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Humic-like substances (HULIS), the hydrophobic part of water-soluble organiccarbon (WSOC), account for a significant fraction of PM2.5 mass. Theirsource studies are so far largely qualitative. In this study, HULIS and WSOCwere determined in 100 PM2.5 samples collected in 2009 at an urban site(Guangzhou) and a suburban site (Nansha) in the Pearl River Delta in SouthChina. The annual average concentration of HULIS was 4.83 and4.71 μg m−3, constituting 8.5 and 10.2% of the PM2.5 mass, whileHULIS-C (the carbon component of HULIS) contributed 48 and 57% ofWSOC at the two sites, respectively. HULIS were found to correlate withbiomass burning (BB) tracers (i.e., levoglucosan and K) and secondaryspecies (e.g., SO42− and NH4+), suggesting itsassociation with BB emissions and secondary formation processes. Sources ofHULIS were investigated using positive matrix factorization analysis ofPM2.5 chemical composition data, including major components andsource markers. In addition to secondary formation process and BB emissions,residual oil combustion related to shipping was identified for the firsttime as a significant source of HULIS. Secondary formation processcontributed the most, accounting for 49–82% of ambient HULIS at the twosites in different seasons. BB emissions contributed a seasonal average of8–28%, with more contributions observed in the winter months(November–February) due to crop residue burning during harvest season.Residual oil combustion was revealed to be an important source at thesuburban site in summer (44% of HULIS-C) due to its proximity to one ofthe ports and the shipping lane in the region. Vehicle emissions were foundto contribute little to HULIS, but had contributions to the hydrophilic WSOCfraction. The contrast in contributions from different combustion sources toHULIS and hydrophilic WSOC suggests that primary sources of HULIS are linkedto inefficient combustion. This source analysis suggests further study ofHULIS be focused on secondary formation process and source characteristicsof HULIS from BB and residual oil combustion

atmospheric chemistry and physics

Sources of humic-like substances in the Pearl River Delta, China: positive matrix factorization analysis of PM&amp;lt;sup&amp;gt;2.5&amp;lt;/sup&amp;gt; major components and source markers