S. L. Gong

Modeling sea-salt aerosols in the atmosphere: 1. Model development

A simulation of the processes of sea-salt aerosol generation, diffusive transport, transformation, and removal as a function of particle size is incorporated into a one-dimensional version of the Canadian general climate model (GCMII). This model was then run in the North Atlantic between Iceland and Ireland during the period of January-March. Model predictions are compared to observations of sea-salt aerosols selected from a review of available studies that were subjected to strict screening criteria to ensure their representativeness. The number and mass size distribution and the wind dependency of total sea-salt aerosol mass concentrations predicted by the model compare well with observations. The modeled dependence of sea-salt aerosol concentration in the surface layer (χ, μg m−3) on 10-m wind speed (U10, m s−1) is given byequation image. Simulations show that both a and b change with location. The value a and b range from 0.20 and 3.1 for Mace Head, Ireland to 0.26, and 1.4 for Heimaey, Iceland. The dependence of χ on surface wind speed is weaker for smaller particles and for particles at higher altitudes. The residence time of sea-salt aerosols in the first atmospheric layer (0–166 m) ranges from 30 min for large particles (r=4–8 μm) to ∼60 hours for small particles (r=0.13–0.25 μm). Although some refinements are required for the model, it forms the basis for comparing the simulations with long-term atmospheric sea-salt measurements made at marine baseline observatories around the world and for a more comprehensive three-dimensional modeling of atmospheric sea-salt aerosols.

Composite global emissions of reactive chlorine from anthropogenic and natural sources: Reactive Chlorine Emissions Inventory

Emission inventories for major reactive tropospheric CI species (particulate CI, HC1, C1NO2, CH3CI, CHCI3, CH3CCI3, C2C14, C2HC13, CH2C12, and CHCIF2) were integrated across source types (terrestrial biogenic and oceanic emissions, sea-salt production and dechlorination, biomass burning, industrial emissions, fossil-fuel combustion, and incinera- tion). Composite emissions were compared with known sinks to assess budget closure; relative contributions of natural and anthropogenic sources were differentiated. Model cal- culations suggest that conventional acid-displacement reactions involving Sov)+O3, S(Iv)+ H202, and H2SO4 and HNO3 scavenging account for minor fractions of sea-salt dechlorina- tion globally. Other important chemical pathways involving sea-salt aerosol apparently pro- duce most volatile chlorine in the troposphere. The combined emissions of CH3CI from known sources account for about half of the modeled sink, suggesting fluxes from known sources were unde:estimated, the OH sink was overestimated, or significant unidentified sources exist. Anthropogenic activities (primarily biomass burning) contribute about half the net CH3CI emitted from known sources. Anthropogenic emissions account for only about 10% of the modeled CHCl3 sink. Although poorly constrained, significant fractions of tropo- spheric CH2C12 (25%), C2HC13 (10%), and C2C14 (5%) are emitted from the surface ocean; the combined contributions of C2C14 and C2HC13 from all natural sources may be substan- tially higher than the estimated oceanic flux.

Canadian Aerosol Module: A size‐segregated simulation of atmospheric aerosol processes for climate and air quality models 1. Module development

A size-segregated multicomponent aerosol algorithm, the Canadian Aerosol Module (CAM), was developed for use with climate and air quality models. It includes major aerosol processes in the atmosphere: generation, hygroscopic growth, coagulation, nucleation, condensation, dry deposition/sedimentation, below-cloud scavenging, aerosol activation, a cloud module with explicit microphysical processes to treat aerosol-cloud interactions and chemical transformation of sulphur species in clear air and in clouds. The numerical solution was optimized to efficiently solve the complicated size-segregated multicomponent aerosol system and make it feasible to be included in global and regional models. An internal mixture is assumed for all types of aerosols except for soil dust and black carbon which are assumed to be externally mixed close to sources. To test the algorithm, emissions to the atmosphere of anthropogenic and natural aerosols are simulated for two aerosol types: sea salt and sulphate. A comparison was made of two numerical solutions of the aerosol algorithm: process splitting and ordinary differential equation (ODE) solver. It was found that the process-splitting method used for this model is within 15% of the more accurate ODE solution for the total sulphate mass concentration and <1% accurate for sea-salt concentration. Furthermore, it is computationally more than 100 times faster. The sensitivity of the simulated size distributions to the number of size bins was also investigated. The diffusional behavior of each individual process was quantitatively characterized by the difference in the mode radius and standard deviation of a lognormal curve fit of distributions between the approximate solution and the 96-bin reference solution. Both the number and mass size distributions were adequately predicted by a sectional model of 12 bins in many situations in the atmosphere where the sink for condensable matter on existing aerosol surface area is high enough that nucleation of new particles is negligible. Total mass concentration was adequately simulated using lower size resolution of 8 bins. However, to properly resolve nucleation mode size distributions and minimize the numerical diffusion, a sectional model of 18 size bins or greater is needed. The number of size bins is more important in resolving the nucleation mode peaks than in reducing the diffusional behavior of aerosol processes. Application of CAM in a study of the global cycling of sea-salt mass accompanies this paper

A Simulated Climatology of Asian Dust Aerosol and Its Trans-Pacific Transport. Part I: Mean Climate and Validation

Abstract The Northern Aerosol Regional Climate Model (NARCM) was used to construct a 44-yr climatology of spring Asian dust aerosol emission, column loading, deposition, trans-Pacific transport routes, and budgets during 1960–2003. Comparisons with available ground dust observations and Total Ozone Mapping Spectrometer (TOMS) Aerosol Index (AI) measurements verified that NARCM captured most of the climatological characteristics of the spatial and temporal distributions, as well as the interannual and daily variations of Asian dust aerosol during those 44 yr. Results demonstrated again that the deserts in Mongolia and in western and northern China (mainly the Taklimakan and Badain Juran, respectively) were the major sources of Asian dust aerosol in East Asia. The dust storms in spring occurred most frequently from early April to early May with a daily averaged dust emission (diameter d < 41 μm) of 1.58 Mt in April and 1.36 Mt in May. Asian dust aerosol contributed most of the dust aerosol loading in the tr...

A general circulation model based calculation of HCl and ClNO2 production from sea salt dechlorination: Reactive Chlorine Emissions Inventory

As part of the Reactive Chlorine Emissions Inventory, a global model of chemical processes in the marine boundary layer (MBL), Marine Aerosol and Gas Phase Interactions (MAGPI), was developed to calculate direct monthly production of HCl and ClNO2 from sea salt dechlorination on a 2.8 × 2.8 latitude-longitude grid. Sea salt mass and size distributions and associated surface exchange fluxes were calculated using the Canadian General Circulation Model; integrated annual production of sea salt Cl− was 1785 Tg Cl yr−1. Corresponding distributions of gas-phase HNO3, SO2, N2O5, H2O2, O3, H2SO4 and NH3 were calculated using different global chemical transport models in which sea salt reactions were not considered. A chemical scheme was developed to estimate the monthly mean steady-state phase partitioning of product and reactant species at each grid point. Average annual gridded fluxes of HCl and ClNO2 varied spatially from 1 to 300 mg Cl m−2 yr−1 and from 1 to 8 mg Cl m−2 yr−1, respectively. Maxima occurred in polluted coastal regions of the North Atlantic, the western North Pacific and the Mediterranean where up to 20% of the total Cl and 80% of the sub-micron Cl volatilized. In remote oceanic regions, available acidity was insufficient to titrate all sea salt alkalinity, thus, significant HCl was not produced via acid displacement. However, in these regions virtually all HNO3 was scavenged by sea salt. The integrated annual global fluxes for HCl and ClNO2 were 7.6 Tg Cl yr−1 and 0.06 Tg Cl yr−1, respectively; virtually all in the Northern Hemisphere. Largest HCl and ClNO2 fluxes occur in northern hemisphere winter due to high sea salt loading and elevated HNO3, SO2 and N2O5 concentrations. 70% of the HCl dechlorination occurs on particles between 0.75 μm and 4 μm radius; ClNO2 volatilized from slightly larger particles. The aerosol pH of each particle size bin equilibrates towards the same value once the alkalinity has been titrated.

Modeling sea-salt aerosols in the atmosphere: 2. Atmospheric concentrations and fluxes

Atmospheric sea-salt aerosol concentrations are studied using both long-term observations and model simulations of Na+ at seven stations around the globe. Good agreement is achieved between observations and model predictions in the northern hemisphere. A stronger seasonal variation occurs in the high-latitude North Atlantic than in regions close to the equator and in high-latitude southern hemisphere. Generally, concentrations are higher for both boreal and austral winters. With the model, the production flux and removal flux at the atmosphere-ocean interface was calculated and used to estimate the global sea-salt budget. The flux also shows seasonal variation similar to that of sea-salt concentration. Depending on the geographic location, the model predicts that dry deposition accounts for 60–70% of the total sea-salt removed from the atmosphere while in-cloud and below-cloud precipitation scavenging accounts for about 1% and 28–39% of the remainder, respectively. The total amount of sea-salt aerosols emitted from the world oceans to the atmosphere is estimated to be in the vicinity of 1.17×1016 g yr−1. Approximately 99% of the sea-salt aerosol mass generated by wind falls back to the sea with about 1–2% remaining in the atmosphere to be exported from the original grid square (300×300 km). Only a small portion of that exported (∼4%) is associated with submicron particles that are likely to undergo long-range transport.

Sensitivity of sulphate aerosol size distributions and CCN concentrations over North America to SO x emissions and H2O2 concentrations

To assess the influence of aerosols on climate, the Northern Aerosol Regional Climate Model (NARCM) is currently being developed. NARCM includes size-segregated aerosols as prognostic and interactive constituents. In this paper, the model is being applied to sulphate aerosol over North America during time periods in July and December 1994. The results give evidence for considerable regional and seasonal variations in sulphate aerosol size distributions over North America. Comparisons of the results with different observations yield a reasonably good agreement in terms of meteorological and physicochemical parameters. Some of the differences in sulphate concentrations and wet deposition rates can be attributed to differences in cloud amounts and precipitation between model results and observations. Indirect tests of the simulated aerosol mass mean diameters are also encouraging. Additional simulations for hypothetical decreases in anthropogenic sulphur emissions and increases in hydrogen peroxide (H2O2) background concentrations are performed for the same time periods to study the responses of concentration, size distribution, and wet deposition of sulphate aerosol to these changes. Also, responses of cloud condensation nuclei (CCN) number concentrations are investigated. The simulation results show that sulphate aerosol concentrations respond almost linearly in both time periods to decreases in sulphur emissions but that CCN number concentrations respond nonlinearly due to decreases in sulphate mass mean diameters. Especially for the December period, increases in hydrogen peroxide background concentrations lead to increases in CCN number concentrations at critical diameters larger than about 0.07 μm. These results lead to the hypothesis that increased in-cloud oxidation in convective clouds due to future increases in oxidant concentrations may produce larger CCN which eventually can be easily activated in subsequently forming stratiform clouds.