Temporal variations and source apportionment of Hulis-C in PM2.5 in urban Shanghai

Humic-like substances (Hulis), the hydrophobic part of water-soluble organic compounds, have been recognized to play important roles in environmental behavior of PM2.5 in atmosphere. In this study, Hulis and other components of PM2.5, collected in urban Shanghai from September 2013 to August 2014 were analyzed. The annual average concentration of Hulis carbon (Hulis-C) was (2.61±2.58) μg/m(3), accounting for about 50% of water-soluble organic carbon (WSOC). The monthly average Hulis-C concentration peaked in December while the lowest was in summer, coinciding with the high and low of levoglucosan and secondary inorganic aerosol (SIA). Biomass burning and secondary formation were both important sources of Hulis-C, and their contributions showed obvious seasonality. In late autumn and winter, the strong inter-correlations among Hulis-C concentrations, NO3(-)/SO4(2-) mass ratios and nitrogen oxidation ratios (NOR) were found, suggesting the atmospheric oxidation of NOx to nitrate and related gas-phase reactions may be inter-linked with Hulis-C formation. In summer, photochemical reaction was clearly the major source of Hulis-C. The investigation by backward trajectory analysis showed that the long-range transport from the Northwest brought large amounts of PM2.5 and enhanced Hulis-C and levoglucosan level, indicating biomass burning as a significant source of Hulis-C under this type of synoptic weather conditions. In comparison, Hulis formation associated with the NOx oxidation pathway was mainly associated with the short-range transport from the neighboring cities. Marine aerosol was relatively clean and had little effects on Hulis-C. The CWT (concentration weighted trajectory) model results suggested that the Yangtze River Delta (YRD) region was an important source region of Hulis-C, while the effect of some northwestern areas was not negligible.