| Aerosols,as an important class of air pollutants,have important impacts on environmental quality,climate change and human health.Compared with primary aerosols,the sources and composition of secondary aerosols,especially secondary organic aerosols(SOA)are more complex.Biogenic volatile organic compounds(BVOCs)are the main contributors to global SOA,but their SOA formation potential could be affected by anthropogenic pollutants.A comprehensive understanding of such complex anthropogenic-biogenic interactions is still lacking,which further hinders the accurate assessment of SOA budget,as well as their climate and health effects from chemistry-climate models.Therefore,relying on more laboratory studies to clarify the response process and mechanisms of biogenic SOA formation to anthropogenic pollutants is a key part to assist the source analysis of atmospheric SOA,improve model performance,and finally formulate effective pollution control measures.In this research,typical terpenes(γ-terpinene,β-pinene,β-caryophyllene)were selected as representatives of BVOCs and typical inorganic gases(NOx,SO2,and NH3)were selected as representatives of anthropogenic pollutants.The effect of anthropogenic-biogenic interactions on SOA formation from terpene ozonolysis was studied.By characterizing key SOA formation parameters including aerosol size distribution,yields,chemical composition,and morphology using smog chamber experimental system,SOA formation process and mechanisms from terpene ozonolysis under single and complex pollution conditions were analyzed.The aim of this study is to deepen the understanding of anthropogenic-biogenic interactions.The main results are as follows:(1)Under NOx-involved anthropogenic-biogenic interactions,SOA formation from different terpene ozonolysis responses differently to NOx.Although the increase of NOx concentration decreased SOA yields of α-pinene ozonolysis,it improved SOA yields of βpinene and y-terpinene ozonolysis.At high NOx concentrations([terpene]0/[NOx]0≤ 5 ppbC ppb-1),SOA yields of γ-terpinenes and β-pinene were increased by more than 0.8 and 5 times,respectively,compared to yields from ozonolysis without NOx.It is suggested that the structure of terpenes,especially the number and position of unsaturated double bonds,affect the effect of NOx on SOA formation.Results from model simulation and mechanism analysis showed that the main way for NOx to influence SOA formation is to form NO3 radicals(NO3·)that could participate in the competitive oxidation of precursors.The oxidation rate of β-pinene by NO3·and SOA yields are higher than its ozonolysis,resulting in increases in SOA growth rate and yields when NOx involves.Organic nitrates derived from β-pinene are mainly formed by direct oxidation of exocyclic double bond by NO3·,while two endocyclic double bonds in yterpinene can be oxidized by NO3· and O3 simultaneously.This means the first-generation oxidation products are still unsaturated and can be further oxidized,forming second-generation multifunctional organic nitrates,which can partition into the particle-phase to increase SOA yields.However,the second-generation oxidation also resulted in the breakage of the carbon skeleton,making the enhancement effect of NOx on the SOA yields of β-pinene more significant.These results indicate that the modification of terpene oxidation pathways by NOx might make NOx play an important role in ONs and SOA formation.(2)Under SO2-involved anthropogenic-biogenic interactions,relative humidity(RH)modifies the role of SO2 in SOA formation from terpene ozonoIysis.When the concentration of SO2 maintained constant(110 ± 3 ppb),SOA yield first decreased(RH<-30%)and then increased with increasing RH(RH>～30%),while sulfuric acid production and SO2 consumption always increased with increasing RH.Kinetic and thermodynamic model simulations showed that changes in aerosol liquid water content,acidity,and SO2 oxidation pathways with RH may jointly result in the nonlinear dependence of SOA yields on RH.Under dry conditions(RH=10 ± 2%),stabilized Criegee intermediates(sCIs)formed during γterpinene ozonolysis can oxidize SO2 to sulfuric acid.The nucleation of sulfuric acid leads to the explosive growth of particle number concentrations.The newly formed particles would provide more surfaces for condensation of gas-phase oxidation products and promote acidcatalyzed heterogeneous reactions,leading to higher SOA yields.The increase in RH suppressed the oxidation of SO2 by sCIs and reduced aerosol acidity.These changes were not conducive to SOA formation.However,RH also increased aerosol liquid water content and the reactive uptake of SO2 into the particle-phase,making the direct reaction between hydroperoxides and SO2 become the main pathway that SO2 participating in SOA formation.Analysis of particle-phase composition using liquid chromatography-high resolution mass spectrometry showed that most organosulfates(OSs)formed under RH=45%conditions were more than five times more abundant than those formed under dry conditions.The reaction of peroxides with dissolved SO2 may be a potential pathway leading to the formation of OSs under high RH conditions.In particular,high oxidation state hydroperoxides formed by autoxidation of peroxy radicals could be precursors of highly oxidized OSs(HOOSs).This study is of great significance to further improve the understanding of HOOS formation mechanism in the atmosphere.(3)When complex pollutants are involved in anthropogenic-biogenic interactions,there are combined effects on SOA formation exist between pollutants.Unlike the promotion effect of one single pollutant,aerosol formation under complex pollution is more complex.When NO2 and SO2 coexisted,aerosol yields were higher than with either pollutant alone.This is mainly because that the nucleation of sulfuric acid could further promote the gasparticle partition of products from NO3·oxidation and particle-phase reactions,thereby leading to a synergistic effect of SO2 and NO2 on aerosol formation.In the coexistence of SO2 and NH3,NH3 weakened the enhancement effect of SO2 on aerosol yields and reduced particle acidity.They acted similarly in β-caryophyllene ozonolysis system and the addition of NH3 further promoted the formation of molecules with high double bond equivalency,oxygen-to-carbon ratios,but suppressed the relative contribution of high-molecular-weight oligomers.These results suggest that the neutralization of aerosol acidity by NH3 may be the main reason for the antagonistic effect of NH3 and SO2 in aerosol formation.In addition,the sulfate formed by SO2 oxidation could be the core of the aerosol,which can be wrapped by the organic shell from gasparticle partition and particle-phase reactions to change aerosol morphology.(4)Anthropogenic-biogenic interactions not only influence formation potential and physical properties of aerosols,but also change their chemical composition.Nitrogen-and sulfur-containing compounds are important markers for the occurrence of anthropogenicbiogenic interactions.In this study,some particulate nitrogen-and sulfur-containing components,including some organic nitrates(C10H15NO5,C10H17NO4,C10H17NO5),OSs(C10H18O5S,C10H16O6S,C10H20O7S,C10H16O7S,C10H18O7S,C15H24O7S)and nitrooxy OSs(e.g.,C10H17NO7S,C10H17NO8S,C10H15NO9S,C10H17NO10S)have the same molecular formula with those observed in ambient aerosols.Their identification in the present study suggests that NOx-and SO2-involved terpene ozonolysis could be their potential sources.The identification and formation mechanism investigation of these characterized molecules reveal their nocturnal formation channels.These results can provide important reference for the source analysis of nitrogen-and sulfur-containing compounds in atmospheric aerosols and the identification of anthropogenic-biogenic interactions. |
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