Formation Mechanisms Of Secondary Inorganic Aerosols In Atmosphere And Its Influence Factors

As one of the important components of PM2 5.the formation mechanism of secondary inorganic aerosol(SIA)is complex,and it is important to clarify its formation process and influencing factors in order to reduce the secondary inorganic formation of PM2.5.However,there is a lack of systematic and quantitative studies on the SIA reaction mechanism and influencing factors.In this study,the atmospheric multiphase chemical box model based on the gas phase(MCM v3.3.1)and aqueous phase chemical reaction mechanism(CAPRAM 2.4)was developed and applied to simulate sulphate(SO42-)and nitrate(NO3-)formation processes in the atmosphere.Based on the model,the sensitivity analysis methods of SO42-and NO3-in the atmosphere of Taiyuan,a key city in the Beijing-Tianjin-Hebei peripheral region for air pollution control,were developed,and the reaction mechanisms and influencing factors of SO42-and NO3-under different scenarios were quantitatively evaluated using the occurrence of COVID-19 as a special case.The main conclusions are as follows:1.The modelling results show that the production of SO42-in PM2 5 in this study in autumn and winter of 2017-2018 was dominated by the reaction mechanisms of gasphase reaction of SO2 with·OH(21.5%),aqueous-phase oxidation of NO2(33.5%)and aqueous-phase oxidation of transition metal ions(TMI)(31.6%).The dominant reaction mechanisms for SO42-formation differed between autumn and winter,with NO2 and ·OH oxidation dominating in autumn(38.5%and 19.9%respectively)and TMI and NO2 oxidation dominating in winter(36.2%and 31.8%respectively).Increased rates of TMI and NO2 oxidation were the main reasons for the increase in SO42-concentrations during heavy pollution.2.The contribution of gas-phase and aqueous-phase oxidation of NOx to NO3production was 47.5%and 52.5%,respectively,during the sampling period,with aqueous-phase oxidation dominating the NO3-production process when NO3concentrations were high,During the periods of heavy pollution,NO3-concentrations increased significantly due to the increase reaction rate of NOx aqueous phase oxidation(i.e.,hydrolysis of N2O5 and NO3 radicals)(the contribution of aqueous phase oxidation increased from 40.6%on clean days to 81.7%during periods of heavy pollution and above).3.Increasing aerosol water content(AWC)accelerated the rate of SO42-and NO3production.At low AWC concentrations(0-5.5 μg/m3),·OH gas-phase oxidation and TMI aqueous-phase oxidation were important for SO42-formation;with increasing AWC concentration.the contribution of NO2 oxidation and TMI oxidation to SO42formation gradually increased.The increase in AWC concentration led to a gradual increase in the relative contribution of the ·OH gas-phase reaction rate to NO3formation,with the highest contribution reaching 92.1%.The highest rates of SO42and NO3-production were found at aerosol pH of 1.6-4.2 and 4.6-5.2.At low pH,TMI and H2O2 oxidation play an important role in SO42-production,and with increasing pH,NO2 oxidation becomes the main mechanism for SO42-production.With increasing pH,the contribution of the gas phase reaction to NO3-production gradually increased and the contribution of the liquid phase reaction gradually decreased.4.Based on the box model,the sensitivity analysis methods of SO42-and NO3production rates to the influencing factors were developed,and the results showed that when SO2 or NOx concentrations were reduced by more than 60%alone or when the proportion of SO2 and NOx concentrations were reduced together by more than 70%,SO42-production rates could be reduced by more than 60%.When O3 or NOx concentrations are reduced by more than 70%alone or when O3 and NOx concentrations are reduced in synergy by more than 80%,the rate of NO3-production can be reduced by more than 60%.The combined reduction of SO2,NOx and O3 concentrations has a significant effect on the synergistic control of SO42-and NO3-.5.Decreased contribution of industrial processes(-5.4%),vehicle exhaust(-7.6%)and resuspended dust(-1.4%)to PM2.5 at the beginning of the lock period(24 January-15 February 2020)was found when compared to the pre-lock period(1 January-23 January 2020);in contrast,the contributions of fireworks burning&biomass(7.1%),secondary formation(6.2%),and coal combustion(1.1%)increased.The dominant mechanism of SO42-generation varied from aqueous phase to OH gas phase oxidation due to the occurrence of the COVID-19 lock,while the dominant mechanism of NO3generation was all aqueous phase oxidation.The reduced contribution of vehicle exhaust during the COVID-19 lock period diminished its role in SIA generation,demonstrating the effectiveness of vehicle exhaust emission control.