bluuebunny/crossref_metadata_2025 huggingface dataset:
Latitudinal gradients in O3 and CO during INDOEX 1999
Authors: Stehr J. W. , Ball W. P. , Dickerson R. R. , Doddridge B. G. , Piety C. A. , Johnson J. E.
Published: October 2002

Measurements of ozone and carbon monoxide (CO) from the INDOEX 1999 experiment show large differences in concentrations of ozone and CO between the Northern Hemisphere and the Southern Hemisphere. These measurements confirm the theory that the Intertropical Convergence Zone (ITCZ) serves as a barrier to mixing over the Indian Ocean, effectively separating the polluted air in the Northern Hemisphere from cleaner air in the Southern Hemisphere. In spite of CO levels similar to those observed off the coast of North America, there is relatively little ozone off the coast of India. The ozone‐to‐CO ratio in air coming from India is 0.14, lower than 0.3 or 0.4 in air being transported to Bermuda from North America. Diurnal cycles are observed in both CO and ozone. INDOEX data taken onboard the R/V Ronald H. Brown show an average diurnal cycle in ozone of 20%, while data from the island of Kaashidhoo in the Republic of Maldives indicate a diurnal variation of 19%, consistent with our analyses of other experiments. Diurnal variations of this magnitude are larger than expected from ozone destruction by conventional HOx chemistry alone, implying that the sink of ozone in the remote marine boundary layer is likely considerably larger than had been assumed. Additional chemical cycles must be fairly substantial—large enough to rival HOx chemistry in ozone destruction.
Tropical tropospheric ozone: Implications for dynamics and biomass burning
Authors: Chandra S. , Ziemke J. R. , Bhartia P. K. , Martin R. V.
Published: October 2002

This paper studies the significance of large‐scale transport and pyrogenic (i.e., biomass burning) emissions in the production of tropospheric ozone in the tropics. Using aerosol index (AI) and tropospheric column ozone (TCO) time series from 1979 to 2000 derived from the Nimbus‐7 and Earth Probe Total Ozone Mapping Spectrometer measurements, our study shows significant differences in the seasonal and spatial characteristics of pyrogenic emissions north and south of the equator in the African region and Brazil in South America. Notwithstanding these differences, most of the observed seasonal characteristics are well simulated by the GEOS‐CHEM global model of tropospheric chemistry. The only exception is the northern African region where modeled and observed TCO differ significantly. In the Indonesian region the most significant increase in TCO occurred during September–December 1997, following large‐scale forest and savanna fires associated with the El Niño induced dry condition. The increase in TCO extended over most of the western Pacific well outside the burning region and was accompanied by a decrease in the eastern Pacific resembling a west‐to‐east dipole about the dateline. These features are well simulated in the GEOS‐CHEM model which suggests that both the biomass burning and changes in meteorological conditions during the El Niño period contributed almost equally to the observed increase in TCO in the Indonesian region. During 1997 the net increase in TCO integrated over the tropical region between 15°N and 15°S was about 6–8 Tg (1 Tg = 1012 g) over the mean climatological value of about 77 Tg. The GEOS‐CHEM model suggests that most of this increase may have been caused by biomass burning in the Indonesian region since dynamical components of El Niño induced changes in TCO tend to cancel out in the area‐averaged data. In addition to biomass burning, the interannual variability in the area‐averaged column ozone in the tropics is influenced by a number of factors including the quasi‐biennial oscillation and solar cycle.
On the temperature structure of the tropical substratosphere
Authors: Thuburn John , Craig George C.
Published: October 2002

The region of the tropical atmosphere between the main convective outflow level around 200–150 hPa and the cold point, the substratosphere, shares properties of the troposphere and the stratosphere. An analogous region exists in single‐column radiative‐convective models between the top of the convectively adjusted region and the cold point, so a radiative‐convective model has been used to investigate the radiative and dynamical factors that allow a substratosphere to exist and to which it is sensitive. A key result is that localized heating in the 15 μm CO2 band, owing to the sharp curvature in the temperature profile near the convection top, forces apart the cold point and convection top creating the substratosphere. A possible radiation‐convective‐transport feedback involving O3 is identified that could amplify the response of the substratosphere and cold point to changes in forcing such as sea surface temperature or stratospheric meridional circulation. A comparison of timescales suggests that the substratosphere is characterized by (1) a radiative timescale shorter than the convective timescale, so that radiation dominates in setting the temperature structure, but (2) a convective transport timescale shorter than the timescales for other processes affecting O3, so that the import of air from the boundary layer keeps O3 mixing ratios low there.
Loss of isoprene and sources of nighttime OH radicals at a rural site in the United States: Results from photochemical models
Authors: Sillman Sanford , Carroll Mary Anne , Thornberry Troy , Lamb Brian K. , Westberg Hal , Brune William H. , Faloona Ian , Tan David , Shepson Paul B. , Sumner Ann Louise , Hastie Donald R. , Mihele Christian M. , Apel Eric C. , Riemer D. D. , Zika Rod G.
Published: October 2002

A one‐dimensional Lagrangian model for atmospheric transport and photochemistry has been developed and used to interpret measurements made at Pellston, Michigan, during the summer of 1998. The model represents a moving vertical column of air with vertical resolution of 25 m near the ground. Calculations have been performed for a series of trajectories, with representation of emissions, vertical mixing, and photochemistry for a 3‐day period ending with the arrival of the air column at Pellston. Results have been used to identify causes of the observed decrease in isoprene at night, to investigate causes of high nighttime OH. Significant OH can be generated at night by terpenes if it is assumed that some fast‐reacting monoterpenes are emitted at rates comparable to inventory emissions for terpenes. However, this nighttime OH is confined to a shallow surface layer (0–25 m) and has little impact on nighttime chemistry. The observed decrease in isoprene at night can be reproduced in models with low OH, and is attributed primarily to vertical dilution. There is also evidence that transport from Lake Michigan contributes to low nighttime isoprene at Pellston. Model results compare well with measured isoprene, NOx, and with isoprene vertical profiles. Significant model‐measurement discrepancies are found for OH, HO2, methylvinylketone, and formaldehyde.
Atmospheric aerosol models for systems including the ions H+, NH4+, Na+, SO42−, NO3−, Cl−, Br−, and H2O
Authors: Wexler Anthony S. , Clegg Simon L.
Published: October 2002

Mole fraction based equations for aqueous phase activities, together with equilibrium constants for the formation of gases and solids, have been combined with a Gibbs free energy minimization algorithm to create equilibrium phase partitioning models of inorganic atmospheric aerosols. The water content, phase state (solid or liquid), and gas/aerosol partitioning are predicted for known ionic composition, relative humidity, and temperature. The models are valid from <200 to 328 K for the subsystems (H+‐SO42−‐NO3−‐Cl−‐Br−‐H2O) and (H+‐NH4+‐SO42−‐NO3−‐H2O), and 298.15 K only for (H+‐NH4+‐Na+‐SO42−‐NO3−‐Cl−‐H2O). The models involve no simplifying assumptions and include all solid phases identified in bulk experiments, including hydrated and double salt forms not treated in most other studies. The Henry's law constant of H2SO4 is derived as a function of temperature, based upon available data, and the model treatment of the solubility of HBr in aqueous H2SO4 is revised. Phase diagrams are calculated for the (NH4)2SO4/H2SO4/H2O system to low temperature. The models are also used to explore the importance of the double salts in urban inorganic aerosols. These Aerosol Inorganics Model (AIM) models can be run on the Web for a variety of problem types at http://mae.ucdavis.edu/wexler/aim.html and http://www.uea.ac.uk/∼e770/aim.html, and their use is summarized here.
A modeling study of the impact of winter–spring arctic outflow on the NOx and O3 budgets of the North Atlantic troposphere
Authors: Hamlin Amy J. , Honrath Richard E.
Published: October 2002

In this study, we determine the impact of winter–spring arctic outflow events on the budgets of tropospheric NOx and ozone over the North Atlantic Ocean. An updated version of the NCAR Master‐Mechanism, a detailed, explicit photochemical model, is applied to trajectories representative of arctic outflow for the months of January through May. During the simulations, peroxyacetyl nitrate (PAN) mixing ratios drop by up to 270 pptv, increasing NOx levels up to 120 pptv. However, the released NOx is quickly destroyed by efficient NOx loss reactions leading to formation of HNO3 and, to a lesser extent, nitroaromatic compounds. As a result of elevated NOx, O3 levels are increased. The resulting net impact on the flux of O3 to the North Atlantic is estimated to be less than 712 Gg O3/month. While this source is similar in magnitude to estimates of ozone produced from aircraft emissions, both are small in comparison to the export of ozone from North America. Therefore, we conclude that while elevated levels of pollutants in arctic outflow increase the levels of NOx and O3 in the North Atlantic region, on a seasonal basis the impact on the budgets of NOx and O3 in this region is small.
Assessment of four methods to estimate surface UV radiation using satellite data, by comparison with ground measurements from four stations in Europe
Authors: Arola Antti , Kalliskota S. , den Outer P. N. , Edvardsen K. , Hansen G. , Koskela T. , Martin T. J. , Matthijsen J. , Meerkoetter R. , Peeters P. , Seckmeyer G. , Simon P. C. , Slaper H. , Taalas P. , Verdebout J.
Published: October 2002

Four different satellite‐UV mapping methods are assessed by comparing them against ground‐based measurements. The study includes most of the variability found in geographical, meteorological and atmospheric conditions. Three of the methods did not show any significant systematic bias, except during snow cover. The mean difference (bias) in daily doses for the Rijksinstituut voor Volksgezondheid en Milieu (RIVM) and Joint Research Centre (JRC) methods was found to be less than 10% with a RMS difference of the order of 30%. The Deutsches Zentrum für Luft‐ und Raumfahrt (DLR) method was assessed for a few selected months, and the accuracy was similar to the RIVM and JRC methods. It was additionally used to demonstrate how spatial averaging of high‐resolution cloud data improves the estimation of UV daily doses. For the Institut d'Aéronomie Spatiale de Belgique (IASB) method the differences were somewhat higher, because of their original cloud algorithm. The mean difference in daily doses for IASB was about 30% or more, depending on the station, while the RMS difference was about 60%. The cloud algorithm of IASB has been replaced recently, and as a result the accuracy of the IASB method has improved. Evidence is found that further research and development should focus on the improvement of the cloud parameterization. Estimation of daily exposures is likely to be improved if additional time‐resolved cloudiness information is available for the satellite‐based methods. It is also demonstrated that further development work should be carried out on the treatment of albedo of snow‐covered surfaces.
Continuous monitoring of the high and persistent chlorine activation during the Arctic winter 1999/2000 by the GOME instrument on ERS‐2
Authors: Wagner T. , Wittrock F. , Richter A. , Wenig M. , Burrows J. P. , Platt U.
Published: October 2002

Measurements of OClO total column amounts during the cold Arctic winter 1999/2000 retrieved from observations by the satellite instrument Global Ozone Monitoring Experiment (GOME) are presented. OClO is formed as a minor product of the reaction of BrO + ClO and thus serves as an indicator for a stratospheric chlorine activation. As a result of the good spatial and temporal coverage of GOME, it is possible to follow the temporal development of the stratospheric chlorine activation during the winter and spring on a daily basis. An initial weak chlorine activation was observed during mid November, shortly after stratospheric temperatures were sufficiently low that formation of polar stratospheric clouds resulted. Strong chlorine activation started around 22 December, when PSC formation was possible over a large altitude range. Chlorine activation was significant and large until the beginning of March, peaking mid February. In the middle of March the chlorine activation steeply decreased and ended around 20 March, when the polar vortex broke up. The duration and the magnitude of the chlorine activation in the Arctic winter 1999/2000 were higher than during all previous Arctic winters since the launch of the GOME instrument in April 1995.
Effect of marine boundary layer clouds on tropospheric chemistry as analyzed in a regional chemistry transport model
Authors: Barth M. C. , Hess P. G. , Madronich S.
Published: October 2002

A regional chemistry transport model is used to investigate whether clouds affect tropospheric species concentrations, particularly ozone. This study focuses on boundary layer clouds in a 20° by 20° region centered over Hawaii. We find that O3 is depleted by 6% in the cloud‐capped boundary layer during a 120‐hour integration period. The O3 loss rate is calculated to be 320 pptv d−1 for the region, which was dominated by low NOx (<10 pptv). Aqueous chemistry and cloud radiative effects contribute nearly equally to the O3 depletion at the pH of 4.5 used in this simulation. When higher pH values are considered, more O3 depletion occurs. For example, at pH of 5.5, O3 is depleted by 17% during the 120‐hour integration. At pH > 4.5 the superoxide ion is favored, and therefore reaction of superoxide and ozone in the cloud drops increases. We discuss the importance of including transport and physical processes (e.g., deposition) on the O3 loss rate. We find that O3‐rich air is transported into the marine boundary layer via subsidence and diffusion, allowing for more O3 to be depleted in an absolute sense compared to when transport and physical processes do not occur. However, the relative change in O3 is smaller in the boundary layer when transport and physical processes are included because large eddies, which are represented by vertical diffusion in the model, maintain relatively high background O3 levels in the boundary layer.
A parameterization of cirrus cloud formation: Homogeneous freezing of supercooled aerosols
Authors: Kärcher B. , Lohmann U.
Published: October 2002

The nucleation and initial growth of ice crystals in cirrus clouds at low (<235 K) temperatures prevailing in the upper troposphere and in the tropopause region is theoretically considered. The analysis explains the dependence of the number density of ice crystals on the vertical velocity and temperature seen in numerical simulations of cirrus formation when the timescale of depositional growth of the pristine ice particles is fast compared to the timescale of the freezing event. In such cases, applicable in many situations, the number of crystals formed via homogeneous freezing of aqueous solution droplets is rather insensitive to details of the aerosol size distribution, but increases rapidly with updraft velocity and decreases with temperature. The derived parameterization is validated with parcel model simulations, and its applicability for use in climate models is discussed. The potential role of aerosol size and heterogeneous freezing processes in altering the predicted cirrus properties is briefly addressed.
Extended Canadian Middle Atmosphere Model: Zonal‐mean climatology and physical parameterizations
Authors: Fomichev V. I. , Ward W. E. , Beagley S. R. , McLandress C. , McConnell J. C. , McFarlane N. A. , Shepherd T. G.
Published: October 2002

This paper describes the energetics and zonal‐mean state of the upward extension of the Canadian Middle Atmosphere Model, which extends from the ground to ∼210 km. The model includes realistic parameterizations of the major physical processes from the ground up to the lower thermosphere and exhibits a broad spectrum of geophysical variability. The rationale for the extended model is to examine the nature of the physical and dynamical processes in the mesosphere/lower thermosphere (MLT) region without the artificial effects of an imposed sponge layer which can modify the circulation in an unrealistic manner. The zonal‐mean distributions of temperature and zonal wind are found to be in reasonable agreement with observations in most parts of the model domain below ∼150 km. Analysis of the global‐average energy and momentum budgets reveals a balance between solar extreme ultraviolet heating and molecular diffusion and a thermally direct viscous meridional circulation above 130 km, with the viscosity coming from molecular diffusion and ion drag. Below 70 km, radiative equilibrium prevails in the global mean. In the MLT region between ∼70 and 120 km, many processes contribute to the global energy budget. At solstice, there is a thermally indirect meridional circulation driven mainly by parameterized nonorographic gravity‐wave drag. This circulation provides a net global cooling of up to 25 K d−1.
Derivation of tropospheric NO3 profiles using off‐axis differential optical absorption spectroscopy measurements during sunrise and comparison with simulations
Authors: von Friedeburg Christoph , Wagner Thomas , Geyer Andreas , Kaiser Norbert , Vogel Bernhard , Vogel Heike , Platt Ulrich
Published: October 2002

Early morning vertical profiles of tropospheric NO3 were derived by spectroscopy of scattered sunlight in off‐axis geometry during sunrise. The measurements were carried out in the urban area of Heidelberg, Germany (April, July, and August 1999). The retrieval algorithm is based on the nitrate radical's rapid photolysis during sunrise, radiative transfer, as well photochemical modeling. We derived NO3 near‐ground concentrations of typically (0.2–18) × 107 cm−3, concentrations above 3 km of (5–50) × 107 cm−3, and a NO3 maximum at a height of ∼350 m with concentrations of (100–900) × 107 cm−3. Assuming the case of very high terpene levels even at higher altitudes, which is very unlikely for the atmospheric conditions during the measurements, we obtain a different mathematical solution of our inversion problem yielding the NO3 maximum at altitudes of ∼2.5 km. The enriched layer was found to hold the bulk of the tropospheric nighttime NO3. The retrieved profile confirms earlier suggestions that ground‐level measurements may be significantly underestimating the oxidative capacity of the boundary layer under stable nocturnal conditions. The NO3 layer is probably formed as a result of the vertical profiles of the NO3 educts (NO2 and O3), with NO2 concentrations falling off more slowly with height than the NO3 scavengers, e.g., anthropogenic NO as well as volatile organic compounds emitted at ground level. Independently from these measurements model simulations with a comprehensive three‐dimensional model system were performed for an area, which includes the measurement site. A pronounced maximum (3.3 × 109 cm−3) of the nocturnal NO3 concentration was found at ∼250 m above the surface, i.e., at the top of the nocturnal stable boundary layer. The average NO3 concentration close to the surface was 5 × 107 cm−3. The only significant difference between the observations and the model results was a steeper decrease of the NO3 concentration above the maximum of the observed profiles.
A parameterization of aerosol activation 3. Sectional representation
Authors: Abdul‐Razzak Hayder , Ghan Steven J.
Published: October 2002

A parameterization of the activation of a lognormal size distribution of aerosols to form cloud droplets is extended to a sectional representation of the aerosol size distribution. For each section, number concentration and chemical composition are uniform functions of particle radius. The parameterization is applied by calculating an effective critical supersaturation of all sections from which the maximum supersaturation of the air parcel is calculated using the previously derived parameterization. The Köhler theory is used to relate the aerosol size distribution and composition to the number activated for each section as a function of maximum supersaturation. For most cases, parametric results are within 10% of those obtained by detailed numerical computations for both idealized and measured aerosol size distributions. The parameterization thus provides an accurate method of treating the activation process for models that use a sectional representation of the aerosol size distribution.
Global distribution and sources of volatile and nonvolatile aerosol in the remote troposphere
Authors: Singh Hanwant B. , Anderson B. E. , Avery M. A. , Viezee W. , Chen Y. , Tabazadeh A. , Hamill P. , Pueschel R. , Fuelberg H. E. , Hannan J. R.
Published: October 2002

Airborne measurements of aerosol (condensation nuclei, CN) and selected trace gases made over areas of the North Atlantic Ocean during Subsonic Assessment (SASS) Ozone and Nitrogen Oxide Experiment (SONEX) (October/November 1997), the south tropical Pacific Ocean during Pacific Exploratory Mission (PEM)‐Tropics A (September/October 1996), and PEM‐Tropics B (March/April 1999) have been analyzed. The emphasis is on interpreting variations in the number densities of fine (>17 nm) and ultrafine (>8 nm) aerosol in the upper troposphere (8–12 km). These data suggest that large number densities of highly volatile CN (104 − 105 cm−3) are present in the upper troposphere and particularly over the tropical/subtropical region. CN number densities in all regions are largest when the atmosphere is devoid of nonvolatile particles. Through marine convection and long‐distance horizontal transport, volatile CN originating from the tropical/subtropical regions can frequently impact the abundance of aerosol in the middle and upper troposphere at mid to high latitudes. Nonvolatile aerosols behave in a manner similar to tracers of combustion (CO) and photochemical pollution (peroxyacetylnitrate (PAN)), implying a continental pollution source from industrial emissions or biomass burning. In the upper troposphere we find that volatile and nonvolatile aerosol number densities are inversely correlated. Results from an aerosol microphysical model suggest that the coagulation of fine volatile particles with fewer but larger nonvolatile particles, of principally anthropogenic origin, is one possible explanation for this relationship. In some instances the larger nonvolatile particles may also directly remove precursors (e.g., H2SO4) and effectively stop nucleation.
Transport of ozone‐depleted air on the breakup of the stratospheric polar vortex in spring/summer 2000
Authors: Piani C. , Norton W. A. , Iwi A. M. , Ray E. A. , Elkins J. W.
Published: October 2002

A high‐resolution three‐dimensional off‐line chemical transport simulation has been performed with the SLIMCAT model to examine transport and mixing of ozone depleted air in the lower stratosphere on breakup of the polar vortex in spring/summer 2000. The model included ozone, N2O, and F11 tracers and used simplified chemistry parameterizations. The model was forced by T106 European Centre for Medium‐Range Weather Forecasts analyses. The model results show that, by the end of June, above 420 K, much of the ozone‐depleted air is transported from polar regions to the subtropics. In contrast, below 420 K, most of the ozone‐depleted air remains poleward of approximately 55°N. It is suggested that the influence of the upper extension of the tropospheric subtropical jet provides a transport barrier at lower levels, while strong stirring on breakup of the polar vortex is important at upper levels. The mean meridional circulation modifies the distribution of ozone‐depleted air by moving it up the subtropics and down in the extratropics. The model simulation is validated by comparing vertical profiles of ozone loss against ozonesonde measurements. The model results are consistent with many of the features present in the ozonesonde measurements. F11‐N2O correlation plots are examined in the model and they show distinct canonical correlation curves for the polar vortex, midlatitudes, and the tropics. Comparison against balloon and aircraft measurements show that the model reproduces the separation between the vortex and midlatitude curves; however, the ratio of N2O to F11 lifetimes is somewhat too small in the model. It is shown that anomalies from the midlatitude canonical correlation curve can be used to identify remnants of polar vortex air which has mixed with midlatitude air. At the end of June there is excellent agreement in the position of air with anomalous F11‐N2O tracer correlation and ozone‐depleted air from the polar vortex.
Intraseasonal variation in the thermoinsulation effect of snow cover on soil temperatures and energy balance
Authors: Sokratov Sergey A. , Barry Roger G.
Published: October 2002

We present the results of an analysis of the observed variation in the energy balance of a soil surface at Barrow, Alaska, during 1993–1998. When combined with snow depth measurements, the data allow several stages to be distinguished in the intraseasonal variation of the snow cover effect on the temperature regime and energy balance of the underlying soil. Each stage corresponds to specific thermoinsulation effects of the snow cover in terms of the energy‐balance dependence of the soil‐surface on snow depth. Stages in the intraseasonal surface energy balance variation can be used as a basis for incorporating detailed snow physics in modeling of the soil freezing/thawing phenomenon and for consideration of the distributed energy balance of snow covered landscapes. Preliminary ideas are presented on applying the results obtained for nonpermafrost regions.
Impact of a climate change on the Rhone river catchment hydrology
Authors: Etchevers P. , Golaz C. , Habets F. , Noilhan J.
Published: October 2002

Within the framework of the Global Energy and Water Cycle Experiment (GEWEX)/Rhone project, a system has been built to estimate the hydrological budget of the Rhone, one of the major European rivers (with a 86,500 km2 surface area for the French part of the catchment). The methodology is based on three models, one for each component of the hydrometeorological system: The atmospheric parameter analysis, the snow cover, the surface water and energy budget, and the underground water transfer and discharge estimation. This tool has been validated for 14 years (from 1981 to 1994) by comparing the daily river flows simulated by the models with measurements from 145 gauging stations. In this study, the results of the ARPEGE‐Climat general circulation model (GCM) have been used to estimate the climate of the Rhone catchment in 60 years. The perturbation of the air temperature and precipitation amount has been used to modify the actual set of meteorological parameters in order to simulate the hydrological budget of the Rhone river. Vegetation and soil structure are supposed to be identical to current values, which is a strong hypothesis. The river discharge and soil water resources under the climatic scenario are compared with the results of the actual simulation. Strong contrasts in the hydrological response of the catchment are noticeable, depending on the location of the subcatchments and the amount of precipitation. Snow cover is the most sensitive hydrological component to the air temperature increase, and the high mountainous river regimes are strongly modified. When considering the soil water content, it appears that the northern part of the domain stays quite wet, whereas drying is enhanced in the south. The uncertainties in the results are estimated by analyzing the model sensitivity to different simple climatic scenarios. In particular, the analysis brings into light the impact of the downscaling of the GCM results.
The subtropical global plume in the Pacific Exploratory Mission‐Tropics A (PEM‐Tropics A), PEM‐Tropics B, and the Global Atmospheric Sampling Program (GASP): How tropical emissions affect the remote Pacific
Authors: Chatfield R. B. , Guo Z. , Sachse G. W. , Blake D. R. , Blake N. J.
Published: October 2002

An extended southern subtropical plume of CO meanders >15,000 km around the world, gradually spreading around ∼20 S. This southern pollution plume is most noticeable in the burning season, southern spring; a similar subtropical plume appears in the northern spring. We use tracer maps to guide the use of trajectories to trace observations of the plume to their origins. The MM5 mesoscale model provides high‐resolution, near‐global synoptic reconstructions of the weather. Two situations are analyzed: NASA's airborne Pacific Exploratory Mission‐Tropics A (PEM‐Tropics A) period, September–October 1996 and the PEM‐Tropics B period, March–April 1999. Similar features are noted for a much earlier mission in 1977, which apparently captured the first, but never‐recognized, samples of the global pollution of the Southern Hemisphere. For PEM‐Tropics A, near‐source pieces of the plume are clearly seen in the Total Ozone Mapping Spectrometer (TOMS) absorbing aerosol product and are well simulated. Downwind, the aircraft sampling of several strands deriving from a single plume seems representative and well simulated. A general mechanism of the plume emerges: The southern plume arises in surface accumulation regions in Africa and South America. Thunderstorm‐scale venting of pollutants usually lofts the plume; however, synoptic‐scale lifting can produce intense outbreaks. The plume flows eastward in the subtropical jet region as a single coherent but articulated current until it is increasingly filamented by storms in the Pacific. A similar northern subtropical plume is described for the PEM‐Tropics B period. The 100‐km model resolution we used seemed to capture much of the variability. However, the model somewhat under predicted the highest values.
Wet scavenging of sulfur compounds and other constituents during the Indian Ocean Experiment (INDOEX)
Authors: Granat L. , Norman M. , Leck C. , Kulshrestha U. C. , Rodhe H.
Published: October 2002

Rainwater samples for chemical analysis were collected over the Indian Ocean during the Indian Ocean Experiment (INDOEX) campaign January–March 1999 on board the research vessels Ronald H. Brown and Sagar Kanya. Samples were analyzed for major ions and some trace metals. The rainwater data are interpreted in terms of transport from potential source regions in Asia using air mass trajectories covering 10 days. A comparison is also made between the rainwater data and the concentration of aerosol components measured simultaneously on the ships. The concentrations of nonsea‐salt (nss)‐SO42−, NO3−, NH4+, nss‐K+, and nss‐Ca2+ in rainwater over the Indian Ocean, while a factor of 2 to 3 lower than over the Indian continent, were still clearly influenced by pollution and soil sources in Asia. The concentration of nss‐Ca2+ decreased more rapidly as the air moved southward from the continent out over the ocean, whereas the concentration of nss‐SO42− became relatively more abundant. This was consistent with the observed higher acidity of the rainwater over the ocean (pH in the range 4.8 to 5.4) than over the Indian subcontinent, with NH4+ as the main cation (rather than Ca2+, as over land). Variations in the concentration of Al and Fe correlated well with those of nss‐Ca2+, indicating a crustal source for these elements. The relation between Na+, Cl−, and Br− in the rainwater was close to that of seawater, implying no excess or deficit of the two halogen ions. The ratio between the concentration in rainwater and the concentration in surface air was systematically larger for aerosol components that exist in the coarse mode of sea‐salt origin (Na+, Mg2+, and Cl−) than those in the fine mode (NH4+, nss‐K+, and nss‐SO42−), indicating that fine‐mode particles are scavenged mainly by in‐cloud processes whereas coarse‐mode sea‐salt particles are scavenged also by falling raindrops under the clouds. Nss‐Ca2+ and NO3− fall in a category in between, indicating that these compounds are not as effectively removed by below‐cloud scavenging as sea‐salt aerosols.
On detection of turnaround and recovery in trend for ozone
Authors: Reinsel Gregory C. , Weatherhead Elizabeth , Tiao George C. , Miller Alvin J. , Nagatani Ronald M. , Wuebbles Donald J. , Flynn Lawrence E.
Published: October 2002

Because of control measures from international agreements, it is predicted that previous decreases in ozone should lessen and eventually turn around as a recovery of ozone. It is important to statistically determine from available ozone data that a change in the rate of downward trend in ozone occurs and that there is an overall “turnaround” in the downward trend. For this purpose, characteristics of a statistical trend model that allows for a change in trend (either a flattening or a turnaround) at some specified date are investigated. This model permits the use of data from before as well as after the change date to be used in the trend analysis and affords the opportunity for detection of a change in the downward ozone trend as an early signal in addition to detection of positive trend (recovery) after the change. Total Ozone Mapping Spectrometer data and ground station total ozone data from the northern midlatitudes are used to estimate the number of years of data required to detect a change in trend and to detect a positive trend recovery, under two different assumptions about future ozone trends. Results show that for midlatitudes (30°–60°) a positive change in trend of reasonably assumed magnitude can be detected within ∼7–8 years from the change date, whereas detection of a positive trend recovery can require roughly 15–20 years for southern midlatitude zonal average data and 20–25 years for northern midlatitude zonal average data. In addition, preliminary trend analysis of recent ground station total ozone data is performed to illustrate estimation using the proposed trend model, with some informative results.
The Los Alamos Sferic Array: A research tool for lightning investigations
Authors: Smith D. A. , Eack K. B. , Harlin J. , Heavner M. J. , Jacobson A. R. , Massey R. S. , Shao X. M. , Wiens K. C.
Published: October 2002

Since 1998 the Los Alamos Sferic Array (LASA) has recorded electric field change signals from lightning in support of radio frequency (RF) and optical observations by the Fast On‐orbit Recording of Transient Events (FORTE) satellite. By “sferic” (a colloquial abbreviation for “atmospheric”), we refer to a remote measurement of the transient electric field produced by a lightning flash. LASA consisted of five stations in New Mexico in 1998 and was expanded to 11 stations in New Mexico, Texas, Florida, and Nebraska in 1999. During the 2 years of operation described in this paper, the remote stations acquired triggered 8‐ or 16‐ms duration, 12‐bit waveforms and GPS‐based sferic time tags 24 hours per day year‐round. Source locations were determined daily using differential time of arrival techniques, and the waveforms from all geolocated events were transferred to Los Alamos National Laboratory (LANL), where they have been archived for further analysis, including event classification and characterization. We evaluated LASA location accuracy by comparing temporally coincident (occurring within 100 μs) LASA and National Lightning Detection Network (NLDN) event locations. Approximately one half of the locations agreed to within 2 km, with better agreement for events that occurred within the confines of LASA subarrays in New Mexico and Florida. Of the ∼900,000 events located by the sferic array in 1998 and 1999, nearly 13,000 produced distinctive narrow bipolar field change pulses resembling those previously identified as intracloud discharges.
Aerosol optical depth retrieval from GOES‐8: Uncertainty study and retrieval validation over South America
Authors: Knapp Kenneth R. , Vonder Haar Thomas H. , Kaufman Yoram J.
Published: October 2002

An algorithm for aerosol optical depth τ retrieval from the Geostationary Observational Environmental Satellite (GOES) series is described, where the darkest pixels are used to create a spatial composite of surface reflectance. The data are calibrated and corrected for atmospheric extinction to retrieve the surface reflectance which is then used to retrieved τ. Analysis suggests that τ retrieval uncertainty is ±18–34% depending on the certainty of the assumed radiative transfer model parameters. Retrieval uncertainty is less over low surface reflectances and at large scattering angles. The retrieval algorithm is validated against Sun‐sky radiometer τ measurements for aerosols emitted by biomass burning in South America during 1995 and 1998. The relative differences between observed and retrieved τ are within the estimated uncertainty, having correlations ranging from 0.78 to 0.97. Further, the GOES retrievals are compared to τ retrieved using the Moderate‐Resolution Imaging Spectroradiometer (MODIS) airborne simulator (MAS). The average relative difference in this comparison is 11%, thus retrieval validations are again within the estimated algorithm uncertainty. These results suggest that the GOES satellite can be used to monitor aerosols over land, while the agreement between MAS and GOES retrievals suggests the ability to combine the spectral abilities of MODIS with the temporal observations of GOES.
Transport out of the Antarctic polar vortex from a three‐dimensional transport model
Authors: Li Shuhua , Cordero Eugene C. , Karoly David J.
Published: October 2002

A three‐dimensional chemical transport model is utilized to study the transport out of the Antarctic polar vortex during the southern hemisphere spring. On average, over five consecutive years between 1993 and 1997, horizontal transport out of the vortex into the midlatitude stratosphere is smaller than vertical transport into the troposphere. However, there is significant interannual variability in the magnitude of mass exchange, which is related to year‐to‐year fluctuations in planetary wave activity. In 1994 the net loss of the vortex tracer mass in September is similar to that in October. However, the relative mass flux entering the midlatitude stratosphere and the troposphere differ between the two months. The ratio of horizontal transport out of the vortex to vertical transport into the troposphere is about 3:7 in September and 5:5 in October, indicating the higher permeability of the vortex in October compared to September. The September mass flux into the troposphere is larger than in October, consistent with the fact that stronger diabatic cooling occurs in September than October over Antarctica. The estimated ozone change at southern midlatitudes due to the intrusion of ozone‐depleted air from high latitudes during September–October 1994 is about −0.44% per decade, which could contribute up to 10% of observed ozone decline at southern midlatitudes in spring. This amount is an underestimate of the dilution effect from high latitudes during the spring season, as it does not include the vortex breakup in late spring.
Quantum yields for production of O(1D) in the ultraviolet photolysis of ozone: Recommendation based on evaluation of laboratory data
Authors: Matsumi Y. , Comes F. J. , Hancock G. , Hofzumahaus A. , Hynes A. J. , Kawasaki M. , Ravishankara A. R.
Published: October 2002

The quantum yield for O(1D) production in the photolysis of ozone in the ultraviolet region as a function of wavelength and temperature is a key input for modeling calculations in the atmospheric chemistry. To provide the modeling community with the best possible information, the available data are critically evaluated, and the best possible recommendations for the quantum yields are presented. Since the authors of this paper are the principal investigators of the groups which have provided most of the recent experimental data for the O(1D) quantum yields, the basic assumptions made by each group, the input parameters used in obtaining the quantum yields, and possible sources of systematic errors are well examined. The fitting expression of the O(1D) yield as a function of photolysis wavelength λ and temperature Tis presented in the ranges of 306 nm ≤ λ ≤ 328 nm and 200 K ≤ T ≤ 300 K. The recommendation values of the O(1D) quantum yield for 290 nm ≤ λ ≤ 306 nm and 328 nm ≤ λ ≤350 nm are also presented. The formation mechanisms of O(1D) in the photolysis of ozone which result in the wavelength and temperature dependence of the O(1D) yields are interpreted.
The effect of deep, tropical convection on the tropical tropopause layer
Authors: Dessler A. E.
Published: October 2002

There is evidence that most detrainment from tropical convection occurs below ∼14 km, about 2 km below the tropopause. On the other hand, it is also generally accepted that convection does occasionally reach the altitude of the tropopause. The question is whether the relatively infrequent penetrations above 14 km are sufficient to influence the composition of the atmosphere above 14 km. Using O3 and CO data as tracers of convective activity, I quantify the height distribution of mass detraining from convection near the tropopause using data obtained in February 1996. The calculation suggests significant detrainment is occurring as high as ∼380 K potential temperature (∼17 km), with as much as 60% of the mass crossing the 380 K surface detraining above 370 K (∼15 km). These findings contrast with the previously published suggestion that convective transport of mass does not significantly influence the composition of the atmosphere above 14 km.
Characterization and parameterization of atmospheric particle number‐, mass‐, and chemical‐size distributions in central Europe during LACE 98 and MINT
Authors: Neusüß C. , Wex H. , Birmili W. , Wiedensohler A. , Koziar C. , Busch B. , Brüggemann E. , Gnauk T. , Ebert M. , Covert D. S.
Published: October 2002

Intensive measurements of chemical and physical properties of the atmospheric aerosol have been performed at two sites in central Europe during the Melpitz‐Intensive (MINT) in November 1997 and the Lindenberg Aerosol Characterization Experiment 1998 (LACE 98) in July and August 1998. Number‐size distributions, hygroscopic particle growth, size‐segregated gravimetric mass, and size‐segregated chemical masses of water‐soluble ions and organic and elemental carbon of aerosol particles have been measured. To obtain information on the quality of the different methods, the number‐derived, gravimetric, and chemically derived mass distributions are compared. Gravimetric mass of fine particles is attributed completely to chemical composition by carbonaceous material and ions, including an estimate of the water content due to hygroscopic compounds. For the characterization of coarse particles, which contribute less to the total mass concentration, insoluble material has to be included in the mass balance. Mass concentrations calculated from the number‐size distributions are well correlated with the gravimetric mass concentration; however, the calculated mass is larger, especially for the Aitken and accumulation modes. The number‐derived mass concentration is most sensitive to the sizing uncertainty of the measured number‐size distribution. Moreover, the impactor cutoffs and the limited knowledge about the density of the particles (especially with high carbon content) account for a major part of the uncertainties. The overall uncertainty of the calculated mass, determined as the standard deviation of the average value in a Monte Carlo approach, is found to be about 10%. Lognormal parameters for the number‐size and volume‐size distributions as well as gravimetric mass‐size distribution and corresponding chemical composition are presented for different air mass types. Most of the modal parameters do not differ significantly between the air mass types. Higher mass concentrations are mostly due to an increase in size (of Aitken and accumulation mode) rather than an increase in the number of particles in a given mode. Generally, the mass percent carbon content increases with decreasing particle size. The most pronounced difference with season is an increase of carbon content from summer to winter as well as an increase in nitrate content, resulting in a decrease of sulfate. For nitrate a strong dependence on air mass direction is observed. Sulfate and nitrate are predominantly neutralized by ammonium. With the results of the two experiments, quality‐controlled mode parameters and corresponding chemical composition of atmospheric aerosol particles in central Europe are now available for application in models.
Influence of Arctic polar ozone depletion on lower stratospheric ozone amounts at Haute‐Provence Observatory (43.92°N, 5.71°E)
Authors: Godin S. , Marchand M. , Hauchecorne A. , Lefèvre F.
Published: October 2002

Lidar and ozone sonde measurements performed at Haute‐Provence Observatory are used together with a high‐resolution potential vorticity (PV) transport model to study the influence of Arctic polar vortex ozone depletion on ozone amounts at this location prior to vortex dilution. The study focuses on four Arctic winters from 1996 to 2000. The transport model is used to investigate the presence of polar air masses above the station. These air masses, which are characterized by high PV values, can extend toward midlatitude regions during winter. Polar air masses are detected using a daily threshold value defined from the PV fields. The monthly average fraction of polar air generally increases with altitude from a few percent in the lower stratosphere to 10% or more above 500 K. The source of presence of polar air is largely dominated by filament events in the lower stratosphere and is equally divided between filaments and vortex excursions above the station in the middle stratosphere. The presence of polar air above the station shows high interannual variability. In the winter of 1999/2000 the least amount of polar air was detected above the station. However, vortex excursions above the station from January to March 2000 led to a significant average ozone reduction, estimated to 1.6% in the 400–650 K potential temperature range using a chemical transport model (CTM) evaluation of ozone loss at the vortex edge. Cold vortex, such as observed in 1999/2000, results in less transport toward midlatitude, but strong ozone depletion within the vortex combined with transient excursions of polar air can have a significant impact on ozone amounts in the midlatitude regions.