Higher contribution of coking sources to ozone formation potential from volatile organic compounds in summer in Taiyuan, China

Regional ozone pollution has become one of the most challenging environmental problems in China. In July 2019, hourly real-time monitoring of ozone (O3) and nitrogen oxides (NOx) and 3-hour off-line measurement of volatile organic compounds (VOCs) during a 10-day intensive campaign were conducted at four sites in Taiyuan, Shanxi Province, China. The average mixing ratio of total VOCs (including alkanes, alkenes, aromatics and acetylene) was 14.8 ± 2.8 ppbv and the dominant VOCs species to O3 formation were alkenes in Taiyuan. According to China Ambient Air Quality Standard Grade II (hourly averaged mixing ratio of 103 ppbv), the studied periods were divided into O3 attainment periods (EP1) and O3 pollution periods (EP2). A continuous O3 pollution event was captured during 12–15 July, with the maximum hourly O3 mixing ratio of 131.7 ppbv. Higher temperature, lower relative humidity, weaker winds and local photochemical reaction were conducive to O3 pollution during EP2. The analysis of VOCs/NOx ratio indicated that the formation of O3 was co-controlled by both VOCs and NOx during the period of 12:00–18:00 LT (high value period of O3), and it was controlled by VOCs during the remaining period. The abundances, compositions of typical VOCs and VOCs/NOx ratio showed clear spatial and temporal variations. Six major sources of VOCs were identified by positive matrix factorization, including coal and biomass combustion (33%), coking sources (28%), vehicular emissions (14%), solvent usage (10%), industrial processes (8%) and biological sources (7%). Backward trajectory analysis found that higher concentration of O3 in air masses from local (70%) and southern areas (22.5%) during EP2. Local (32%) and southern (30%) coking sources were the main contributors of ozone formation potential (OFP) during EP2.