Seasonal variability of VOCs in Nanjing, Yangtze River delta: Implications for emission sources and photochemistry

Online measurements of volatile organic compounds (VOCs) during the entire year of 2016 were conducted at an urban site in Nanjing, Jiangsu Province of China. Seasonal variation characteristics in ambient VOCs levels, ratios, and sources were then analyzed to investigate which factors controlled seasonal cycles of VOCs levels during 2016. Impact of photochemistry was evaluated based on monthly average concentrations of OH radical ([OH]) determined by ratios of ethene versus acetylene and o-xylene versus ethylbenzene. [OH] exhibited higher values during summer and lower values during winter, similar with the seasonality of ozone concentrations and ultraviolet radiation, indicating the existence of photochemical influence on VOCs. Further variance analysis on VOCs levels suggested that seasonality of photochemistry played a minor role in explaining VOCs seasonal cycles. Monthly changes in VOCs sources were then analyzed using the Positive Matrix Factorization (PMF) receptor model. Seven factors were identified, including two factors related to traffic emission and other combustion processes (i.e. vehicular exhaust + combustion processes, gasoline evaporation + vehicular exhaust), two factors related to petrochemical industry (i.e. petrochemical industry#1-propane and petrochemical industry#2-propene), two factors attributed to paints and solvents use (ie. paints and solvents use#1-toluene, paints and solvents use#2-C8 aromatics), and a factor related to biogenic emission. Gasoline evaporation + vehicular exhaust and biogenic emission both exhibited higher relative contributions during summertime. Meanwhile, relative contributions of petrochemical industry and vehicular exhaust + combustion processes showed maximum during winter and minimum during summer. Relative contributions of two factors related to paints and solvents use showed higher values in March April and August September. Conditional probability function (CPF) analysis identified the influence of transport on seasonal changes in VOC source contributions. Finally, the PMF results of this study were compared with recent publications. The discrepancy of VOCs sources reported by different studies implied that there is still large uncertainty in our knowledge on VOCs emissions in Nanjing, and therefore further research on evaluation of VOCs emission inventories and source apportionment is needed in future.