LAUSR

Abstract To investigate chemical characteristics and sources of humic-like substances (HULIS) in the background region of Southwest China, daily PM2.5 samples were collected on Weizhou Island from 14 March 2015 to 14 April 2015. Water soluble inorganic ions, organic carbon (OC), elemental carbon (EC), water-soluble organic carbon (WSOC), HULIS, oxalate, levoglucosan and mannosan were analyzed. Average concentration of PM2.5 was 32.5 ± 12.6 μg/m3 with SO42−, OC and NH4+ as the three most abundant components. Average concentration of HULIS was 5.00 ± 2.21 μg/m3 with daily values ranged from 1.46 to 10.36μg/m3. The carbon content of HULIS (HULIS-C) contributed 35.6% ± 7.2% of OC and 76.8% ± 12.9% of WSOC, indicating HULIS-C as one of the most important species in OC and WSOC in this region. The concentration of HULIS correlated well with those of non-sea salt (nss) K+, levoglucosan, oxalate, OC, EC, WSOC, SOC and WSOC/OC, suggesting dominant source factors from both biomass burning and secondary aerosol formation. Contributions from crust and sea salt source factors to HULIS were minimal as indicated by its poor correlation with nss-Ca2+, Mg2+ and Na+. Research flight observations showed increasing biomass-burning indicators with increasing altitude, likely due to aerosols produced from biomass burning in Southeast Asia, which were then uplifted by deep convection to the middle troposphere and transported to this area. This hypothesis was supported by negative correlations between HULIS and sea level pressure when high-altitude air masses were transported from the Southeast Asian. Air mass backward trajectory analysis suggested Southeast Asian as the dominant source region for HULIS on Weizhou Island.