Connecting oxidative potential with organic carbon molecule composition and source-specific apportionment in PM2.5 in Xi'an, China

Epidemiological studies have suggested that exposure to excessive amounts of particle-bound reactive oxygen species (ROS) pollution can lead to a range of health problems. To identify the chemical composition and sources of particulate matter of diameter <2.5 μm (PM2.5)-bound ROS, high-time-resolution PM2.5 samples were collected in Xi'an, China in the periods July 24 to August 5, 2017 (summer), and January 1 to 6, 2017 (winter). Dithiothreitol (DTT) assay was used to measure the oxidative potential of PM2.5-bound ROS. The associations of six types of molecular compositions with ROS were evaluated, and source-specific contributions to ROS were quantified. The results indicated greater activity and diurnal fluctuations in PM2.5-bound ROS in winter than in summer. The mass-normalised DTT consumption rate (DTTm) reached a peak at 8:00 a.m. in summer for polluted and nonpolluted days. In winter, levels of ROS were higher during the day than night, regardless of PM2.5 pollution level. In summer, CHNO and CHOS had the greatest effect on DTTm on polluted days and CHO and CHNOS had the greatest effect on DTTm on nonpolluted days. On polluted days in winter, all molecules (CHO, CHNO, CHOS, CHNOS, CHN and CHNS) investigated in this study had limited effects on DTTm. DTTm on nonpolluted days was mainly affected by CHNO and CHNS. CHNO and CHNOS molecules from secondary reactions favoured ROS production. Results from source apportionment based on positive matrix factorization showed that in summer, vehicle emission was the primary contributor to ROS generation (41.3%), followed by secondary formation (33.7%), fugitive dust (18.6%), and coal combustion (6.3%), whereas in winter, biomass burning and secondary formation were the primary contributors (>58%) to ROS. Our work helps clarify the mechanisms of ROS formation and contributes to theoretical research on PM2.5 toxicity.