Study On The Characteristics Of PM_2.5 Acidity Based On The High Temporal Resolution Observations

Rapid economic development and urbanization in recent years have resulted in serious air pollution in many cities in China, especially particulate matter pollution. Haze events happen frequently. Acidic aerosol is widely observed in the atmosphere, and plays an important role in acidity-dependent heterogeneous atmospheric processes, acid deposition, climate change, and health effects. Therefore, great importance has been attached to the study on acidity of particulate matter. Although high temporal resolution observations have been applied to the study on the temporal variations in the chemical composition and concentration of PM2.5, the study on PM2.5 acidity based on the high temporal resolution observations is still relatively scarce.Hourly measurements of PM2.5 ionic compositions were conducted by a Monitor for AeRosols and Gases in Ambient air (MARGA) at Shanghai Fudan University, Shanghai Baoshan and Hangzhou sampling sites. Combing with the meteorological factors and gaseous precursors in the atmosphere, the temporal variations in the concentrations of the water soluble ions, the PM2.5 acidity and its causes and influencing factors, and the speciation of sulfate and nitrate were studied in detail. At the same time, compared with the situation in the non-haze events, the characteristics of PM2.5 acididy in haze events were explored. This study would improve understanding of atmospheric aerosol acidity and the formation of particulate nitrate in acidic PM2.5, and has important implications for the controlling of aerosol pollution and the protection of human health.The major results were summarized as follows:1. At the Fudan sampling site, the total water-soluble ions (TWSI) showed obvious seasonal variation:winter> spring> summer> autumn. The secondary inorganic ions including SO42=, NO3= and NH4 were the dominant component in TWSI, accounting for 85.09%of TWSI. NH4+ was the main substance for the neutralization of SO42= and NO3=. The main sources of the pollutants were the stationary sources during the observation, but the contribution of mobile sources is increasing. Further simulations using Extended Aerosol Inorganic Model IV (E-AIM 4) showed that the average value of in situ PM2.5 pH was 1.90, indicating that the PM2.5 particles during this period are highly acidic. In situ acidity showed dirunal variations and was mainly affected by RH and H2O in the particulate pase. Further analysis showed that particulate SO42- and NO3= in the spring, autumn and winter mainly existed in the form of NH4HSO4 and NH4NO3, while particulate SO42- and NO3- in the summer mainly existed in the form of H2SO4, NH4NO3 and a small amount of NH4HSO4.2. Three haze events were observed during the observation period. The fine particle pollution at the Baoshan site was more serious than that at Fudan site. The PM2.5 concentration in haze days was 1.96 times higher than that in non-haze days. The secondary inorganic ions in the haze days contributed more to the TWSI than that in the non-haze days. Due to Baoshan’s own characteristics of the air pollution, the PM2.5 aerosol acidity in the haze days was less acidic than that in the non-haze days. Furthermore, the intercept of the regression line with the axis of [NH4+]/[SO42-] was 1.68, indicating that the excess NH4+ became available for NO3- formation after neutralizing most of the SO42- and HSO4- when [NH44]/[SO42-]≥1.68. When [NH4+]/[SO42-]<1.68, [H+]/[SO42-] and [H+]T/SO42-] molar ratios rapidly decreased, indicating that free hydrogen ions [H+]1 in the particle was rapidly neutralized by gaseous ammonia ([H-]+NH3â†'NH-); when [NH4+]/[SO42-]≥1.68, the neutralization of free hydrogen ions by ammonium was almost stopped. The haze events during the observation mainly originated from the secondary particukte matter pollution,and the SO42- and NO3- were mainly produced through the conversion SO2 and NO2 from their precursors, and the sulfur and nitrogen oxidation rate (SOR and NOR) in the haze days is greater than that in the non-haze days.3. At the Hangzhou sampling site, the results showed the average measured total particle acidity ([H+]T) was 124.2 (24.7-360.6) nmol/m3 for the period of July 12-16, 2011. Further simulations using E-AIM 4 revealed that the in situ particle pH ranged from 0.86 to 2.89, with an average at 1.85, indicating the PM2.5 particles in Hangzhou during this period are highly acidic. The in situ particle pH showed a well-defined diurnal profile. The amount of particle liquid water content can significantly influence in situ particle acidity and the concentrations of bisulfate ions. Further analysis indicated that the sulfate aerosol phase was mainly composed of H2SO4, with some NH4HSO4 (or NH4+ and HSO4- solution).4. At the three sampling sites, the results indicated that most of NO3- coupled with excess NH44- in PM2.5 except the NH4+ bound to sulfate during the sampling periods, which indicated ammonium nitrate formatioa However, there were still some of the excess NH4+ bounded to species other than NO3-, implying that some of excess NH4+ might couple with Cl- or organic anions, or some heterogeneous reactions resulting in the formation of nitrate occurred without involving NH3.