A large international field experiment and use of transport modeling has yielded physical, chemical, and radiative properties of the abundant aerosols originating from Asia.

Abstract. This study combines laboratory measurements and modeling analysis to quantify the role of heterogeneous reactions of gaseous nitrogen dioxide and nitric acid on mineral oxide and mineral dust particles in tropospheric ozone formation. At least two types of heterogeneous reactions occur on the surface of these particles. Upon initial exposure of the oxide to NO2 there is a loss of NO2 from the gas phase by adsorption on the particle surface, i.e., NO2(g) --> NO2(a). As the reaction proceeds, a reduction of gaseous NO2 to NO, NO2 (g) --> NO (g) is found to occur. Initial uptake coefficients ¾0 for NO2 on the surface of these particles have been measured at 298 K using a Klmdsen cell reactor coupled to a mass spectrometer. For the oxides studied, ct,¾-A1203, ct,¾-Fe203, TiO2, SiO2, CaO, and MgO, ¾0 ranges from < 4 x 10 © for SiO2 to 2 x 10 -5 for CaO with most values in the 10 -6 range. For authentic samples of China 10ess and Saharan sand, similar reactivity to the oxides is observed with ¾0 values of 2 x 10 -6 and 1 x 10 -6, respectively. For HNO3 the reactivity is 1-2 orders of magnitude higher. Using these laboratory measurements, the impact of heterogeneous reactions of NO2 and HNO3 on mineral dust in tropospheric ozone formation and on O3-precursor relationships is assessed using a time-dependent, multiphase chemistry box model. Simulations with and without heterogeneous reactions were conducted to evaluate the possible influence of these heterogeneous reactions on ambient levels. Results show that values of the initial uptake for NO2 and HNO3, adjusted for roughness effects, must be greater than 10 -4 to have an appreciable impact on NOx, HNO3, and 03 concentrations for the conditions modeled here.Thus the measured uptake coefficients for NO2 on dry surfaces are just below the lower limit to have an impact on the photochemical oxidant cycle, while the heterogeneous reactivity of HNO3 is sufficiently large to have an effect. Under conditions of high mineral dust mass loadings and/or smaller size distributions the importance of these reactions (both NO2 and HNO3) is expected to increase.

Abstract. The evolution of sea-salt and dust particles in East Asia is investigated using a threedimensional transport and chemistry model. A kinetic approach under thermodynamic constraint is utilized to model the condensation/evaporation processes, and other important aerosol processes and influential components (e.g., dust/sea-salt generation, NH3 emissions, gravitational settling, nucleation) are taken into account in this analysis. The model is used to study the Pacific Exploratory Mission-West B period (March 1-6, 1994). It is found that (1) during strong continental outflow, in general, the fine aerosol mode (< 2 pm in aerodynamic diameter) accommodates sulfate and ammonium and the cation-rich coarse mode (> 2 pm in aerodynamic diameter) attracts nitrate. However, in the dust plume, sulfate preferentially resides in the coarse mode due to larger coarse mode mass loading; (2) particulate nitrate coupled with particulate ammonium in the fine mode is predicted over regions where high gaseous NH3 mixing ratios are present (lower courses of the Huang river); (3) dust and sea-salt particles provide important reaction surfaces for sulfate production in the troposphere and increase sulfate production rates by 20 -80%; and (4) soil dust and sea salt provide an important source of boundary layer and free trnnnqnh•r• nlknlin• rnnt•rinl nnd tho

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[1] The dispersion and chemical evolution of NO x in ship plumes has been investigated for marine boundary layer (MBL) conditions. This effort has involved combining a plume dispersion algorithm with a time-dependent photochemical box model. The analysis has considered several factors, all of which can influence the net impact of NO x on the background environment. These include the following: season of the year, latitude of point of release, meteorological setting, and ship NO x emission rate. Reaction rates within a plume were shown to be a nonlinear function of the levels of NO x , leading to relative estimates of ship plume NO x lifetimes that were factors of 2.5-10 times shorter than for ambient marine conditions. The shortened ship-plume NO x lifetime reflects both elevated daytime levels of OH and nighttime levels of NO 3 and N 2 O 5 , all of which were estimated to be several times larger than those typical of ambient marine conditions. During daylight hours, elevated ship plume OH resulted in the net photochemical production of O 3 , with peak concentrations being 5-65% higher than background values, depending on latitude. The areal integrated O 3 effect, however, is estimated to be quite small due to further plume dilution. In addition, because of the shorter estimated lifetime for NO x , it would seem reasonable that the integrated O 3 production from the current Lagrangian modeling effort would be significantly lower than that predicted by global 3-D grid models. The current predicted shortened lifetime for NO x is quite significant in terms of assessing a ship plume's impact on background marine levels of NO x . In fact, these results would seem to explain a significant fraction of the overprediction of NO x levels in and near shipping lanes recently estimated using 3-D Eulerian global models.

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[1] The Atmospheric Brown Cloud-East Asia Regional Experiment (ABC-EAREX) was conducted under the UNEP/ABC-Asia project to intercompare the aerosol and gas measurements in springtime from various instruments from late February to April 2005 at the Gosan Supersite on Jeju Island, Korea. Satellite instruments such as the Moderate Resolution Imaging Spectroradiometer (MODIS) and the Ozone Monitoring Instrument (OMI) provide a large-scale regional view of the aerosol during the ABC-EAREX period. This study shows the temporal and spatial distribution of four major aerosol types (dust, carbonaceous, sea salt and sulfate) retrieved by MODIS-OMI Algorithm and FourChannel Algorithm utilizing data from MODIS and OMI over east Asia during the ABC-EAREX campaign. Results from two different retrieval show that a complexity of aerosol types and sources exist over east Asia: Some aerosols are emitted while others are transported. Nevertheless, the results show reasonable consistency in the distribution according to aerosol type. The agreement of aerosol type classification for each aerosol type ranges from 32% to 81% depending on the type. These results were compared with the results from a three-dimensional aerosol transport radiation model, SPRINTARS. Dust type aerosol is usually found to be mixed with carbonaceous type aerosol. It implies that the dust type aerosol is loaded and transported with polluted air mass. The evidence that polluted air masses in the continent can be transported long distance is also captured; that is, sea salt type mixed with the sulfate aerosol is detected over a remote ocean.

[1] In this paper, aerosol composition and size distributions in east Asia are simulated using a comprehensive chemical transport model. Three-dimensional aerosol simulations for the TRACE-P and ACE-Asia periods are performed and used to help interpret actual observations. The regional chemical transport model, STEM-2K3, which includes the on-line gas-aerosol thermodynamic module SCAPE II, and explicitly considers chemical aging of dust, is used in the analysis. The model is found to represent many of the important observed features. The Asian outflow during March and April of 2001 is heavily polluted with high aerosol loadings. Under conditions of low dust loading, SO 2 condensation and gas phase ammonia distribution determine the nitrate size and gas-aerosol distributions along air mass trajectories, a situation that is analyzed in detail for two TRACE-P flights. Dust is predicted to alter the partitioning of the semivolatile components between the gas and aerosol phases as well as the size distributions of the secondary aerosol constituents. Calcium in the dust affects the gas-aerosol equilibrium by shifting the equilibrium balance to an anion-limited status, which benefits the uptake of sulfate and nitrate, but reduces the amount of aerosol ammonium. Surface reactions on dust provide an additional mechanism to produce aerosol nitrate and sulfate. The size distribution of dust is shown to be a critical factor in determining the size distribution of secondary aerosols. As much of the dust mass is found in the supermicron mode (70-90%), appreciable amounts of sulfate and nitrate are found in the supermicron particles. For sulfate the observations and the analysis indicate that 10-30% of sulfate is in the supermicron fraction during dust events; in the case of nitrate, more than 80% is found in the supermicron fraction.