| Secondary organic aerosol(SOA)is one of the most important pollutants in atmospheric aerosols,accounting for 75%of the total concentration of PM2.5,which also is one of the primary substances for haze formation.Dicarboxylic acids and related compounds are a ubiquitous water-soluble component of SOAs,having low vapour pressure and high solubility can act as cloud condensation nuclei(CCN),affecting atmospheric radiation force and regional even global climate change.Shandong province is one of the most polluted areas in China.However,studies of dicarboxylic acids and related compounds were rarely conducted in background regions in Shandong province.In recent years,although some conclusions about formation mechanisms of dicarboxylic acids and related compounds have been reported in the world,there are no direct chemical reaction pathways to support these findings.For these essential factors,field measurement was performed with offline detection technology in Mountain Tai(Mt.Tai)in Shandong province.Molecular distributions of dicarboxylic acids and related compounds were analysed,and their formation mechanisms were investigated using multiphase chemical model.In addition,contributions and sources of biogenic and anthropogenic hydrocarbons to secondary organic aerosols at Mt.Tai in 2014 were identified.Dicarboxylic acids and related compounds in PM2.5 were measured at Mt.Tai in summer 2014.The total concentration of dicarboxylic acids and related compounds was 1043 ng m-3.C2,C4 and C3 were the most abundant species with mean concentration of 523,124 and 82.3 ng m-3,respectively,and their sum concentration contributed 70%to dicarboxylic acids and related compounds.Their concentration level is lower than that in urban sites in China,which reflects the serious secondary organic aerosol pollution at Mt.Tai.Almost four-fifths of day-/night-time concentrations ratios of dicarboxylic acids and related compounds ranged from 0.9-1.1,suggesting that most dicarboxylic acids and related compounds had similar average concentrations between the day and night.Correlation analysis of dicarboxylic acids and related compounds,their gas precursors-VOCs,the correlation between C2 and sulfate show that precursor emissions and aqueous phase oxidations probably play an important role.Biomass burning impact was remarkable only in the first half sampling period.Average concentrations of dicarboxylic acids and related compounds in the first half period were about two times higher than those in the second half period.PCA results showed anthropogenic activities followed by photochemical processing was the dominant source for dicarboxylic acids and related compounds at Mt.Tai.To identify multiphase chemistry formation mechanisms and key gas phase precursors of dicarboxylic acids and related compounds,in this study,we performed using air parcel model SPACCIM(SPectral Aerosol Cloud Chemistry Interaction Model).Modeled concentrations of radical oxidant,non-radical oxidant and dicarboxylic acids and related compounds indicate that SPACCIM model can reasonably predict oxidant budget and concentrations of dicarboxylic acids and related compounds at Mt.Tai.A net formation flux of C2 is mainly observed in non-cloud periods,while a net degradation is mainly found in the day coming under non-cloud periods.The most important sources of C2 in the aqueous phase under cloud conditions are aqueous oxidation of CHOCOO-during the day and night.In contrast to the cloud conditions,the C2 formations in day/night deliquescent particles are dominated by aqueous oxidation of hydrated glyoxylic acid.The main sinks of C2 under day cloud condition were iron-oxalate complexes photolysis.On the other hand,the aqueous reaction of oxalate with NO3 and oxalate monoanion with NO3 was the most important sink during the night.Under non-cloud conditions,the sinks of C2 in the night dominated by the oxalate monoanion reaction with NO3,whereas in the day,the sinks of C2 are only significant in the day coming and dominant by photolysis of iron-oxalate complexes.The modeled source and sink fluxes of glyoxylic acid show a net formation flux mainly during cloud conditions,as well as a net degradation during the non-cloud periods.In the cloud conditions,the aqueous reactions of hydrated glyoxal represent the main formation pathway of glyoxylic acid.Both during the day/night non-cloud conditions,the most important sources are the reactions of hydrated glyoxal and gas phase oxidant of 3-methyl-6-nitrocatechol.Under cloudy conditions,the day ?C2 sinks are dominated by aqueous reaction between hydrated glyoxylate with OH,while in the night,they are an aqueous reaction between hydrated glyoxylate with NO3,OH.Different from the cloud conditions,the reaction of gas phase ?C2 photolysis,gas phase reaction of ?C2 with OH and aqueous phase reaction of hydrated ?C2 with OH radical act as critical sinks for ?C2 in the day non-cloud conditions.Under nighttime deliquescent particle conditions,the sinks of?C2 were low and can be ignored.Sensitivity studies indicate that modelled concentrations of selected dicarboxylic acids and related compounds are strongly sensitive to emission data.Alkenes and aromatics are the critical gas phase precursors for C2 and ?C2,among them,isoprene and toluene are the most important species.To understand contributions and source identification of biogenic and anthropogenic VOCs to secondary organic aerosols at Mt.Tai,using the SOA tracer and parameterisation method,we estimated the contributions from biogenic and anthropogenic VOCs,respectively.The results showed that the average concentrationof biogenic SOA was 1.08?g m-3,among which isoprene SOA tracers were dominant.The anthropogenic VOC-derived SOA was 7.03 ?g m-3 and 1.92 ?g m-3 under low-and high-NOx conditions,respectively,and aromatics made the most significant contribution.However,the sum of biogenic and anthropogenic SOA only contributed 18.1-49.1%of the total SOA.Source apportionment by positive matrix factorisation(PMF)revealed that secondary oxidation and biomass burning were the primary sources of biogenic SOA tracers.Anthropogenic aromatics mainly came from solvent use,fuel and plastics combustion and vehicular emissions.However,for>C6 alkanes and cycloalkanes,vehicular emissions and fuel and plastics combustion were the most significant contributors.The potential source contribution function(PSCF)identified the Bohai Sea Region(BSR)as the major source area for organic aerosol compounds and VOC species at Mt.Tai.This paper studied pollution characteristics and formation mechanisms of dicarboxylic acids and related compounds at background site-Mt.Tai and Tuoji Island at Shandong province.The contributions and sources of biogenic and anthropogenic VOCs to SOA were identified at Mt.Tai.These provide data support and scientific basis for comprehending the SOA pollution situation,and theoretical basis for making control strategies of SOA pollution. |
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