Pollution Characteristics Of VOCs In Urban And Rural Areas Of Shanghai,and Their Influence On SOA Formation

Volatile organic compounds（VOCs）are one of the most important air pollutants in China,which can be formed into secondary organic aerosol（SOA）through a series of complicated photochemical reactions.Although the research on non-methane hydrocarbons（NMHCs）has been intensively performed in China in the past years,the research on oxygenated VOCs（OVOCs）is still lacking.Recently,many studies have reported that the PM_2.5 pollution in China has been effectively mitigated due to the strict implementation of various emission reduction strategies.Besides,these results also showed that the relative contribution of SOA to particulate matter pollution has been increasing in the country,and SOA has become a driver of haze pollution in China.However,the formation process of SOA is still unclear.To clarify the pollution characteristics of the major NMHCs and OVOCs in the Chinese atmosphere,as well as their influence on SOA formation,this study investigated the pollution characteristics of VOCs,and the chemical compositions in PM_2.5 in the urban and rural atmosphere of Shanghai in summer and winter by combining the online and offline measurements.Seasonality and diurnal variations in concentrations,compositions,sources of VOC,and formation mechanisms of SOA in the urban and rural areas of Shanghai were explored.Based on the above results,major conclusions derived from this study are summarized as follows:（1）Concentrations of total measured VOCs,NMHCs,and OVOCs in Minhang,which is a representative of the urban area of Shanghai,were 25±16 ppbv,1.6±0.62ppbv,and 22±13 ppbv in summer,respectively.Compared to those in summer,total VOCs and OVOCs in winter were slightly lower while the NMHCs in the urban area during winter was 3.6 times higher.In summer total measured VOCs,NMHCs,and OVOCs on Chongming Island,which is a representative of rural areas of Shanghai,were 18±11 ppbv,3.6±1.6 ppbv,and 13±5.9 ppbv,respectively,while compared to those in summer the total measured VOCs,NMHCs,and OVOCs on Chongming Island in winter increased by 1.9 times,4.2 times,and 1.4 times,respectively.Such a different seasonality is most likely due to the differences in VOC emission sources between the urban and rural areas.NMHC concentrations in both urban and rural areas decreased during the day in winter and summer.The concentration of daytime OVOCs in the urban and rural areas showed a unimodal pattern in summer and a bimodal pattern in winter,respectively.Aromatics in the two areas mostly originated from motor vehicle exhaust emissions in both seasons,whereas all the measured OVOCs in the two areas during the summer originated mostly from non-anthropogenic sources.In the urban area during winter acetaldehyde,formic acid,and methylglyoxal（MGLY）were largely derived from non-anthropogenic sources while acetone,acetic acid and MEK were largely derived from anthropogenic sources.In contrast,in the rural area the wintertime OVOCs were largely derived from non-anthropogenic sources.（2）During the 2019 China International Import Exhibition（CIIE）event,all the aromatic measured in CIIE were reduced by 32%～35%compared to those after the CIIE period,resulting in 40%and 10%decreases in the concentrations of their gaseous ring-retaining and ring-breaking products,respectively.Except for acetone,the concentrations of non-anthropogenic OVOCs were reduced by 18%to 38%during the CIIE in comparison with those after the CIIE.（3）Based on the wintertime observation on Chongming Island,we found that the oxidation degree of NMHCs under the real atmospheric conditions was determined by the atmospheric oxidation capacity,and the formation of gas-phase oxidation products was proportional to the concentration ratio of NMHCs to NOx.Furthermore,by combining the online VOC observation and the offline filter sample analysis,we found that in the rural area of Shanghai under high NOx conditions,which is corresponding to a NO₂ threshold of 25 ppbv and NOx threshold of 35 ppbv,the oxidation channel of aromatics in winter shifted from the bicyclic RO₂ oxidative pathway to the phenolic oxidative pathway.（4）The summer and winter secondary organic carbon（SOC）concentrations were2.7±1.5·g m^-3and 5.5±1.7·g m^-3 at the Minhang urban site,while those on Chongming Island were 3.3±2.4·g m^-3 and 3.9±2.4·g m^-3,respectively.The percentages of SOC in organic carbon（OC）were about 50%in different seasons in both urban and rural areas.The summer SOA in Minhang and Chongming Island during the observation period was mainly formed in the gas phase,in which both the atmospheric oxidation capacity and NOx are responsible for the SOA formation.In contrast,in addition to the atmospheric oxidation capacity and NOx,the aerosol water content and meteorological conditions also took a nonnegligible role in the formation process of SOA in winter.（5）The SOA in winter on Chongming Island can be categorized into gas SOA and aqSOA.We found that the gas SOA was formed mostly by the oxidation of aromatics and NOx under low relative humidity（RH<80%）conditions,and its concentration was determined by the concentration of aromatics during the day but by the NOx level at night.The aqSOA was formed largely under high relative humidity（RH>80%）conditions.Water-soluble OVOCs from photochemical reactions are the main precursors of aqSOA during the day,while VOCs directly emitted from biomass combustion are the main precursors of the aqSOA at night.Under high NOx conditions,gas SOA can further be converted to aqSOA.In summary,it was discovered that VOCs have significant spatial and temporal characteristics in Shanghai with OVOC concentrations much higher than NMHC,and both aromatics and non-anthropogenic OVOCs responded quickly to the pollution control measures during the CIIE event.During daytime aromatics are the precursors of gas SOA,while OVOCs are the precursors of aqSOA.The gas SOA can be converted to aqSOA due to the change in the oxidation pathway of aromatics under high NOx conditions.Therefore,further strengthening the reduction of NOx along with the synergistic reduction of VOCs and NOx can reduce the type of SOA pollution and effectively simplify the treatment of aerosol pollution.