Formation Mechanism And Radiative Effects Of Secondary Organic Aerosols Over China

Secondary organic aerosol(SOA)is generated from the condensation of volatile organic compounds(VOC)oxidation products,and is one of the most abundant fine particles.The precursors of SOA are in great variety,and its formation mechanism remains unclear.The climate effects of SOA are stronger than coarse particles and SOA can serve as cloud condensation nuclei(CCN)efficiently.Organic aerosols may become the most important fine particles in the future because of the reduction of sulfate and nitrate.Thus,it is necessary to study the formation mechanism,the spatial and temporal distribution,and the climte effects of SOA.In this thesis,improvements are made to a box model CAABA/MECCA-3.0 and a regional climate model RegCM4.Gas and aqueous phase reactions of aldehydes have been included in the box model.A SOA module called volatile basis set(VBS)is implemented in the regional model.The optical parameters calculations,the CCN parameterization,and the precipitation auto-conversion parameterization have been improved.The main results are as follows:SOA is formed from both gas and aqueous reactions.With the modified box model,the aqueous phase reactions of glyoxal,methylglyoxal,and glycolaldehyde,which are the oxidation products of VOCs are studied.According to the sensitivity study,liquid water content,relative humidity,and temperature are the primary factors on influencing SOA aqueous phase generation.Isoprene,toluene and xylene are three major contributors to aqueous phase SOA formation,the yields are 2.5%,0.7%,and 0.5%,respectively.With the anthropogenic and biomass burning emission in 2006 and an off-line biogenic emission(MEGAN-MACC),the simulation is conducted for year 2006,using the modified RegCM4.The results indicate that SOA concentration decrease with increased vapor pressure.Year-averaged anthropogenic SOA(ASOA)is distributed mainly in Sichuan Basin,Hubei,Hunan,Guizhou and Guangxi provinces,and biogenic SOA(BSOA)is distributed mainly in Yunnan,Guizhou,Hunan,Jiangxi,Guangxi and Guangdong provinces.SOA are distributed mainly over North China in Summer,and over South China in Winter.The ASOA concentration is higher in Autumn and Winter than Spring and Summer.As for BSOA,the concentration is higher in Summer and Autumn.The total SOA surface concentrations are 5.50,4.31,4.05 and 5.95 ?g m-3 in Winter,Spring,Summer,and Autumn,respectively.Aromatic and ?-pinene VOCs are the main contributors to ASOA and BSOA.The results of process analysis suggest that chemical reactions,dry deposition and vertical turbulence are the three most important processes of SOA generation.The optical parameters of toluene and ?-pinene SOA retrieved from laboratory are used to represent ASOA and BSOA optical parameters.Direct RF sensitivity to optical parameters is studied.For Direct RF at top of atmosphere(TOA),it will increase by 156%and 161%for ASOA and BSOA by using POA optical parameters from Global Aerosol Data Set(GADS)instead of toluene and ?-pinene SOA optical parameters.If the POA is treated as purely scattering,the RF will increase by 203%and 204%.The TOA RF will increase by 62%and 49%for ASOA and BSOA by using bimodal log-normal size distribution instead of single modal in optical parameters calculation.The RF is not sensitive to hygroscopic parameters change.The simulated AOD distribution is consistent with aerosol column burden,which is concentrated in Sichan Basin,Jiangxi and Hunan provinces.The clear sky TOA and surface RF are as high as-4 and-10 W m-2 in some parts of China,and the domain-averaged RF are-1.55 and-3.46 W m-2.The averaged all sky TOA and surface RF are 0.09 and-2.16 W m-2,respectively.The seasonal change of clear sky TOA RF is consistent with column burden change,which is that the column burden in larger in Autumn and Winter than in Spring and Summer.Due to the direct RF,the averaged surface temperature will decrease by 0.07 K,and the surface precipitation flux decrease by 2.71%.For the first indirect RF,GI scheme and Qian scheme are used.The TOA and surface RF in GI scheme is as high as-24 and-16 W m-2 in Guizhou,Guangxi,Hunan,and Shandong provinces,and the averaged TOA and surface RF are-1 1.78 and-7.45 W m-2.For Qian scheme,the averaged RF are-9.56 and-5.51 W m-2,respectively.Due to the first indirect RF,the surface temperature decreases by 0.42 and 0.37 K,and the surface precipitation flux decreases by 12.30%and 7.59%for GI and Qian scheme.The first indirect RF in Spring and Summer is higher than that in Autumn and Winter.The total indirect radiative effects increase the surface temperature by 0.83K and decrease the precipitation flux by 3.69 × 10-6 kg m-2 s-2.Under the future RCP45 climate scenarios,the increased temperature and water vapor lead to increased ozone concentration in the north part of Yangzi River,and decreased ozone will decrease in the south part.According to the change of ozone,surface SOA concentration will increase in the north part of Yangzi River,and decrease in the south part.Averaged surface ASOA concentration will decrease by 2.6%and BSOA will decrease by 1.5%.Though the domain averaged values are small,the future climate has larger impact on SOA in regional scale.Under RCP26,RCP45,RCP60 and RCP85 emission scenarios,ASOA surface concentrations will decrease by 80%,60%,40%and 45%and BSOA will decrease by 21%,22%,11%and 12%,respectively.Under RCP26 emission scenarios,the reduction of POA emission decrease the SOA concentration by 6%,and the reduction of NOx emission decrease the SOA concentration by 2%and 11%for ASOA and BSOA.The production and the use of solvent is the biggest contributor to SOA concentration change in the future.Above all,the modified regional climate model has a good performance in SOA modeling,especially in OC seasonality.The aqueous phase production of SOA is smaller than the gas phase production.In China,ASOA has a larger contribution than BSOA.The direct and first indirect effects cool the surface,increase the atmosphere stability,and keep more moisture,thus decrease the precipitation.The second indirect effects produce a stronger longwave heating than solar cooling,thus increase the surface temperature.The indirect effects are stronger than the direct effects.Regardless of biogenic emission change,SOA will increase in the north part of Yangzi River,and decrease in the south part,because of the higher temperature and water vapor in future.SOA will decrease because of the reduction of VOC,POA,and NOx.