Observation Of Secondary Organic Aerosols In Different Regions Of China

Secondary organic aerosol(SOA)was produced by the reaction of volatile organic compounds(VOCs)with atmospheric oxidant,and formed through condensation onto and/or uptake by pre-existing particles.As the important component of fine particle,SOA contributed more than 50%of organic aerosol(OA),and even contributed up to70%of OA during the severe haze events in China.SOA could affect the global radiation balance,regional air quality and human health.At present,due to the lack of comprehensive understanding of the precursors source,chemical composition and formation mechanism of SOA,there were still large discrepancy between the results of model simulation and field observations,the composition and spatial-temporal distribution of SOA were also different in each model.Thus,the composition and spatial-temporal characteristics of SOA in different regions of China still need to be studied.SOA formation was affected by anthropogenic emissions,but how did the anthropogenic emissions affect the SOA formation under the polluted actual atmosphere was not clear.SOA tracers could trace the formation processes of SOA from different precursors and reveal the spatial-temporal evolution characteristics of SOA.Based on the observation results of SOA tracers in different regions of China,we could clarify the spatial-temporal characteristics of SOA and provide calibration data for model simulation.The research on the influencing factors of SOA formation in polluted areas could provide new understanding for the SOA formation mechanism,and provide scientific basis for air pollution control.Based on SOA tracers,this study found out the temporal and spatial distribution characteristics of typical biogenic and anthropogenic SOA through one-year network observation covering 12 sites in 6 regions of China.This study also revealed the impacts of anthropogenic emissions on the biogenic SOA formation through one-year continuous observation at 9 sites in the Pearl River Delta(PRD).In addition,through the one-year comparative observation of three typical urban sites in China,our work discussed the key mechanism of isoprene SOA formation in the polluted urban air.The main conclusions of this work were listed as follows:1.The annual average concentrations of isoprene,monoterpene and?-caryophyllene SOA tracers in 12 sites over China were in the range of 6.67-122 ng m^-3,9.80-49.0 ng m^-3and 1.72-7.72 ng m^-3 respectively,and the concentrations of them were higher in the Southern China.The concentrations of isoprene and monoterpene SOA tracers were mainly higher in summer,however,the unexpected increase of concentrations were also found in winter at Dunhuang and Hailun sites.And the concentration of?-caryophyllene SOA tracer was almost higher in winter.The unexpected increase of these biogenic SOA(BSOA)tracers concentrations in fall-winter were largely due to the elevated biomass burning emission.The annual average concentration of aromatic SOA tracer(2,3-Dihydroxy-4-oxopentanoic acid,DHOPA)ranged from 1.23 to 8.83 ng m^-3.The concentration of DHOPA in the Northern China was higher in urban sites,while in the Southern China,DHOPA was higher in rural sites.The concentrations of DHOPA at 12 sites were all significantly elevated in winter.The significant increase of DHOPA in the Northern China was mainly affected by biofuels and bulk coal combustion for heating,while that in the Southern China was mainly affected by open biomass burning.We estimate the concentrations of SOA in China by the“SOA-tracer approach”.The total annual average concentration of SOA in China was 2.49±1.09?g m^-3 and it was highest in Southwest China.The concentration of BSOA(1.66±0.79?g m^-3)was higher than aromatic SOA(0.87±0.49?g m^-3).The contribution of aromatic SOA was highest in fall-winter,and sesquiterpene SOA also had significant increase in fall-winter.In summer,isoprene SOA was the highest,while monoterpene SOA was the majority product in Spring.2.The annual average concentrations of BSOA tracers were in the range of 45.4to 109 ng m^-3 at Pearl River Delta(PRD)region,with the majority of products of monoterpenes(47.2±9.29 ng m^-3),then of the isoprene(23.1±10.8 ng m^-3)and?-caryophyllene(3.85±1.75 ng m^-3).Isoprene SOA formation pathway was influenced by NO_x emission and the ratio of 2-methylglyceric acid(High-NO_x product)to 2-methyltetrols(Low-NO_x products)(2-MGA/2-MTLs)could trace the effect of NO_x on the isoprene SOA formation pathways.Compared with the first generation products of monoterpene SOA,its later generation products were more significantly affected by the oxidant(O_x)and sulfate,resulting in more aging monoterpene SOA in the PRD.The unexpected increase of?-caryophyllene SOA tracer in winter was highly associated with the enhancement of biomass burning,O_x and sulfate component in the PRD.The annual average concentration of BSOA in PRD was 1.68±0.40?g m^-3,with the highest concentration in fall and the lowest concentration in spring.Monoterpene SOA dominated the BSOA mass all year round.Anthropogenic pollutants emission(such as the precursors of sulfate and O_x)had significant promotion on BSOA formation and it highlighted that we could reduce the BSOA through controlling on the anthropogenic emissions in polluted regions.3.At three typical urban sites in China,the annual average concentrations of isoprene SOA oxygen-containing tracers(iOTs)were in the range of 10.3-28.3 ng m^-3,isoprene organosulfates(iOSs)were in the range of 1.49-2.92 ng m^-3,high-NO_x SOA products were in the range of 3.75-6.04 ng m^-3 and low-NO_x SOA products were in the range of 10.8-31.7 ng m^-3.The concentrations of isoprene SOA products were highest at Kunming,followed by Hefei and lowest at Beijing,and higher levels of isoprene SOA products were found in summer-fall.The ratios of organosulfates to oxygen-containing tracers formed from isoprene(iOSs/iOTs)at three sites were higher in winter,lower in summer.The seasonal variation of iOSs/iOTs was affected by the reaction rate of isoprene epoxy intermediate reacting with sulfate and liquid water(k_{aq SO42-},k_aq,H2O).Isoprene was more likely to react with liquid water rather than sulfate.However,isoprene might also more likely to react with sulfate in winter at Beijing site.Low-NO_x channel was the dominant pathway for isoprene SOA formation in highly polluted urban atmosphere.The ratio of high NO_x to low NO_x SOA products of isoprene was higher in winter and lower in summer and the seasonal variation of the ratio was mainly affected by NO_x level and the reactive uptake rate of isoprene epoxy intermediate.