In order to evaluate the pollution characterization of PM2.5 (particles with aerodynamic diameters less than or equal to 2.5 μm) and secondary aerosol formation under the different photochemical activity levels, CO was used as a tracer for primary aerosol, and hourly maximum of O3 (O3,max) was used as an index for photochemical activity. Results showed that under the different photochemical activity levels of L, M, LH and H, the mass concentration of PM2.5 were 29.8 ± 17.4, 32.9 ± 20.4, 39.4 ± 19.1 and 42.2 ± 18.9 μg/m3, respectively. The diurnal patterns of PM2.5 were similar under the photochemical activity and they increased along with the strengthening of photochemical activity. Especially, the ratios of estimated secondary aerosol to the observed PM2.5 were more than 58.6% at any hour under the photochemical activity levels of LH and H. The measured chemical composition included water soluble inorganic ions, organic carbon (OC), and element carbon (EC), which accounted for 73.5 ± 14.9%, 70.3 ± 24.9%, 72.0 ± 21.9%, and 65.8 ± 21.2% in PM2.5 under the photochemical activities of L, M, LH, and H, respectively. Furthermore, the sulfate (SO42-) and nitrate (NO3-) were nearly neutralized by ammonium (NH4+) with the regression slope of 0.71, 0.77, 0.77, and 0.75 between [NH4+] and 2[SO42-] + [NO3-]. The chemical composition of PM2.5 was mainly composed of SO42-, NO3-, NH4+ and secondary organic carbon (SOC), indicating that the formation of secondary aerosols significantly contributed to the increase in PM2.5. The formation mechanism of sulfate in PM2.5 was the gas-phase oxidation of SO2 to H2SO4. Photochemical production of nitric acid was intense during daytime, but particulate nitrate concentration was low in the afternoon due to high temperature.
Keywords: atmospheric fine particles; photochemical reaction; secondary aerosol.