Secondary formation and influencing factors of WSOC in PM2.5 over Urumqi, NW China

Exploring the secondary formation of water-soluble organic carbon (WSOC) and its influencing factors is critical to understand the impact of WSOC on climate change and air quality. In this study, PM2.5 samples were collected from Sep.2017 to Aug.2018 in Urumqi, Northwest China. These samples were analyzed for WSOC, organic and element carbon (OC, EC), and water-soluble ions. The levels of WSOC ranged from 0.57 to 19.7 mu g C/m3 with an annual mean of 6.08 +/- 3.50 mu g C/m3, accounting for 53% of OC. The observed ratios of WSOC to OC (0.35-0.77) were within the range of those for biomass burning and secondary organic aerosol (SOA). In addition, WSOC was positively correlated with the biomass burning tracer (K+) and secondary species (sulfate, nitrate, and ammo-nium). All of these observational evidences indicated that atmospheric WSOC over Urumqi were mainly from biomass burning and SOA. Based on a K-tracer approach, we estimated WSOC from biomass burning (WSOCBB) and SOA (WSOCsec). Our results suggested that WSOCsec was dominated over WSOCBB in four seasons with the highest level in winter (8.22 +/- 3.82 mu g C/m3) and the lowest in summer (2.91 +/- 1.63 mu g C/m3). The WSOCsec concentrations and WSOCsec fractions in OC were high under the conditions of high relative humidity, low temperature and wind speed. WSOCsec was positively correlated with liquid water content (LWC) but exhibited no dependence on aerosol acidity (in-situ pH). This indicated that the heterogeneous formation of WSOCsec in the air of Urumqi was largely associated with LWC rather than acidity.