. Chemical and microphysical properties of marine stratiform cloud in the North Atlantic. Journal of Geophysical Research 103(D17): 22,073-22,085 (1998).

The chemical and microphysical properties of marine stratiform clouds were measured at a ridgetop elevation of 992 m above mean sea level (AMSL) on Tenerife in the Canary Islands in the eastern North Atlantic during the summers of 1995 and 1996. The results show an inverse relationship between hourly-averaged cloud droplet diameter and droplet number concentration, which ranged from 116 to 1355 cm^-3. Strong relationships were observed between droplet number and equivalent clear air concentrations of non-sea-salt sulfate, nitrate, and elemental carbon in the droplets. Droplet sizes inferred from radiances measured by satellite for clouds offshore and upwind agreed with droplet sizes derived for clouds over the mountain sampling site, and also with those measured in cloud 4-5 hours later. Estimated cloud short-wave radiative forcing was enhanced by 8% in radiative model studies of polluted versus clean clouds with droplet concentrations of 786 and 127 cm^-3 and droplet effective radii of 6 to 10 mm, respectively.

BUTLER, J.H., S.A. MONTZKA, A.D. CLARKE, J.M. LOBERT, and J.W. ELKINS. Growth and distribution of halons in the atmosphere. Journal of Geophysical Research 103(D1):1503-1511 (1998).

The atmospheric burden of halons has continued to increase in recent years, despite an international ban on their production and sales in developed nations as of January 1, 1994. Halon emissions persist because of a lack of suitable substitutes for critical uses as fire extinguishants. As of January 1, 1997, halons H-1301 (CBrF₃), H-1211 (CBrClF₂), and H-2402 (CBr₂F₄) were present in the troposphere at 2.3 ± 0.1, 3.5 ± 0.1, and 0.45 ± 0.03 pmol mol^-1. During 1995-1996 the tropospheric mole fraction of H-1301 increased at 0.044 ± 0.011 pmol mol^-1 yr^-1, while that for H-1211 grew at 0.16 ± 0.016 pmol mol^-1 yr^-1. These increases are significant and of concern because of the efficiency of bromine in depleting stratospheric ozone and because of the long atmospheric lifetimes of these gases. Given the current atmospheric record and the reported amount of halon produced before the ban on their production, emission of H-1301 at the 1995-1996 rate could continue for another 40 years, but H-1211 would be depleted in 8-12 years. Exemptions to the ban on production may extend these periods. Tropospheric H-2402 is increasing at 9 ± 1 fmol mol^-1 yr^-1, but historical data on its production and use are lacking.

Chan, L.Y., H.Y. Liu, K.S. Lam, T. Lang, S.J. OLTMANS, and J.M. HARRIS. Analysis of the seasonal behavior of trophpspheric ozone at Hong Kong. Atmospheric Environment 32(2):159-168 (1998).

Ozonesondes have been launched at Hong Kong (22.2°N, 114.3°E) since 1993. The results of data analysis of the ozone profiles are presented, with a focus on the seasonal cycle in tropospheric ozone. The results show that the tropospheric ozone column has an obvious maximum in spring and a minimum in summer. The former. The former is a common feature at many locations in the Northern Hemisphere. The summer minimum is attributed to the onset of the summer monsoon when air flow from the Asian continent is replaced by air from the South China Sea or the tropical Pacific. The tropospheric ozone has an important influence on the seasonal cycle of total ozone at Hong Kong. The seasonal cycle of ozone mixing ratio below 2 km is bimodal with ozone peaks in spring and autumn. A frequently observed feature in late autumn and winter is a relative minimum of ozone mixing ratio (as low as 30-40 ppbv) in the upper troposphere (from about 9 to 16 km). Trajectory analysis shows this relative minimum of ozone is associated with air masses coming from the tropical region. It is proposed that the East Asia local Hadley circulation is responsible for this feature.

Cooper, O.R., J.L. Moody, J.C. Davenport, S.J. OLTMANS, B.J. JOHNSON, X. Chen, P.B. Shepson, and J.T. Merrill. Influence of springtime weather systems on vertical ozone distributions over three North American sites. Journal of Geophysical Research 103(D17): 22,001-22,013 (1998).

The Atmosphere/Ocean Chemistry Experiment (AEROCE) ’96 Springtime Intensive was designed to quantify major atmospheric chemical species transported from the United States east coast to the North Atlantic Ocean. During the study ozonesondes were launched almost every day between March 22 and May 3, 1996, from Charlottesville, Virginia; Purdue University, Indiana; and Bermuda. Whenever possible, the Charlottesville sondes were timed to fly behind passing cold fronts into midtropospheric to upper tropospheric bands of dry air. The dry air was hypothesized to contain ozone of stratospheric origin and was detected with color-enhanced satellite water vapor imagery. Soundings were placed in three categories: sondes launched ahead of an approaching cold front, sondes launched behind a passing cold front, and sondes launched into cutoff lows. This stratification explained much of the vertical ozone variation at each site. Tropospheric mean ozone increased with height at all three sites under postfrontal conditions, with relatively little increase with height under prefrontal conditions. Backward trajectories from Charlottesville indicated that the postfrontal air masses originated at high elevations to the northwest, while prefrontal air masses came from relatively lower elevations to the southwest. Transport also explained the lower tropospheric ozone differences between Charlottesville and Bermuda. The enhanced ozone observed in the upper troposphere over Charlottesville compared with the other sites may be linked to upwind conditions more favorable for tropopause folding. However, we believe that selective launching based on the operational use of satellite water vapor imagery allowed us to sample the full range of tropospheric ozone over Charlottesville.

Curtis, J., G. Wendler, R. STONE, and E. DUTTON. Precipitation decrease in the western Arctic, with special emphasis on Barrow and Barter Island, Alaska. International Journal of Climatology 18:1687-1707 (1998).

Over the Arctic during the last few decades a decrease in annual precipitation and snow depths have been observed; this decrease is especially pronounced during the winter months. This decrease was not only found over northern Alaska but also over the high latitude Canadian stations and Russian drift stations. Further, satellite monitoring of North America snow cover has revealed a significant decreasing trend in mid-spring cover since 1972. The temperature increased during the last few decades in the Arctic, hence the simplest explanation—normally increased temperature leads to high precipitation—is not valid. A casual explanation for these trends had been related to the shift of the Aleutian low and Arctic high. This study, with special emphasis on the surface observation data from Barrow and Barter Island indicates: (1) not only the frequency, but the mean intensity of precipitation has decreased; (2) the amount of total cloud cover and in particular, low cloudiness, has decreased with time; (3) sea-level pressure did not show any significant trends. Variability in atmospheric pressure, however, decreased with time, suggesting that either the intensity and/or frequency of cyclones has decreased; and (4) a shift in seasonal resultant winds at Barrow has been observed.

Deshler, T., and S.J. OLTMANS. Vertical profiles of volcanic aerosol and polar stratospheric clouds above Kiruna, Sweden: Winters 1993 and 1995. Journal of Atmospheric Chemistry 30:11-23 (1998).

Vertical profiles of aerosol were measured in February 1993 and January-March 1995 using balloonborne particle counters released from Kiruna, Sweden. Condensation nuclei (CN) and aerosol with radii ³0.15–10.0 mm were measured in 8-12 size classes. The three flights in 1993 were within the polar vortex. Temperatures were below polar stratospheric cloud (PSC) threshold temperatures on one flight and a thin PSC was observed. The volcanic aerosol in the 1993 vortex was similar to that in 1992. In 1993, surface areas were 10-20 mm² cm^-3 and volumes 1-3 mm³ cm^-3. In 1995 three of five flights were within the polar vortex. The volcanic aerosol had decreased to 3-7 mm² cm^-3 and 0.1-0.4 mm³ cm^-3. The top of the volcanic aerosol layer in both years was near 500 K potential temperature (~20 km). A thick nitric acid and water PSC was observed in January 1995. In the thickest region of this PSC nearly all CN were observed to be activated and surface areas of 5-20 mm² cm^-3 were calculated. The volumes observed in this PSC were closer to what would be expected for particles composed of nitric acid trihydrate than for ternary solution droplets. In 1993 the opposite was observed, the volumes in the thin PSC were closer to what would be expected for ternary solution droplets.

DLUGOKENCKY, E.J., K.A. MASARIE, P.M. LANG, and P.P. TANS. Continuing decline in the growth rate of the atmospheric methane burden. Nature 393:447-450 (1998).

The global atmospheric methane burden has more than doubled since pre-industrial times, and this increase is responsible for about 20% of the estimated change in direct radiative forcing due to anthropogenic greenhouse-gas emissions. Research into future climate change and the development of remedial environmental policies therefore require a reliable assessment of the long-term growth rate in the atmospheric methane load. Measurements have revealed that although the global atmospheric methane burden continues to increase with significant interannual variability, the overall rate of increase has slowed. Here we present an analysis of methane measurements from a global air sampling network that suggests that, assuming constant OH concentration, global annual methane emissions have remained nearly constant during the period 1984-1996, and that the decreasing growth rate in atmospheric methane reflects the approach to a steady state on a timescale comparable to methane’s atmospheric lifetime. If the global methane sources and OH concentration continue to remain constant, we expect average methane mixing ratios to increase slowly from today’s 1730 nmol mol^-1 to ~1800 nmol mol^-1, with little change in the contribution of methane to the greenhouse effect.

EVANS, R.D., D.M. QUINCY, W.D. Komhyr, and G.L. KOENIG. Results of four international Dobson spectrophotometer intercalibrations held since 1992. Proceedings XVIII Quadrennial Ozone Symposium, II, R.D. Bojkov and G. Visconti (eds.), pp. 899-902, Parco Scientifico e Technologico d'Abruzzo, Italy (1998).

Since the last Quadrennial Ozone Symposium in 1992, four international Dobson spectrophotometer intercomparisons have been held, under sponsorship of the World Meteorological Organization and in collaboration with the International Ozone Commission in efforts to improve the quality of ground-based ozone measurements. These intercomparisons provide information on existing Dobson instrument calibration levels, and allow improvements in succeeding measurements by improving the instruments calibrations and operators’ knowledge of the measurements, routine maintenance, and standard data processing techniques. The 35 instruments calibrated are a substantial part of the international network of Dobson spectrophotometers in the Global Ozone Observing System coordinated by the WMO. In general, the calibration of most of the instruments was highly satisfactory, within 1% of the standard. Individual instrument results are presented, as well as comparisons with Brewer ozone spectrophotometers, a SOAZ spectrometer, and an Automated Filter Ozonometer. Differences in the ozone profiles obtained from Umkehr measurements made with different instruments on the same morning are also presented.

Glosík, G. Bánó, E.E. FERGUSON, and W. Lindinger. Selected ion flow drift tube study of the formation and dissociation of CO⁺.N₂ ions in nitrogen buffer gas: the CO⁺.N₂ bond energy. International Journal of Mass Spectrometry 176:177-188 (1998).

Efficient vibrational quenching of CO⁺ (n = 1) by N₂ led to the deduction of an unexpectedly strong attractive potential between CO⁺ and N₂. This was subsequently confirmed by ab initio quantal calculations of the CO⁺.N₂ bond strength. The bond energy of CO⁺.N₂ has now been determined in a selected ion flow drift tube, by measuring collisional destruction and formation of CO⁺ .N₂. The bond energy is found to be 0.7 ± 1.2 eV. This is sufficiently large to rationalize the fast vibrational quenching but somewhat less than an ab initio value. The large value of D(CO⁺.N₂) can unambiguously be explained by resonance interaction between CO⁺ .N₂ and the charge-transfer CO.N₂⁺ states.

Grant, W.B., R.B. Pierce, S.J. OLTMANS, and E.V. Browell. Seasonal evolution of total and gravity wave induced laminae in ozonesonde data in the tropics and subtropics. Geophysical Research Letters 25(11):1863-1866 (1998).

ECC ozonesonde ozone and potential temperature profile data for several stations from 25.05°S to 19.6°N were processed to look for the type, seasonal behavior, and vertical distribution of the laminar structure in the troposphere and lower stratosphere. The fractions of ozone laminae associated with Rossby and gravity wave induced laminae were determined from correlations between ozone and potential temperature structure. Gravity-wave-induced laminae in the stratosphere were found to be associated with deep convection in the winter months and winds from the southwest in the summer months for the Hilo station, and orographic wave activity for the Pretoria station in September. No significant Rossby-wave activity was found for these stations. Lower tropospheric laminae of mixed characteristics were found to be prevalent at Hilo related to transport from the northeast and the continental tropical stations in the June-August period which are associated with biomass burning.

Halthore, R.N., S. Nemesure, S.E. Schwartz, D.G. Imre, A. Berk, E.G. DUTTON, and M.H. BERGIN. Models overestimate diffuse clear-sky surface irradiance: A case for excess atmospheric absorption. Geophysical Research Letters 25(19):3591-3594 (1998).

Radiative transfer models consistently overestimate surface diffuse downward irradiance in cloud-free atmospheres by 9 to 40% at two low altitude sites while correctly calculating direct-normal solar irradiance. For known systematic and random measurement errors and for realistic aerosol optical properties, the discrepancy can be resolved by a reduction in the vertical aerosol optical thickness (AOT) inferred from sunphotometric measurements by an average 0.02 ± 0.01 for 32 cases examined, together with a compensating increase in a continuum-like atmospheric absorptance over the solar spectrum of ~5.0% ± 3.0%. This phenomenon is absent at two high altitude sites, where models and measurements agree to within their mutual uncertainties. Examination of apparent AOT at several locations around the globe also indicates presence of such excess atmospheric absorption. The proposed absorption and corresponding reduction in AOT would have important consequences for climate prediction and remote sensing.

Hansel, A., M. Glantschnig, C.H. Scheiring, W. Lindinger, and E.E. FERGUSON. Energy dependence of the isomerization of HCN⁺ to HNC⁺ via ion molecule reactions. Journal of Chemical Physics 109(5):1743-1747 (1998).

The mechanism for isomerization of HNC⁺ ions to the more stable HNC⁺ in collision with CO₂ at thermal energy has been confirmed to be a double proton transfer in the collision complex. First a proton is transferred from C in HCN⁺, then, following CN rotation in the complex, the proton is transferred to the N atom, both proton transfers being exothermic. The mechanism has been established by measuring the reaction of HCN⁺ with CO₂ as a function of energy from thermal up to 0.7 eV relative kinetic energy in a selected ion flow drift tube. The isomers are distinguished by the use of monitors that react differently with the two isomers. A similar HCN⁺ to HNC⁺ isomerization occurs in thermal collisions with CO.

Hansel, A., Ch. Scheiring, M. Glantschnig, W. Lindinger, and E.E. FERGUSON. Thermochemistry of HNC, HNC⁺, and CF₃⁺. Journal of Chemical Physics 109(5):1748-1750 (1998).

In the course of mechanistic studies on HCN⁺ isomerization in reaction with CO and CO₂, we have fortuitously determined the heats and formation of HNC and HNC⁺ with good precision. The uncertainty is the uncertainty in the heat of formation of HCN, ± 1 kcal mol^–1. This appears to be the first precise experimental determination of these energies. Several prior theoretical calculations have produced reasonable energy values. We also observed the reaction of HCN⁺ with CF₄ to be slightly endothermic, which allows the determination of an upper limit on the threshold energy for CF₄_®CF₃⁺ + F of 14.28 eV, in excellent agreement with several prior determinations but in sharp disagreement with the most recent determination.

HARRIS, J.M, S.J. OLTMANS, E.J. DLUGOKENCKY, P.C. NOVELLI, B.J. JOHNSON, and T. MEFFORD. An investigation into the source of the springtime tropospheric ozone maximum at Mauna Loa Observatory. Geophysical Research Letters 25(11) 1895-1898, 1998.

Measurements of CH₄, CO, O₃, and H₂O vapor from Mauna Loa Observatory (MLO) are examined in conjunction with isentropic trajectories to investigate the cause of a maximum in tropospheric O₃ consistently observed during spring. CO and O₃ have been found to be positively correlated in pollution plumes containing O₃ precursors downwind of industrialized regions. However, we report that during continental transport from Asia, O₃ is not correlated with either CO or CH₄, although CO and CH₄ are strongly correlated. The relationship between CO and CH₄ suggests common source regions. The lack of correlation between these species and O₃ probably indicates an O₃ source distinct from that of CO and CH₄. While Asian pollution does not appear to be a strong candidate for causing the spring increase in O₃, transport characteristics and H₂O vapor measurements are consistent with both an upper-tropospheric/stratospheric contribution and an enhancement from transport across O₃gradients.

Herman, R.L., D.C. Scott, C.R. Webster, R.D. May, E.J. Moyer, R.J. Salawitch, Y.L. Yung, G.C. Toon, B. Sen, J.J. Margitan, K.H. Rosenlof, H.A. Michelsen, and J.W. ELKINS. Tropical entrainment time scales inferred from stratospheric N₂O and CH₄ observations. Geophysical Research Letters 25(15):2781-2784 (1998).

Simultaneous in situ measurements of N₂O and CH₄ were made with a tunable diode laser spectrometer (ALIAS II) aboard the Observations from the Middle Stratosphere (OMS) balloon platform from New Mexico, Alaska, and Brazil during 1996 and 1997. We find different compact relationships of CH₄ with N₂O in the tropics and extra-tropics because mixing is slow between these regions. Transport into the extra-tropics from the tropics or the polar vortex leads to deviations from the normal compact relationship. We use measured N₂O and CH₄ and a simple model to quantify entrainment of midlatitude stratospheric air into the tropics. The entrainment time scale is estimated to be 16 (+17, -8) months for altitudes between 20 and 28 km. The fraction of tropical air entrained from the extra-tropical stratosphere is 50% (+18%, -30%) at 20 km, increasing to 78% (+11%) at 28 km.

Heymsfield, A.J., L.M. Miloshevich, C. Twohy, G. Sachse, and S. OLTMANS. Upper-tropospheric relative humidity observations and implications for cirrus ice nucleation. Geophysical Research Letters 25(9):1343-1346 (1998).

Relative humidity (RH) measurements acquired in orographic wave cloud and cirrus environments are used to investigate the temperature-dependent RH required to nucleate ice crystals in the upper troposphere, Rh_nuc(T). High ice-supersaturations in clear air--conducive to the maintenance of aircraft contrails yet below Rh_nuc and therefore insufficient for cirrus formation--are not uncommon. Earlier findings are supported that Rh_nuc in midlatitude, continental environments decreases from water-saturation at temperatures above -39°C to 75% RH at -55°C. Uncertainty in determining Rh_nuc below -55°C results in part from size detection limitations of the microphysical instrumentation but analysis of data from the SUCCESS experiment indicates that Rh_nuc below -55° C is between 70 and 88%. A small amount of data acquired off-shore suggests the possibility that Rh_nuc may also depend on properties of the aerosols.

Hintsa, E.J., P.A. Newman, H.H. Jonsson, C.R. Webster, R.D. May, R.L. Herman, L.R. Lait, M.R. Schoeberl, J.W. ELKINS, P.R. WAMSLEY, G.S. DUTTON, T.P. Bui, D.W. Kohn, and J. G. Anderson. Dehydration and denitrification in the Arctic polar vortex during the 1995-1996 winter. Geophysical Research Letters 25:501-504, 1998.

Dehydration of more than 0.5 ppmv water was observed between 18 and 19 km (q ~450-465 K) at the edge of the Arctic polar vortex on February 1, 1996. More than half the reactive nitrogen (No_y) had also been removed, with layers of enhanced No_y at lower altitudes. Back trajectory calculations show that air parcels sampled inside the vortex had experienced temperatures as low as 188 K within the previous 12 days, consistent with a small amount of dehydration. The depth of the dehydrated layer (~1 km) and the fact that trajectories passed through the region of ice saturation in one day imply selective growth of a small fraction of particles to sizes large enough (>10 m to be irreversibly removed on this timescale. Over 25% of the Arctic vortex in a 20-30 K range of q is estimated to have been dehydrated in this event.

HOFMANN, D.J., R.S. STONE, M.E. Wood, T. Deshler, and J.M. HARRIS. An analysis of 25 years of balloonborne aerosol data in search of a signature of the subsonic commercial aircraft fleet. Geophysical Research Letters 25(13):2433-2436 (1998).

The University of Wyoming balloonborne condensation nuclei (CN) record for 1973-1997 was analyzed to determine possible effects of commercial aircraft on the 8.6-12.7 km altitude range of the atmosphere, which includes the primary commercial air lanes between 29 and 41 kft. Thin layers of highly concentrated CN are often observed in this region of the atmosphere. Generally, aircraft flight information is not available for past balloon soundings making it impossible to ascribe a source to any specific observed CN layers. However, a CN layer observed in March 1997 was traced to a particular aircraft thus supporting the hypothesis that at least some of the observed layers are contrail remnants. Using the Laramie data set, a method was developed to quantify the enhancement in CN concentration induced by aircraft in comparison with natural background levels. We estimate conservatively that the contribution of the commercial aircraft fleet in the vicinity of Laramie, Wyoming, averaged over the period of record, amounts to about 10% of the background CN concentration. The frequency of occurrence of the CN layers approximately doubled from 1980 to 1992.

HURST, D.F., P.S. BAKWIN, and J.W. ELKINS, Recent trends in the variability of halogenated trace gases over the United States. Journal of Geophysical Research 103(D19):25,299-25,306 (1998).

Recent trends in the atmospheric variability of seven halogenated trace gases are determined from 3 years (November 1994 through October 1997) of hourly gas chromatographic measurements at a 610 m tower in North Carolina and 17 months (June 1996 through October 1997) of similar measurements at a 450 m tower in Wisconsin. Production of five of these gases, CCl₃F (CFC-11), CCl₂F₂ (CFC-12), CCl₂FCClF₂ (CFC-113), CH₃CCl₃ (methyl chloroform), and CCl₄ (carbon tetrachloride), is now strictly regulated in the United States and other developed countries under international legislation. C₂Cl₄ (tetrachloroethene) and SF₆ (sulfur hexafluoride) are currently produced without restriction, but requests for voluntary cutbacks in C₂Cl₄ emissions have been made, at least in the United States. Atmospheric variability of these gases is examined at several sampling heights on the towers, but trends are deduced using only nighttime data at the top sampling level of each tower to minimize variability driven by local emissions and the diurnal cycle of the planetary boundary layer, leaving regional emissions as the main source of day-to-day variability. Significant downward trends are determined for CFC-12, CFC-113, CH₃CCl₃, and C₂Cl₄ variability at both towers, reflecting decreased emissions of these gases in two regions of the United States. Trends in CFC-11, CCl₄, and SF₆ variability at both towers are not significantly different from zero.

Kallenborn, R., M. Oehme, D.D. Wynn-Williams, M. Schlabach, and J. HARRIS. Ambient air levels and atmospheric long-range transport of persistent organochlorines to Signy Island, Antarctica. The Science of the Total Environment 220:167-180 (1998).

Levels of persistent organic pollutants (POPs), such as polychlorinated biphenyls and pesticides have been determined in ambient air at Signy Island, Antarctica, over a period of 17 weeks. Mean concentrations for single polychlorianted biphenyls (0.02-17 pg/m³), for chlordanes (0.04-0.9 pg/m³), DDT compounds (0.07-040 pg/m³) and g -hexachlorocyclohexane (HCH, 22 pg/m³) were comparable to those in arctic air. However, a -HCH levels were approximately one order of magnitude lower. Compared to the Arctic, differences were also observed in the concentration ratios of a -/g -HCH and chlordane compounds. Two possible atmospheric long-range transport episodes from South America were found by comparing 10-day back trajectories with observed concentration changes. The lower limits of determination (LOD) were mainly governed by the field blanks. They were satisfactory for the most volatile PCBs. However, many concentrations for DDT and chlordane compounds were below the LODs (range 0.1-1 pg/m³) or even the instrumental detection limit (0.01-0.03 pg/m³).

Kinne, S., R. Bergstrom, O.B. Toon, E. DUTTON, and M. Shiobara. Clear-sky atmospheric solar transmission: An analysis based on FIRE 1991 field experiment data. Journal of Geophysical Research 103(D16):19,709-19,720 (1998).

We consider explanations for disagreements between models and measurements of clear-sky solar transmission. Data from four cloud-free days during the FIRE 1991 field experiment are studied. Our model simulations of solar broadband fluxes exceed measurements by about 10%. About half of this discrepancy occurs in the near-infrared spectral region. About half of this discrepancy can be linked to the direct irradiance. These deviations suggest (1) model underestimates in near-infrared trace-gas absorption and (2) errors in visible diffuse fluxes. The near-infrared error in other models is generally less than the one associated with our model. Other models suggest more trace-gas absorption. However, variations among models are so large that they swamp current measurement errors such as uncertainties of the water vapor column. The visible diffuse flux error cannot be easily explained. We illustrate that a poor cosine response of solar flux instruments could have contributed. Also, underestimates of aerosol absorption could be a factor. There is clearly a need for measurements and comparisons at high spectral resolution to better quantify the sources of these errors. However, these comparisons will only be fruitful if we have confidence in the accuracy of our models and the accuracy of the data.

Knapp, K.G., M.L. Jensen, B.B. Balsley, J.A. Bognar, S.J. OLTMANS, T.W. Smith, and J.W. Birks. Vertical profiling using a complementary kite and tethered balloon platform at Ferryland Downs, Newfoundland, Canada: Observation of a dry, ozone-rich plume in the free troposphere. Journal of Geophysical Research 103(D11):13,389-13,397 (1998).

Vertical profiles of ozone mixing ratio, water vapor mixing ratio, and temperatures obtained during August 3-13, 1995, at Ferryland Downs, Newfoundland, Canada, as part of the North Atlantic Regional Experiment (NARE) demonstrate the use of high-altitude tethered balloons and kites as measurement platforms. In combination with a novel, fast winching system, previously developed for use with kites, tethered balloons were used for profiling chemical species and meteorological parameters to altitudes as high as 7.6 km. Tethered balloon profiling complements profiling with kite systems by expanding the range of wind conditions over which data can be obtained. A total of 65 profiles were collected using these platforms. Elevated ozone mixing ratios highly anticorrelated with water vapor mixing ratios observed during August 4-7 suggest an upper atmospheric source for ozone-rich air during summertime in the Maritime Provinces of Canada. Ozone mixing ratios reaching 100 parts per billion by volume (ppbv) in this air mass were associated with water vapor mixing ratios as low as 0.1 g kg^-1. Profiles collected during August 11-13 demonstrate the use of kite and balloon systems to obtain vertical profiles over a wide range of wind conditions. Ozone mixing ratios of 65 ppbv were observed at 1 km altitude on August 12, while profiles collected on August 13 exhibit no evidence of elevated ozone. These results document the rapid transport of an air mass containing elevated ozone levels over the sampling site that would not have been possible with either system alone.

KOENIG, G.L., S.J. OLTMANS, J.A. LATHRUP, R.D. EVANS, B.J. JOHNSON, D.M. QUINCY, and M.A. CLARK. Umkeher observations at three automated Dobson spectrophotometer stations from 1983 to 1995. Proceedings XVIII Quadrennial Ozone Symposium, II, edited by R.D. Bojkov and G. Visconti, pp. 135-138, Parco Scientifico e Technologico d'Abruzzo, Italy (1998).

Umkehr observations with automated Dobson spectrophotometers have now been made for between 9 and 14 years at six stations in the NOAA Climate Monitoring and Diagnostics Laboratory (CMDL) network. A time series for each location is prepared with allowances made for the adverse effects from the Pinatubo eruption. The data are vertical ozone distributions retrieved by the conventional Umkehr method. Averaged seasonal and annual profiles are presented. A significant difference between profiles obtained with the 1964 algorithm and the 1991 algorithm is that the new inversion shifts the ozone maximum down by a layer. At four of the locations where Umkehr observations are made there are nearby ozonesonde sites. At Mauna Loa Observatory, Hawaii, the two records are of nearly equal length and span about a decade. A comparison of these records found the largest difference in layer 2 with the Umkehrs being the larger. In the other layers, the difference was in the range of -10% to +12%. During the MLO3 intercomparison campaign held in Hawaii during August 1995 about 20 near simultaneous Umkehr and ozonesonde profiles were obtained. The largest differences were in the region of the maximum where the Umkehr tends to underestimate the ozone amount relative to the sondes.

MASARIE, K., and P.TANS. Guidelines for atmospheric trace gas data management. WMO GAW TD 907 World Meteorological Organization, Geneva, 46 pp. (1998).

No abstract.

MC INNES, L., M. BERGIN, J. OGREN, and S. Schwartz. Apportionment of light scattering and hygroscopic growth to aerosol composition. Geophysical Research Letters 25(4): 513-516 (1998).

During a recent campaign at the NOAA CMDL monitoring station on Sable Island, Canada (43.93º N, 60.01º W) a dual-nephelometer humidigraph measured the hygroscopic growth factor of aerosol scattering, f_RH(s_sp), one of the key parameters necessary for estimating short-wave aerosol radiative forcing. Measurements revealed less growth for anthropogenically influenced aerosols than for marine, f_RH(s_sp), of 1.7 ± 0.1 vs. 2.7 ± 0.4, where f_RH(s_sp) = s_sp(85%)/s_sp(40%). A combined measurement-modeling approach was used to estimate s_sp and its RH-dependence, based on the measured particle size distribution and composition. The model suggested that differences in the particle size distribution, assuming the same aerosol composition, could not explain the observed differences in f_RH(s_sp). We have confirmed with individual particle analysis, that aerosol composition was indeed responsible for the difference in f_RH(s_sp). As well, the scattering contribution of organic carbon for the influenced case is at least as much as sulfate aerosol.

Munger, J.W., S.-M. Fan, P.S. BAKWIN, M.L. Goulden, A.H. Goldstein, A.S. Colman, and S.C. Wofsy. Regional budgets for nitrogen oxides from continental sources: Variations of rates for oxidation and deposition with season and distance from source regions. Journal of Geophysical Research 103(D7):8355-8368 (1998).

Measurements of nitrogen deposition and concentrations of NO, NO₂, No_y (total oxidized N), and O₃ have been made at Harvard Forest in central Massachusetts since 1990 to define the atmospheric budget for reactive N near a major source region. Total (wet plus dry) reactive N deposition for the period 1990-1996 averaged 47 mmol m^-2 yr^-1 126 mmol m^-2 d^-1, 6.4 kg N ha^-1 yr^-1, with 34% contributed by dry deposition. Atmospheric input adds about 12% to the N made available annually by mineralization in the forest soil. The corresponding deposition rate at a distant site, Schefferville, Quebec, was 20 mmol m^-2 d^-1 during summer 1990. Both heterogeneous and homogeneous reactions efficiently convert No_x to HNO₃ in the boundary layer. HNO₃ is subsequently removed rapidly by either dry deposition or precipitation. The characteristic (e-folding) time for No_x oxidation ranges from 0.30 days in summer, when OH radical is abundant, to ~1.5 days in the winter, when heterogeneous reactions are dominant and O₃ concentrations are lowest. The characteristic time for removal of No_x oxidation products (defined as No_y minus No_x) from the boundary layer by wet and dry depositions is ~1 day, except in winter when it decreases to 0.6 day. Biogenic hydrocarbons contribute to N deposition through formation of organic nitrates but are also precursors of reservoir species, such as peroxyacetylnitrate, that may be exported from the region. A simple model assuming pseudo first-order rates for oxidation of No_x, followed by deposition, predicts that 45% of No_x in the northeastern U.S. boundary layer is removed in 1 day during summer and 27% is removed in winter. It takes 3.5 and 5 days for 95% removal in summer and winter, respectively.

NOVELLI, P.C., K.A. MASARIE, and P.M. LANG. Distributions and recent changes of carbon monoxide in the lower troposphere. Journal of Geophysical Research 103(D15): 19,015-19,033 (August 1998).

Since 1988, the distribution of carbon monoxide (CO) in the lower troposphere has been determined using a globally distributed air sampling network. Site locations range from 82°N to 90°S, with wide longitudinal coverage, and represent the marine boundary layer, regionally polluted atmospheres, and the free troposphere. These measurements present a unique, intercalibrated, and internally consistent data set that are used to better define the global temporal and spatial distribution of CO. In this paper, times series from 49 sites are discussed. With an average lifetime of ~2 months, CO showed significant concentration gradients. In the marine boundary layer, mixing ratios were greatest in the northern winter (200-220 ppb) and lowest in the southern summer (35-45 ppb). The interhemispheric gradient showed strong seasonality with a maximum difference between the high latitudes of the northern and southern hemispheres (160-180 ppb) in February and March and a minimum in July and August (10-20 ppb). Higher CO was found in regions near human development relative to those over more remote areas. The distributions provide additional evidence of the widespread pollution of the lower atmosphere. Remote areas in the high northern hemisphere are polluted by anthropogenic activities in the middle latitudes, and those in the southern hemisphere are heavily influenced by the burning of biomass in the tropics. While tropospheric concentrations of CO exhibit periods of increase and decrease, the globally averaged CO mixing ratio over the period from 1990 through 1995 decreased at a rate of approximately 2 ppb yr^-1.

NOVELLI, P.C., V.S. Connors, H.G. Reichle, Jr., B.E. Anderson, C.A.M. Brenninkmeijer, E.G. Brunke, B.G. Doddridge, V.W.J.H. Kirchhoff, K.S. Lam, K.A. MASARIE, T. Matsuo, D.D. Parrish, H.E. Scheel, and L.P. Steele. An internally consistent set of globally distributed atmospheric carbon monoxide mixing ratios developed using results from an intercomparison of measurements. Journal of Geophysical Research 103(D15): 19,285-19,293 (1998).

The Measurement of Air Pollution from Satellite (MAPS) instrument measures carbon monoxide (CO) in the middle troposphere from a space platform. In anticipation of the deployment of MAPS aboard the space shuttle Endeavor for two 10-day missions in 1994, plans were made to prepare a set of correlative measurements which would be used as part of the mission validation program. Eleven laboratories participated in the correlative measurement program by providing NASA with the results of their CO field programs during April and October 1994. Measurements of CO in the boundary layer, while not used in the MAPS validation, provide a picture of CO in the lower troposphere. Because measurements of CO made by different laboratories have been known to differ significantly, all correlative team members participated in an intercomparison of their measurements to define potential differences in techniques and calibration scales. While good agreement was found between some laboratories, there were differences between others. The use of similar analytical techniques and calibration scales did not always provide similar results. The results of the intercomparisons were used to normalize all ground-based measurements to the National Oceanic and Atmospheric Administration/Climate Monitoring and Diagnostics Laboratory CO reference scale. These data provide an internally consistent picture of CO in the lower atmosphere during spring and fall 1994.

Ohmura, A., E.G. DUTTON, B. Forgan, C. Fröhlich, H. Gilgen, H. Hegner, A. Heimo, G. König-Langlo, B. McArthur, G. Müller, R. Philipona, R. Pinker, C.H. Whitlock, K. Dehne, and M. Wild. Baseline surface radiation network (BSRN/WCRP): New precision radiometry for climate research. Bulletin of the American Meteorological Society 79(10):2115-2136 (1998).

To support climate research, the World climate Research Programme (WCRP) initiated a new radiometric network, the Baseline Surface Radiation Network (BSRN). The network aims at providing validation material for satellite radiometry and climate models. It further aims at detecting long-term variations in irradiances at the earth’s surface, which are believed to play an important role in climate change. The network and its instrumentation are designed (1) to cover major climate zones, (2) to provide the accuracy required to meet the objectives, and (3) to ensure homogenized standards for a long period in the future. The limits of the accuracy are defined to reach these goals. The suitable instruments and instrumentation have been determined and the methods for observations and data management have been agreed on at all stations. Measurements of irradiances are at 1 Hz, and the 1-min statistics (mean, standard deviation, and extreme values) with quality flags are stored at a centralized data archive at the WCRP’s World Radiation Monitoring Center (WRMC) in Zurich, Switzerland. The data are quality controlled both at stations and at the WRMC. The original 1-min irradiance statistics will be stored at the WRMC for 10 years, while hourly mean values will be transferred to the World Radiation Data Center in St. Petersburg, Russia. The BSRN, consisting of 15 stations, covers the earth’s surface from 80°N to 90°S, and will soon be joined by seven more stations. The data are available to scientific communities in various ways depending on the communication environment of the users. The present article discusses the scientific base, organizational and technical aspects of the network, and data retrieval methods; shows various application possibilities; and presents the future tasks to be accomplished.

OLTMANS, S.J., A. S. Lefohn, H. E. Scheel, J. M. HARRIS, H. Levy II, I. E. Galbally, E.-G. Brunke, C. P. Meyer, J. A. Lathrop, B. J. JOHNSON, D. S. Shadwick, E. Cuevas, F. J. Schmidlin, D. W. Tarasick, H. Claude, J. B. Kerr, O. Uchino, V. Mohnen. Trends of Ozone in the Troposphere. Geophysical Research Letters 25(2):139-142, 1998.

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Using a set of selected surface ozone (nine stations) and ozone vertical profile measurements (from six stations), we have documented changes in tropospheric ozone at a number of locations. From two stations at high northern hemisphere (NH) latitudes there has been a significant decline in ozone amounts throughout the troposphere since the early 1980s. At midlatitudes of the NH where data are the most abundant, on the other hand, important regional differences prevail. The two stations in the eastern United States show that changes in ozone concentrations since the early 1970s have been relatively small. At the two sites in Europe, however, ozone amounts increased rapidly into the mid-1980s, but have increased less rapidly (or in some places not at all) since then. Increases at the Japanese ozonesonde station have been largest in the lower troposphere, but have slowed in the recent decade. The tropics are sparsely sampled but do not show significant changes. Small increases are suggested at southern hemisphere (SH) midlatitudes by the two surface data records. In Antarctica large declines in the ozone concentration are noted in the South Pole data, and like those at high latitudes of the NH, seem to parallel the large decreases in the stratosphere.

Peterson, M.C., R.E. Honrath, D.D. Parrish, and S.J. OLTMANS. Measurements of nitrogen oxides and a simple model of NO_y fate in the remote North Atlantic marine atmosphere. Journal of Geophysical Research 103(D11):13,489-13,503 (1998).

NO and No_y (total reactive oxidized nitrogen) were measured at a site in the Azores (27.322°W, 38.732°N, 1 km altitude) over a 3-week period in August and September 1993, during the first summer intensive of the North Atlantic Regional Experiment (NARE). These measurements were performed to determine background reactive nitrogen oxide levels and to assess the impact that long-range transport of reactive nitrogen oxides associated with human activities have on these levels. Median No_y mixing ratios during background marine boundary layer (MBL) periods ranged from 59 to 93 parts per trillion by volume (pptv), with an overall median of 73 pptv. Analysis of back trajectories, low and uncorrelated CO and O₃ levels, and low levels of MBL No_y indicate that the central North Atlantic region was not influenced by direct transport of anthropogenic emissions during the period of this study. Changes in No_y levels during two MBL periods with adjacent in and out-of-cloud events indicated that up to 45-47% of MBL No_y was scavenged by clouds. However, mean No_y levels during all in-cloud periods (~70 pptv) and all out-of-cloud periods (~80 pptv) were not significantly different, apparently because of variability in MBL No_y levels. In addition to the MBL periods, there were two periods when the site was within the free troposphere (FT), as indicated by vertical soundings and weather conditions at the site. FT No_y mixing ratios were ³ 280 pptv and ³ 400 pptv during these two periods. The median clear-sky FT NO level during the hour centered on solar noon was 16 pptv. A mass balance model considering FT/MBL exchange and MBL removal processes is used to find the No_y MBL effective first-order loss lifetime (~1.2 days) and the No_y MBL e-folding response time due to both effective first-order loss processes and subsidence-induced ventilation of the MBL (~0.9 days). The apparent rapid loss of MBL No_y implies that it will respond rapidly to changes in the overlying FT, but the correlations of NO_y with trace gases with slower MBL removal, such as O₃ and CO, will be degraded within the subsidence-influenced MBL.

Philipona, R., C. Frohlich, K. Dehne, J. DeLuisi, J. Augustine, E. DUTTON, D. NELSON, B. Forgan, P. Novotny, J. Hickey, S.P. Love, S. Bender, B. McArthur, A. Ohmura, J.H. Seymour, J.S. Foot, M. Shiobara, F.P.J. Valero, and A. W. Strawa. The baseline surface radiation network pyrgeometer round-robin calibration experiment. Journal of Atmospheric and Oceanic Technology 15(3):687-696 (1998).

With the aim of improving the consistency of terrestrial and atmopsheric longwave radiation measurements within the Baseline Surface Radiation Network, five Eppley Precision Infrared Radiometer (PIR) pyrgeometers and one modified Meteorological Research Flight (MRF) pyrgeometer were individually calibrated by 11 specialist laboratories. The round-robin experiment was conducted in a "blind" sense in that the participants had no knowledge of the results of others until the whole series of calibrations had ended. The responsivities C(mV/W m^-2) determined by 6 of the 11 institutes were within about 2% of the median for all five PIR pyrgeomters. Among the six laboratories, the absolute deviation around the median of the deviations of the five instruments is less than 1%. This small scatter suggests that PIR pyrgeometers were stable at least during the 2 years of the experiment and that the six different calibration devices reproduce the responsivity C of PIR pyrgeometers consistently and within the precision required for climate applications. The results also suggest that the responsivity C can be determined without simultaneous determination of the dome correction factor k, if the temperature difference between pyrgeometer body and dome is negligible during calibration. For field measurements, however, k has to be precisely known. The calibration of the MRF pyrgeometer, although not performed by all institutes, also showed satisfactory results.

Polissar, A.V., P.K. Hopke, J.M. HARRIS, B.A. BODHAINE, and E.G. DUTTON. Source regions for atmospheric aerosol measured in the western Arctic. Journal of Aerosol Science 29:S513-S514 (1998).

Aerosol data including condensation nuclei (CN), black carbon (BC), and aerosol light scattering (SC) measurements at Barrow, Alaska, for the period from 1977 to 1994 have been analyzed. The description of the monitoring site at Barrow and instrumentation has been presented by Bodhaine (1989). A receptor modeling method called the potential source contribution function (PSCF) (Hopke et al., 1995) that combines the aerosol data with meteorological information in the form of ten-day air parcel backward trajectories was applied to identify possible source areas and the preferred pathways that gives rise to the observed high aerosol concentrations. Ten-day back trajectories arriving twice daily at 500 m above sea level were calculated for the five-year period from 1989 to 1993 by using the isentropic transport model (Harris and Kahl, 1994).

Ridley, B., J. Walega, G. Hübler, D. Montzka, E. Atlas, D. Hauglustaine, F. Grahek, J. Lind, T. Campos, R. Norton, J. Greenberg, S. Schauffler, S. OLTMANS, and S. Whittlestone. Measurements of NO_x and PAN and estimates of O₃ production over the seasons during Mauna Loa Observatory Photochemistry Experiment 2. Journal of Geophysical Research 103(D7):8323-8339 (1998).

Measurments of peroxyacetyl nitrate (PAN) and NO_x and a variety of other constituents were made over approximately 1-month-long intensives in the autumn of 1991 and the winter, spring, and summer of 1992 during the second Mauna Loa Observatory Photochemistry Experiment (MLOPEX2). PAN and NO_x in the free troposphere had maximum abundances in spring in concert with the well-known maximum in O₃. The ratio of the spring to summer averages was a factor of 4.1 for PAN, a factor of 1.6 for O₃, and only a factor of 1.4 for NO_x. During most intensives, variations over periods of a few days to a week were often larger than the average seasonal amplitude. In free tropospheric air masses local to Hawaii, average PAN/NO_x ratios were a maximum in winter through spring but in the range of 0.25-0.86 in all intensives. PAN decomposition is unlikely to be the major net source of NO_x in local air masses in summer and fall. The low HNO₃/NO_x ratios determined during MLOPEX 1 were confirmed during MLOPEX2. Intensive average ratios of 1.6-3.8 over the year are lower than some model predictions. Both the low ratio and the magnitude of NO_x imply a shortcoming in our understanding of the transformations and sources of NO_y constituents in the central Pacific. The 3- to 4-km altitude region near Hawaii was a net importer of O₃, on average, over the year. The average net rate of production of O₃ in free tropospheric air was near zero in winter, -0.4 to –0.8 ppbv/d in spring, -1.4 ppbv/d in summer, and –0.6 ppbv/d in autumn. Thus the spring maximum in O₃ is not due to local photochemistry. We believe, as has been concluded from the long-term measurements of long-lived constituents by the Climate Monitoring and Diagnostics Laboratory, that the variation of ozone precursors over the year and on shorter timescales of a few days to a week is controlled predominantly by changes in long-range transport: more frequent sampling of higher-latitude and higher-altitude air masses in winter and spring versus more frequent sampling of well-aged air from lower altitudes and latitudes in summer and autumn.

SHERIDAN, P. J., and R.B. Norton. Determination of the passing efficiency for aerosol chemical species through a typical aircraft-mounted, diffuser-type aerosol inlet system. Journal of Geophysical Research 103(D7):8215-8225 (1998).

To assess the particle transmission efficiency of a conventional aircraft-mounted, diffuser-type inlet (CI), a new design inlet containing an internal filter basket assembly (aerosol filter inlet, or AFI) was constructed. All interior surfaces of the AFI were covered with filter material, and air was actively pulled through these filter walls during aerosol sampling. The AFI was demonstrated in the laboratory to trap nearly all particles entering its nozzle orifice, so it was considered usable as a baseline to judge the performance of other inlets. Wind tunnel studies were conducted at three different wind velocities that approximated typical research aircraft speeds. As wind velocity increased, particle transmission through the CI relative to the AFI decreased, as evidenced by chemical analysis of the filter deposits. Aircraft studies of the two inlets showed that particle transmission varied significantly with the measured species. Typical coarse-particle species such as Ca++, Mg++, Na+ and K+ showed 50-90% mass losses through a conventional diffuser-type inlet/curved intake tube system. Predominantly fine particle species such as O₄⁼ and NH₄⁺ passed the CI system with much higher efficiencies, with aerosol mass losses of 0-26% for most flights. Since the AFI traps nearly all particles aspirated into its nozzle orifice, these values indicate that on average, 80-90% of the SO₄⁼ and NH₄⁺ aerosol mass passes through the CI and curved intake tube during airborne sampling. This finding suggests that the capability to sample fine (i.e., submicrometer) aerosols from aircraft is perhaps not as bad as has been previously reported, given that adequate attention is paid to inlet design, location, and orientation issues.

STONE, R.S. Variations in western Arctic temperatures in response to cloud radiative and synoptic-scale influences. Journal of Geophysical Research 102(D18):21,769-21,776 (1998).

The analysis focuses on Barrow, Alaska, a site that is sensitive to changing conditions because it is located near cryospheric boundaries and is influenced by both extratropical and Arctic synoptic activity. Surface and upper air meteorological data for a 31-year period (1965-1995) are used to evaluate temperature variations as they relate to dynamical and radiative processes. Both annual and monthly analyses indicate a tendency toward warming overall. However, the annual warming is not monotonic over time and varies seasonally. Comparisons of temperature time series from four sites along the Siberian-Alaskan coastline show that Barrow is a representative site to evaluate climate change in the western Arctic coastal zone. Regionally, the warming is dominated by significant temperature increases during winter and spring, but cooling is indicated for autumn. These results are not entirely consistent with model predictions of a more uniform high-latitude warming during the cold season in response to increasing concentrations of greenhouse gases in the atmosphere. Rather, the observed changes are attributed to well-known natural processes that affect regional cloud distributions in response to changing circulation patterns. Coincident daily and hourly meteorological and radiation data are also used to demonstrate empirically how clouds modulate Arctic temperatures.

WAMSLEY, P.R., J.W. ELKINS, D.W. Fahey, G.S. DUTTON, C.M. VOLK, R.C. MYERS, S.A. MONTZKA, J.H. BUTLER, A.D. Clarke, P.J. Fraser, L.P. Steele, M.P. Lucarelli, E.L. Atlas, S.M. Schauffler, D.R. Blake, F.S. Rowland, W.T. Sturges, J.M. Lee, S.A. Penkett, A. Engel, R.M. Stimpfle, K.R. Chan, D.K. Weisenstein, M.K.W. Ko, and R.J. Salawitch. Distribution of halon-1211 in the upper troposphere and lower stratosphere and the 1994 total bromine budget. Journal of Geophysical Research 103(D1):1513-1526. (1998).

We report here on the details of the first, in situ, real-time measurements of H-1211 (CBrClF₂) and sulfur hexafluoride (SF₆) mixing ratios in the stratosphere up to 20 km. Stratospheric air was analyzed for these gases and others with a new gas chromatograph, flown aboard a National Aeronautics and Space Administration ER-2 aircraft as part of the Airborne Southern Hemisphere Ozone Experiment/Measurements for Assessing the Effects of Stratospheric Aircraft mission conducted in 1994. The mixing ratio of SF₆, with its nearly linear increase in the troposphere, was used to estimate the mean age of stratospheric air parcels along the ER-2 flight path. Measurements of H-1211 and mean age estimates were then combined with simultaneous measurements of CFC-11 (CCl₃F), measurements of brominated compounds in stratospheric whole air samples, and records of tropospheric organic bromine mixing ratios to calculate the dry mixing ratio of total bromine in the lower stratosphere and its partitioning between organic and inorganic forms. We estimate that the organic bromine-containing species were almost completely photolyzed to inorganic species in the oldest air parcels sampled. Our results for inorganic bromine are consistent with those obtained from a photochemical, steady state model for stratospheric air parcels with CFC-11 mixing ratios greater than 150 ppt. For stratospheric air parcels with CFC-11 mixing ratios less than 50 ppt (mean age ³5 years) we calculate inorganic bromine mixing ratios that are approximately 20% less than the photo-chemical, steady state model. There is a 20% reduction in calculated ozone loss resulting from bromine chemistry in old air relative to some previous estimates as a result of the lower bromine levels.

Weatherhead, E.C., G.C. Reinsel, G.C. Tiao, X.-L. Meng, D. Choi, W.-K. Cheang, T. Keller, J. DeLuisi, D.J. Wuebbles, J.B. Kerr, A.J. Miller, S.J. OLTMANS, and J.E. Frederick. Factors affecting the detection of trends: Statistical considerations and applications to environmental data. Journal of Geophysical Research 103(D14):17,149-17,161 (1998).

Detection of long-term, linear trends is affected by a number of factors, including the size of trend to be detected, the time span of available data, and the magnitude of variability and autocorrelation of the noise in the data. The number of years of data necessary to detect a trend is strongly dependent on, and increases with, the magnitude of variance (s _N²) and the autocorrelation coefficient (f ) of the noise. For a typical range of values of s _N² and f the number of years of data needed to detect a trend of 5%/decade can vary from ~10 to >20 years, implying that in choosing sites to detect trends some locations are likely to be more efficient and cost-effective than others. Additionally, some environmental variables allow for an earlier detection of trends than other variables because of their low variability and autocorrelation. The detection of trends can be confounded when sudden changes occur in the data, such as when an instrument is changed or a volcano erupts. Sudden level shifts in data sets, whether due to artificial source, such as changes in instrumentation or site location, or natural sources, such as volcanic eruptions or local changes to the environment, can strongly impact the number of years necessary to detect a given trend, increasing the number of years by as much as 50% or more. This paper provides formulae for estimating the number of years necessary to detect trends, along with the estimates of the impact of interventions on trend detection. The uncertainty associated with these estimates is also explored. The results presented are relevant for a variety of practical decisions in managing a monitoring station, such as whether to move an instrument, change monitoring protocols in the middle of a long-term monitoring program, or try to reduce uncertainty in the measurements by improved calibration techniques. The results are also useful for establishing reasonable expectations for trend detection and can be helpful in selecting sites and environmental variables for the detection of trends. An important implication of these results is that it will take several decades of high-quality data to detect the trends likely to occur in nature.

Weinheimer, A.J., D.D. Montzka, T.L. Campos, J.G. Walega, B.A. Ridley, S.G. Donnelly, E.R. Keim, L.A. Del Negro, M.H. Proffitt, J.J. Margitan, K.A. Boering, A.E. Andrews, B.C. Daube, S.C. Wofsy, B.E. Anderson, J.E. Collins, G.W. Sachse, S.A. Vay, J.W. ELKINS, P.R. WAMSLEY, E.L. Atlas, F. Flocke, S. Schauffler, C.R. Webster, R.D. May, M. Loewenstein, J.R. Podolske, T.P. Bui, K.R. Chan, S.W. Bowen, M.R. Schoeberl, L.R. Lait, and P.A. Newman. Comparison between DC-8 and ER-2 species measurements in the tropical middle troposphere: NO, NO_y, O₃, CH₄, and N₂O. Journal of Geophysical Research 103(D17):22,087-22,096 (1998).

We compare measurements of six species taken aboard NASA DC-8 and ER-2 aircraft during two flight legs in the tropical middle troposphere near Hawaii. NO, NO_y, O₃, CH₄, and N₂O measurements agree to within the limits set by the known systematic errors. For CO₂, which can be measured with better relative precision than the other five species, differences in measured values from the two platforms are slightly larger than expected if the air masses sampled by the two aircraft were indeed similar in CO₂ composition to better than 0.08%.

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(303) 497-6074