The chemical properties and secondary components of PM2.5 were investigated in the city of Zhengzhou, China. Water-soluble ionic species (Na+, NH4+, K+, Mg2+, Ca2+, F–, Cl–, NO3– and SO42–) contents, carbonaceous components (organic carbon (OC) and elemental carbon (EC)) in PM2.5 were measured for three years. The EC tracer method was used to estimate the secondary organic carbon (SOC) content, and the Interagency Monitoring of Protected Visual Environments formula was used to estimate light extinction due to the chemical composition of PM2.5.
The annual mean concentrations of PM2.5 were 186, 180 and 218 µg m–3 in 2011, 2012 and 2013, respectively. These concentrations were 5–6 times greater than the National Ambient Air Quality Standards of China (annual value of 35 µg m–3) and indicated the presence of severe PM2.5 pollution in Zhengzhou. Particulate organic matter (OM) contributed the most (18–26%) to the annual average PM2.5, followed by SO42– (14–19%), NO3– (10–11%), NH4+ (8–9%) and EC (3%). From 2011 to 2013, the contributions of OM and SO42– increased by 8% and 3%, respectively. The higher sulfur oxidation ratio indicated the formation of significant amounts of secondary inorganic aerosols (SIA), particularly during the summer and spring. Obvious SOC enrichment occurred during the winter and autumn. In addition, SIA and secondary organic aerosols accounted for 26–50% and 4–21% of the PM2.5 by mass, respectively. An investigation of the secondary species revealed that secondary aerosols played a dominant role in the total PM2.5 mass and the decrease in visibility. The secondary aerosols ((NH4)2SO4 + NH4NO3 + SOC) accounted for 80% of bext. The main secondary aerosols that led to poor visibility in Zhengzhou were (NH4)2SO4 and NH4NO3.