Unresolved complex mixture (UCM) accounts for a substantial fraction of particulate organic matters and plays an important role in forming secondary organic aerosol (SOA), yet their abundance, sources and atmospheric processes are not well understood. In this study, filter-based ambient PM _2.5 samples were collected concourrently at an urban site and a rural site in the Pearl River Delta (PRD) region, south China, to characterize semi-volatile UCM. Dust, urban tunnel exhaust, ship exhaust and biomass burning samples were also collected to characterize UCM from typical primary emission sources. No obvious UCM humps were found in total ion chromatograms (TIC) of the PM _2.5 samples collected during July–October, while the determined UCM reached 6.51 ± 4.92 μg/m ³ at the urban site and 6.75 ± 4.78 μg/m ³ at the rural site during November–May, accounting for ~9% of PM _2.5 mass at both sites. The missing UCM humps in the hot months were due to much large fraction of these semi-volatiles partitioning to gas phase and their much faster atmospheric oxidation. In addition, the lower organic matter (OM) was also a non-negligible factor contributing to less particulate UCM in summer. Five major sources for PM _2.5-bound UCM were identified by positive matrix factorization (PMF) involving organic and inorganic molecular source tracers. Vehicle exhaust accounted for 44.4% and 30.3% of UCM at the urban and the rural site, respectively. Biomass burning contributed more to UCM at the rural site (28.0%) than at the urban site (19.2%). Ship emission was found to contribute substantially to UCM (28.9% at the urban and 17.3% at the rural site) in the PRD harbor megacity. Coal combustion and dust altogether contributed much less at the urban site (7.5%) than at the rural site (24.4%).