Characteristics And Gas-to-Particle Partitioning Of Water-Soluble Organic Compounds In The Atmosphere

Water-soluble organic compounds（WSOC）are important components of fine particulate matter in the atmosphere,which significantly affect air quality,global climate change and human health.However,the sources,formation mechanisms and physicochemical properties of atmospheric WSOC are still elusive,thus our knowledegs on their atmospheric behaviors are very limited.As a typical developed city in the Yangtze River Delta,Shanghai has received special attention for its air pollutions due to many kinds of local emission sources.In this study,gas-to-particle partitioning process of atmospheric WSOC in Shanghai during winter and summer were studied for the first time through simultaneously determinining both gaesou WSOC（WSOCg）and particulate WSOC（WSOCp）with a high time-resolution.Spatial and temporal variation characteristics of WSOC on Chongming Island,which is a rual area of Shanghai,and Minghang District,which is an uran area of Shanghai,were investigated.The optical properties and sources of WSOCp in the two areas were quantitatively explored.In this study,a series of statistic methods such as linear regression analysis,random forest analysis and generalized additive model analysis were used to quantitatively analyze the controlling factors of gas-to-particle partitioning of WSOC in different types of haze events in Shanghai.By combining the observation on the data from the literatures and the analysis of thermodynamic S-curve of low molecular weight organic acids,the spatial distribution and historical trend of the relative contents of NH₃,nitrate,sulfate and WSOCp were analyzed in China on a national scale.Major conclusions of this study are summarized as follows:（1）The average mass concentrations of fine particulate matter(PM_2.5)in Chongming and Minhang of Shanghai are close,which are 41±34μg m^-3 and 39±29μg m^-3,respectively.The major water-soluble components in PM_2.5 showed obvious seasonal and daily variations.WSOCg and low molecular weight（LMW）organic acid concentrations on Chongming Island were higher in summer and lower in winter,while major inorganic salts(SNA=SO₄^2-+NO₃^-+NH₄⁺)and WSOCp showed an opposite seasonal variation trend.In contrast,SNA,gas-and particle-phase WSOC and LMW organic acid concentrations in Minhang were higher in winter and lower in summer.WSOCg in Chongming and Minhang showed a unimodal diurnal vartiation pattern,which is higher in daytime but lower in nighttime in both seasons.In Minhang,WSOCp showed a bimodal variation pattern in winter.（2）The partitioning coefficient of WSOC（Fp）during the high-RH periods（RH≥80%）in winter on Chongming Island was higher than that during the low-RH periods（RH<80%）.WSOCg in the dry periods dominantly partitioned to the organic phase,which was controlled mainly by organic matters concentration and temperature.On contrast,in the humid periods,WSOCg on Chongming Island during the observation campaign largely partitioned to the aerosol liquid water（ALW）phase,which was collaboratively controlled by aerosol liquid water content（ALWC）,pH and NH₃.We found that NH₃ increased ALWC and pH,thus promoting the aqueous partitioning of WSOC.In addition,NH₃ and its gas-to-particle partitioning promoted the absorption of WSOCg by forming carboxylate through acid-base neutralization and liquid-phase reaction.Additionally,we found that the relative abundances of WSOCp and NH₄NO₃ showed a strong linear correlation throughout China with a spatial distribution consistent with that of NH₃,further indicating that the ammonia-rich conditions in the country provided a favorable environment for the partitioning of WSOCg,thus enhancing the formation of secondary organic aerosols（SOA）.（3）The gas-to-particle partitioning coefficient（Fp）of WSOC during the nitrate-dominated（ND）episodes（0.36±0.04）was 40%higher than that during the sulfate-dominated（SD）episodes（0.25±0.05）.These results were largely driven by ALWC and pH during ND and SD episodes,respectively.Statistical model analysis further showed that during the campaign,pH of PM_2.5was regulated by the changes in concentrations of SO₄^2-and total ammonia,RH,and T,whereas ALWC was regulated by the coupling effect of total nitrate and RH.The thermodynamic model simulation for Fp and the trend analysis for PM_2.5 composition revealed that the gas-to-particle-phase partitioning of atmospheric WSOCs throughout China in the past 20 years has increased with the increase in the relative abundance of nitrate to sulfate.This indicates that the formation of SOA in China will be more efficient in the near future owing to the increasingly efficient partitioning of WSOC.（4）The optical characteristics of WSOC(Abs₃₆₅ and MAC₃₆₅)in the Chongming pollution events were higher at nighttime but lower during the daytime,while Abs₃₆₅and MAC₃₆₅ in Minhang pollution periods showed bimodal distribution with two peaks in the morning and evening rush hours.Source apportionment analysis showed that secondary formation of pollutants emitted from biomass burning（68%）was the main source of light-absorbing WSOC in Chongming and fossil fuel combustion of vehicle exhaust（75%）was the main source in Minhang in the two rears,respectively.This study also found that Abs₃₆₅ and MAC₃₆₅ on Chongming Island increased with the enhancement of gas-to-particle partitioning of WSOC during the high RH periods.Compared with that of Minhang area,the atmospheric environment of Chongming Island is favorable for the aqueous uptake of WSOC,which can promote the ammonia-related aqueous reaction due to the high RH,pH and ALWC,thus producing more WSOC with stronger light absorption ability.