Characteristics and Sources of Inorganic Ions and Organic Acids in Precipitation in the Northern Suburb of Nanjing, China

Abstract

Rainwater samples were collected in Nanjing from December 2016 to November 2017. Water-soluble ion and organic acid content in rainwater samples was determined to analyze the chemical characteristics of precipitation and their seasonality. The positive matrix factorization (PMF) model was employed to identify the potential sources of precipitation. The results show that the volume-weighted mean of pH in precipitation was 5.6, which was higher than the results of previous studies conducted in Nanjing. The volume-weighted mean of total ions was 297.3 μmol·L-1, and the concentrations of each species were in the order of NH4+ > Ca2+ > K+ > Na+ > Mg2+ for cations and NO3- > SO42- > Cl- > F- for anions. The volume-weighted mean of organic acids was 2.86 μmol·L-1, with organic acids accounting for 2.2% of the total anions. CHO2-, C2H3O2-, and C2O42- were the main organic acids in precipitation with annual volume-weighted means of 1.35, 1.05, and 0.26 μmol·L-1, respectively. A significant seasonality was observed for the ions and organic acids. The volume-weighted mean of inorganic ions was higher in winter and spring compared to those in summer and autumn. On the other hand, the volume-weighted mean of total organic acids was the highest in summer, followed by spring, and the lowest in winter. High concentrations of organic acids in the summer can be attributed to the biogenic emissions from plants. The ratio of formic and acetic (F/A) showed that organic acids mainly originated from primary emissions (e.g., biogenic emissions, combustion of organics, and traffic emissions) rather than atmospheric oxidation processes. Using the PMF model, we found that marine sources and secondary inorganic products (40.0%) were the predominant sources of inorganic ions and organic acids in precipitation, followed by burning of biomass (22.2%), continental origin and waste incineration (22.0%), secondary organic products (14.5%), and biological emissions along with their secondary products (1.3%).