Anthropogenic sulfate aerosol is a major contributor to shortwave radiative forcing of climate change by direct light scattering and by perturbing cloud properties and to local concentrations of atmospheric particulate matter. Here we analyze results from previously published calculations with an Eulerian transport model for atmospheric sulfur species in the Northern Hemisphere in June–July, 1997 to quantify the absolute and relative contributions of specific source regions (North America, Europe, and Asia) and SO2-to-sulfate conversion mechanisms (gas-phase, aqueous-phase and primary sulfate) to sulfate and SO2 column burdens as a function of location and time. Although material emitted within a given region dominates the sulfate and SO2 column burden in that region, examination of time series at specific locations shows that material imported from outside can make a substantial and occasionally dominant contribution. Frequently the major fraction of these exogenous contributions to the sulfate column burden was present aloft, thus minimally impacting air quality at the surface, but contributing substantially to the burden and, by implication, to radiative forcing and diminution of surface irradiance. Although the dominant sulfate formation pathway in the domain as a whole is aqueous-phase reaction in clouds (62%), in regions with minimum opportunity for aqueous-phase reaction gas-phase oxidation is dominant, albeit with considerable temporal variability depending on meteorological conditions. These calculations highlight the importance of transoceanic transport of sulfate, especially at the western margins of continents under the influence of predominantly westerly transport winds.

A high‐resolution (1° × 1°, 27 vertical levels) Eulerian chemical transport and transformation model for sulfate, SO2, and related species driven by analyzed forecast meteorological data has been run for the Northern Hemisphere for June–July 1997 and extensively evaluated with observational data, mainly from air quality and precipitation chemistry networks. For ∼5000 evaluations, 50% of the modeled sulfate 24‐hour mixing ratios were within a factor of 1.85 of the observations; 50% of ∼328 concurrent subgrid observations were within a factor of 1.33. Much greater subgrid variation for 24‐hour SO2 mixing ratios (50% of ∼3552 observations were within a factor of 2.32) reflects high variability of this primary species; for ∼12600 evaluations, 50% of modeled mixing ratios were within a factor of 2.54 of the observations. These results indicate that a substantial fraction of the modeled and observed differences is due to subgrid variation and/or measurement error. Sulfate mixing ratios are identified by source type (biogenic, volcanic, and anthropogenic) and production mechanism (primary and by gas‐phase and aqueous‐phase oxidation). Examination of key diagnostics showed substantial variation for the different types of sulfur, e.g., SO2 aqueous‐phase oxidation rates of 29–102% d−1 and sulfate residence times of 4–9 days. Volcanic emissions contributed 10% of the sulfate burden and 6% of emissions, because the elevated release allows large fractional conversion of SO2 and long residence time. Biogenic SO2 was generally at lower concentrations than H2O2, resulting in efficient aqueous‐phase oxidation; this source type contributed 13% of emissions but only 5% of sulfate burden. Anthropogenic sources were the dominant contributors to sulfur emissions (80%) and sulfate burden (84%).

High resolution measurements of sulfate during the ACE‐2 campaign (June–July 1997) permit detailed evaluation of the performance of a chemical transport models driven by analyzed meteorological data. At Tenerife, Canary Islands, (minimal proximate sources) the median ratio characteristic spread between model and observations, Sm/o ∼1.3, was comparable to the spread of three sets of collocated observations and to the spread of observations at stations separated by ∼13 km within a single model grid cell (1° × 1°). Somewhat greater Sm/o, ∼1.6, at Sagres, Portugal is attributed to nonrepresentativeness of a single measurement site influenced by proximate sources. At Tenerife contributions from European, North American, and biogenic sources to sulfate mixing ratios are comparable, with North American sources dominating (up to ∼85%) under conditions of a strong Azores high; thus substantial contributions of sulfate, and by implication other aerosols, can result from long‐range midlatitude transport across the Atlantic Ocean.

Abstract Anthropogenic aerosols are hypothesized to decrease cloud drop radius and increase cloud droplet number concentration enhancing cloud optical depth and albedo. Here results have been used from a chemical transport model driven by the output of a numerical weather prediction model to identify an incursion of sulfate-laden air from the European continent over the mid–North Atlantic under the influence of a cutoff low pressure system during 2–8 April 1987. Advanced Very High Resolution Radiometer (AVHRR) measurements of visible and near-infrared radiance are used to infer microphysical properties of low-altitude (T = 260–275 K) maritime clouds over the course of the event. Examination of the cloud optical depth, drop radius, and drop number concentration on the high- and low-sulfate days has allowed identification of the increase in cloud droplet number concentration and decrease in cloud drop radius associated with the sulfate incursion. These observations are consistent with the Twomey mechanism o...

Here the authors have used the output of a chemistry/transport model to identify a situation in which sulfate aerosol from industrial sources may be expected to exert a strong influence on cloud microphysical and radiative properties in an oceanic area that is well displaced from source regions. Pertinent cloud microphysical properties (optical depth and cloud drop radius) are inferred from radiance data obtained from satellite remote sensing. Comparison of these quantities in situations where the model indicates the presence or absence of industrial sulfate has allowed identification of the expected signature of one aerosol indirect effect--an increase in droplet number concentration and concomitant decrease in droplet radii, on a synoptic scale. Although the information obtained on changes in cloud optical depth is too meager to draw conclusions regarding radiative forcing, there is no doubt that the cloud microphysical properties are influenced by the incursion of continental sulfate aerosol in a way that is consistent with that expected by the Twomey indirect forcing mechanism.

Atmospheric aerosols, suspensions of solid or liquid particles, are an important multi-phase system. Aerosols scatter and absorb shortwave (solar) radiation, affecting climate (Charlson et al., 1992; Schwartz, 1996) and visibility; nucleate cloud droplet formation, modifying the reflectivity of clouds (Twomey et al., 1984; Schwartz and Slingo, 1996) as well as contributing to composition of cloudwater and to wet deposition (Seinfeld and Pandis, 1998); and affect human health through inhalation (NRC, 1998). Existing and prospective air quality regulations impose standards on concentrations of atmospheric aerosols to protect human health and welfare (EPA, 1998). Chemical transport and transformation models representing the loading and geographical distribution of aerosols and precursor gases are needed to permit development of effective and efficient strategies for meeting air quality standards, and for examining aerosol effects on climate retrospectively and prospectively for different emissions scenarios. Important aerosol properties and processes depend on their size distribution: light scattering, cloud nucleating properties, dry deposition, and penetration into airways of lungs. The evolution of the mass loading itself depends on particle size because of the size dependence of growth and removal processes. For these reasons it is increasingly recognized that chemical transport and transformation models must represent not just the mass loading of atmospheric particulate matter but also the aerosol microphysical properties and the evolution of these properties if aerosols are to be accurately represented in these models. If the size distribution of the aerosol is known, a given property can be evaluated as the integral of the appropriate kernel function over the size distribution. This has motivated the approach of determining aerosol size distribution, and of explicitly representing this distribution and its evolution in chemical transport models.

The Twomey effect of enhanced cloud droplet concentration, optical depth, and albedo caused by anthropogenic aerosols is thought to contribute substantially to radiative forcing of climate change over the industrial period. However, present model-based estimates of this indirect forcing are highly uncertain. Satellite-based measurements would provide global or near-global coverage of this effect, but previous efforts to identify and quantify enhancement of cloud albedo caused by anthropogenic aerosols in satellite observations have been limited, largely because of strong dependence of albedo on cloud liquid water path (LWP), which is inherently highly variable. Here we examine satellite-derived cloud radiative properties over two 1-week episodes for which a chemical transport and transformation model indicates substantial influx of sulfate aerosol from industrial regions of Europe or North America to remote areas of the North Atlantic. Despite absence of discernible dependence of optical depth or albedo on modeled sulfate loading, examination of the dependence of these quantities on LWP readily permits detection and quantification of increases correlated with sulfate loading, which are otherwise masked by variability of LWP, demonstrating brightening of clouds because of the Twomey effect on a synoptic scale. Median cloud-top spherical albedo was enhanced over these episodes, relative to the unperturbed base case for the same LWP distribution, by 0.02 to 0.15.

The loading of tropospheric aerosols is highly variable spatially and temporally as a consequence of large spatial variability in sources, temporal variability in transport winds, and short residence times (days), and additionally for secondary sulfate, intermittent production associated with aqueous‐phase reaction in clouds, and intermittent removal, mainly by precipitation. We have used a chemical transport and transformation model for atmospheric sulfur driven by observationally derived meteorological data to calculate the geographical distribution of sulfate and sulfur dioxide (SO2) over North America, the North Atlantic, and Europe as a function of time for April 1987. We present the results of these calculations, mainly as animations showing the time dependence of the column burden (vertical integral of concentration) and of sulfate wet deposition, and interpret the temporal evolution in terms of the controlling meteorological phenomena as discerned from synoptic analyzes at 925 and 500 hPa. The analyzes show highly localized and episodic buildup and removal of sulfate and SO2 and circulations under the influence of high‐ and low‐pressure systems as well as instances of rapid meridional and zonal transport over distances of thousands of kilometers. Key meteorological phenomena giving rise to large amounts of sulfate include not only slow moving high‐pressure systems, as previously recognized, but also low‐pressure systems. Whether or not a given low‐pressure system results in large sulfate loading depends strongly on its location relative to sources of sulfur species and of the water vapor that is required for formation of clouds and precipitation. We present a detailed analysis of the evolution of sulfate and SO2 over the eastern North Atlantic April 3–8, 1987, under the influence of a cutoff low‐pressure system during which sources in northern Europe continued to feed a system that repeatedly circulated over source regions. Animations such as these facilitate identification of episodes of large aerosol loading and relating these loadings to the controlling meteorological phenomena.

We describe new developments in the application of the Quadrature Method of Moments (QMOM) [1]. These include the first application of the QMOM in a 3-D chemical transformation and transport model on the sub-hemispheric scale [2]. The QMOM simultaneously tracks an arbitrary (even) number of moments of a particle size distribution directly in space and time without the need for explicitly representing the distribution itself. The present implementation evolves the six lowest-order radial moments for each of several externally-mixed aerosol populations. From these moments we report modeled geographic distributions of several aerosol properties, including a shortwave radiative forcing obtained using the Multiple Isomomental Distribution Aerosol Surrogate (MIDAS) technique [3]. These results demonstrate the capabilities of these moment-based techniques to simultaneously represent aerosol nucleation, condensation, coagulation, dry deposition, wet removal, cloud activation, and transport processes in a large-scale model, and to yield aerosol optical properties and radiative influence from the modeled aerosol moments. We report on recent extensions of the method for simulation of internal mixtures and generally-mixed aerosols, and on a bivariate extension of the QMOM for modeling simultaneous coagulation and sintering of particle populations [4].

We examine the balance between processes that contribute to the global and regional distributions of sulfate aerosol in the Earth’s atmosphere using a set of simulations from the NCAR CCM3. The analysis suggests that the seasonal cycle of SO and SO u0004 are controlled by a complex interplay between transport, chemistry and deposition processes. The seasonal cycle of these species are not strongly controlled by temporal variations in emissions, but by seasonal variations in volume of air processed by clouds, mass of liquid water serving as a site for aqueous chemistry, amount of oxidant available for the conversion from SO to SO u0004 , vertical transport processes, and deposition. A tagging of the sulfate by emission region (Europe, N. America, Asia, and rest of world [ROW]), chemical pathway (gaseous vs. in-cloud), and type of emissions (anthropogenic vs. biogenic) is used to differentiate the balance of processes controlling the production and loading from this material. Significant differences exist in the destiny of SO molecules emitted from the several regions. An SO molecule emitted from the ROW source region has a much greater potential to form sulfate than one emitted from, for example, Europe. A greater fraction of the SO molecules are oxidized that originate from ROW compared with other areas, and once formed, the sulfate has a longer residence time (that is, it is not readily scavenged). The yield of sulfate from ROW sources of SO is a factor of four higher than that of Europe. A substantially higher fraction of the SO emitted over Europe is oxidized to sulfate through the ozone pathway compared to other regions. The analysis suggests that there are significant differences in the vertical distribution, and horizontal extent, of the propagation of sulfate emitted from the several source regions. Sulfate from Asian source regions reaches the farthest from its point of origin, and makes a significant contribution to burdens in both hemispheres, primarily from plumes reaching out in the upper troposphere. Sulfate from other source regions tend to remain trapped in their hemisphere of origin. The National Center for Atmospheric Research is sponsored by the National Science Foundation. Printed: 27 July 1999 Introduction: Sulfur emissions from industry, and from natural (biogenic and volcanic) sources are believed to influence the earths climate in a num-

This letter describes the first application of the Quadrature Method of Moments (QMOM) [McGraw, 1997] in a 3‐D chemical transformation and transport model. The QMOM simultaneously tracks an arbitrary (even) number of moments of a particle size distribution directly in space and time without the need for explicitly representing the distribution itself. The host 3‐D model, the Global Chemistry Model driven by Observation‐derived meteorological data (GChM‐O), has been previously described [Benkovitz et al., 1994]. The present implementation evolves the six lowest‐order radial moments for each of several externally‐mixed aerosol populations. From these moments we report modeled geographic distributions of several aerosol properties, including a shortwave radiative forcing obtained using the Multiple Isomomental Distribution Aerosol Surrogate (MIDAS) technique [Wright, 2000]. These results demonstrate the capabilities of these moment‐based techniques to simultaneously represent aerosol nucleation, condensation, coagulation, dry deposition, wet removal, cloud activation, and transport processes in a large scale model, and to yield aerosol optical properties and radiative influence from the modeled aerosol.

Sulfur chemistry has been incorporated in the National Center for Atmospheric Research Community Climate Model in an internally consistent manner with other parameterizations in the model. The model predicts mixing ratios of dimethylsulfide (DMS), SO2, SO42−, H2O2. Processes that control the mixing ratio of these species include the emissions of DMS and SO2, transport of each species, gas‐ and aqueous‐phase chemistry, wet deposition, and dry deposition of species. Modeled concentrations agree quite well with observations for DMS and H2O2, fairly well for SO2, and not as well for SO42−. The modeled SO42− tends to underestimate observed SO42− at the surface and overestimate observations in the upper troposphere. The SO2 and SO42− species were tagged according to the chemical production pathway and whether the sulfur was of anthropogenic or biogenic origin. Although aqueous‐phase reactions in cloud accounted for 81% of the sulfate production rate, only ∼50–60% of the sulfate burden in the troposphere was derived from cloud chemistry. Because cloud chemistry is an important source of sulfate in the troposphere, the importance of H2O2 concentrations and pH values was investigated. When prescribing H2O2 concentrations to clear‐sky values instead of predicting H2O2, the global‐averaged, annual‐averaged in‐cloud production of sulfate increased. Setting the pH of the drops to 4.5 also increased the in‐cloud production of sulfate. In both sensitivity simulations, the increased in‐cloud production of sulfate decreased the burden of sulfate because less SO2 was available for gas‐phase conversion, which contributes more efficiently to the tropospheric sulfate burden than does aqueous‐phase conversion.

We examine the balance between processes that contribute to the global and regional distributions of sulfate aerosol in the Earth's atmosphere using a set of simulations from the National Center for Atmospheric Research Community Climate Model, Version 3. The analysis suggests that the seasonal cycle of SO2 and SO42− are controlled by a complex interplay between transport, chemistry and deposition processes. The seasonal cycle of these species is not strongly controlled by temporal variations in emissions but by seasonal variations in volume of air processed by clouds, mass of liquid water serving as a site for aqueous chemistry, amount of oxidant available for the conversion from SO2 to SO42−, vertical transport processes, and deposition. A tagging of the sulfate by emission region (Europe, North America, Asia, and rest of world [ROW]), chemical pathway (gaseous versus in‐cloud), and type of emissions (anthropogenic versus biogenic) is used to differentiate the balance of processes controlling the production and loading from this material. Significant differences exist in the destiny of SO2 molecules emitted from the several regions. An SO2 molecule emitted from the ROW source region has a much greater potential to form sulfate than one emitted from, for example, Europe. A greater fraction of the SO2 molecules is oxidized that originate from ROW compared with other areas, and once formed, the sulfate has a longer residence time (that is, it is not readily scavenged). The yield of sulfate from ROW sources of SO2 is a factor of 4 higher than that of Europe. A substantially higher fraction of the SO2 emitted over Europe is oxidized to sulfate through the ozone pathway compared to other regions. The analysis suggests that there are significant differences in the vertical distribution, and horizontal extent, of the propagation of sulfate emitted from the several source regions. Sulfate from Asian source regions reaches the farthest from its point of origin and makes a significant contribution to burdens in both hemispheres, primarily from plumes reaching out in the upper troposphere. Sulfate from other source regions tends to remain trapped in their hemisphere of origin.

Much if not all of the chlorine present in fossil fuels is released into the atmosphere as hydrogen chloride (HCl) and chloromethane (CH 3 Cl, methyl chloride). The chlorine content of oil‐based fuels is so low that these sources can be neglected, but coal combustion provides significant releases. On the basis of national statistics for the quantity and quality of coal burned during 1990 in power and heat generation, industrial conversion and residential and commercial heating, coupled with information on the chlorine contents of coals, a global inventory of national HCl emissions from this source has been constructed. This was combined with an estimate of the national emissions of HCl from waste combustion (both large‐scale incineration and trash burning) which was based on an estimate of the global quantity released from this source expressed per head of population. Account was taken of reduced emissions where flue gases were processed, for example to remove sulphur dioxide. The HCl emitted in 1990, comprising 4.6 ± 4.3 Tg Cl from fossil fuel and 2 ± 1.9 Tg Cl from waste burning, was spatially distributed using available information on point sources such as power generation utilities and population density by default. Also associated with these combustion sources are chloromethane emissions, calculated to be 0.075 ± 0.07 Tg as Cl (equivalent) from fossil fuels and 0.032 ± 0.023 Tg Cl (equivalent) from waste combustion. These were distributed spatially exactly as the HCl emissions, and a further 0.007 Tg Cl in chloromethane from industrial process activity was distributed by point sources.

Emission inventories for major reactive tropospheric Cl species (particulate Cl, HCl, ClNO2, CH3Cl, CHCl3, CH3CCl3, C2Cl4, C2HCl3, CH2Cl2, and CHClF2) were integrated across source types (terrestrial biogenic and oceanic emissions, sea‐salt production and dechlorination, biomass burning, industrial emissions, fossil‐fuel combustion, and incineration). Composite emissions were compared with known sinks to assess budget closure; relative contributions of natural and anthropogenic sources were differentiated. Model calculations suggest that conventional acid‐displacement reactions involving S(IV) + O3, (IV) + O3 H2O2, and H2SO4 and HNO3 scavenging account for minor fractions of sea‐salt dechlorination globally. Other important chemical pathways involving sea‐salt aerosol apparently produce most volatile chlorine in the troposphere. The combined emissions of CH3Cl from known sources account for about half of the modeled sink, suggesting fluxes from known sources were underestimated, the OH sink was overestimated, or significant unidentified sources exist. Anthropogenic activities (primarily biomass burning) contribute about half the net CH3Cl emitted from known sources. Anthropogenic emissions account for only about 10% of the modeled CHCl3 sink. Although poorly constrained, significant fractions of tropospheric CH2Cl2 (25%), C2HCl3 (10%), and C2Cl4 (5%) are emitted from the surface ocean; the combined contributions of C2Cl4 and C2HCl3 from all natural sources may be substantially higher than the estimated oceanic flux.