Assessment of the global impact of aerosols on tropospheric oxidants

Abstract

[1] We present here a fully coupled global aerosol and chemistry model for the troposphere. The model is used to assess the interactions between aerosols and chemical oxidants in the troposphere, including (1) the conversion from gas-phase oxidants into the condensed phase during the formation of aerosols, (2) the heterogeneous reactions occurring on the surface of aerosols, and (3) the effect of aerosols on ultraviolet radiation and photolysis rates. The present study uses the global three-dimensional chemical/ transport model, Model for Ozone and Related Chemical Tracers, version 2 (MOZART-2), in which aerosols are coupled with the model. The model accounts for the presence of sulfate, soot, primary organic carbon, ammonium nitrate, secondary organic carbon, sea salt, and mineral dust particles. The simulated global distributions of the aerosols are analyzed and evaluated using satellite measurements (Moderate-Resolution Imaging Spectroradiometer (MODIS)) and surface measurements. The results suggest that in northern continental regions the tropospheric aerosol loading is highest in Europe, North America, and east Asia. Sulfate, organic carbon, black carbon, and ammonium nitrate are major contributions for the high aerosol loading in these regions. Aerosol loading is also high in the Amazon and in Africa. In these areas the aerosols consist primarily of organic carbon and black carbon. Over the southern high-latitude ocean (around 60°S), high concentrations of sea-salt aerosol are predicted. The concentration of mineral dust is highest over the Sahara and, as a result of transport, spread out into adjacent regions. The model and MODIS show similar geographical distributions of aerosol particles. However, the model overestimates the sulfate and carbonaceous aerosol in the eastern United States, Europe, and east Asia. In the region where aerosol loading is high, aerosols have important impacts on tropospheric ozone and other oxidants. The model suggests that heterogeneous reactions of HO 2 and CH 2 O on sulfate have an important impact on HO x (OH + HO 2 ) concentrations, while the heterogeneous reaction of O 3 on soot has a minor effect on O 3 concentrations in the lower troposphere. The heterogeneous reactions on dust have very important impacts on HO x and O 3 in the region of dust mobilization, where the reduction of HO x and O 3 concentrations can reach a maximum of 30% and 20%, respectively, over the Sahara desert. Dust, organic carbon, black carbon, and sulfate aerosols have important impacts on photolysis rates. For example, the photodissociation frequencies of ozone and nitrogen dioxide are reduced by 20% at the surface in the Sahara, in the Amazon, and in eastern Asia, leading to 5-20% reduction in the concentration of HO x and to a few percent change in the O 3 abundance in these regions.

Trace gas and aerosol interactions in the fully coupled model of aerosol‐chemistry‐climate ECHAM5‐HAMMOZ: 1. Model description and insights from the spring 2001 TRACE‐P experiment

“…They found no significant change in the global O 3 burden, but significant regional variations in surface O 3 concentrations due to heterogeneous reactions and to the aerosol photochemical effect (e.g., O 3 decrease by 10 -15% over northeastern China and Yellow Sea and by 10-40% over India). Tie et al [2005] found a decrease in surface O 3 concentration ranging from 5 to 15% because of heterogeneous reactions of N 2 O 5 , HO 2 , O 3 , CH 2 O and HNO 3 on sulfate, soot and dust particles in polluted regions and ranging from 10 to 30% in dust source regions. The effect of heterogeneous reactions on dust on O 3 concentration remains largely uncertain.…”

“…Some papers also assessed the effect of aerosols on trace gases through the modification of photolysis rates. In general this effect on surface O 3 concentrations is found to be globally small [Bian and Zender, 2003;Tie et al, 2005], but can be significant in aerosol source regions such as eastern Asia, Europe, United States and Sahara [Tang et al, 2003;Martin et al, 2003]. Note that the comparison between these studies remains somewhat difficult as the representation of aerosols (e.g., bulk approach For relative humidity (RH) !…”

“…In general, those models include the same processes, but they differ substantially in terms of the aerosol parameterizations (resulting in varying aerosol number concentrations, size distributions, loads and spatial distributions), uptake coefficients, parameterizations of heterogeneous chemistry, etc. In particular, Dentener et al [1996], Bian and Zender [2003], Bauer et al [2004], Martin et al [2003], and Tie et al [2005] consider externally mixed particles while both internally and externally mixed particles are considered by Liao et al [2003Liao et al [ , 2004, and Liao and Seinfeld [2005]; the prognostic size distribution and mixing state of the aerosols is only described in HAM [Stier et al, 2005].…”

“…Again we suggest this is due to differences in the heterogeneous reaction scheme. In the work of Tie et al [2005], N 2 O 5 and HO 2 react on both sulfate and mineral dust aerosols with a constant uptake coefficient of 0.04 and 0.2, respectively. In our model HO 2 can react with the same uptake coefficient but only on wet aerosol particles, not on externally mixed dry mineral dust particles.…”

Exploring Possible Missing Sinks of Nitrate and Its Precursors in Current Air Quality Models —A Case Simulation in the Pearl River Delta, China, Using an Observation-Based Box Model

“…Aerosol Surface Area (SA) was calculated from the observed aerosol mass concentrations. We assumed that aerosols were spherical, and the mean radius and mass density of sulfate and soot were taken from Tie et al (2005) (sulfate: 0.24 µm, 1.7 g cm −3 ; soot: 0.04 µm, 1.0 g cm −3 ), with a lognormal distribution. Dust and sea salts were divided into 4 bins (0−1 m; 1−2.5 m, 2.5−5 m, 5−10 m), with mass density of 2.2 g cm −3 and 2.65 g cm −3 , respectively.…”