Source and formation characteristics of water-soluble organic carbon in the anthropogenic-influenced Yellow River Delta, North China

High intensity measurement of water-soluble organic carbon (WSOC) in PM2.5 was conducted at Yellow River Delta (YRD), North China, from 29 May to 1 July 2013. On average, concentration of WSOC was 3.09 ± 2.45 μg m−3 with a relative high WSOC/OC mass ratio (56.39%), implying organic aerosol in YRD was aged. WSOC concentration in day time was obviously higher than night time, which was mainly attributed to the decrease of source emission. While secondary formation of WSOC was strengthen in night time under stable atmospheric condition. The significant relationship between WSOC and SOC indicated WSOC was mostly secondary formation product. Furthermore, WSOC formation was enhanced at high level of acidity, providing direct evidence for the great impact of aerosol acidity on WSOC formation. WSOC correlated well with nss-K+, nss-SO42−, NO3−, Zn and Cu, suggesting a major part of observed WSOC and/or its precursors was of biomass burning and fossil fuel combustion origin. Moreover, vehicle emission may make great proportion in the fossil fuel combustion. Conditional probability function (CPF) analysis showed significant contribution of WSOC occurred when wind came from southerly (135-195°) and northwesterly (285, 345°) directions. In order to further confirm the source of WSOC, two merged samples representing the two directions were selected for radiocarbon (14C) measurement. 14C results demonstrated the average value of ƒc(WSOC) was 0.57 ± 0.01, implying biogenic and biomass burning (B&B) was the major source of WSOC. However, fossil fuel contribution could not be ignored in North China in summer.