RESUMO
Tropospheric ozone (O3) is mainly produced through a series of photochemical reactions of nitrogen oxides (NOx) and volatile organic compounds (VOCs). The reaction process presents complex non-linear relationships. In this work, datasets of atmospheric ozone and volatile organic compounds (VOCs) observed during the summer of 2018 in Nanjing were used. Combining with the framework for 0-D atmospheric model-master chemical mechanism (F0AM-MCM), the characteristics of photochemical reactions for ozone (O3) formation in Nanjing during the O3 episode days and non-episode days were investigated. The results showed that φ(O3) and φ(TVOCs) in the O3 episode days were 47.8×10-9 and 49.0×10-9, respectively, exceeding those in the non-episode days by factors of 1.8 and 1.6. Furthermore, F0AM, the empirical kinetic modeling approach (EKMA), and relative incremental reactivity (RIR) were utilized for the calculation of ozone chemical sensitivity. It was found that O3 formation in Nanjing was attributed to both VOCs and NOx limitation. In addition, the modeled ·OH and HO2 concentrations in the O3 episode days were 1.3 and 1.8 times higher than those in the non-episode days. The higher formation and loss rates of ·OH and HO2 were also found during O3 episode days. These findings reflected that the enhancements of atmospheric oxidation capacity resulted in increased production rates of O3, providing an explanation for the enhancements of O3 concentrations in Nanjing during the O3 episode days. The findings also improved the understanding of the O3 photochemical characteristics over Nanjing in the summer during the O3 episode days.
RESUMO
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%).
