Size-resolved light-absorbing organic carbon and organic molecular markers in Nanjing, east China: Seasonal variations and sources.

Owing to the potential influence of light-absorbing organic carbon (OC), also termed "brown carbon" (BrC), on the planetary radiation budget, many studies have focused on its absorption in single-sized ranges of particulate matter (PM). However, the size distribution and organic tracer-based source apportionment of BrC absorption have not been extensively investigated. In this study, size-resolved PM samples were collected using multi-stage impactors from eastern Nanjing during each season in 2017. The light absorption of methanol-extractable OC at 365 nm (Abs, Mm) was determined using spectrophotometry, and a series of organic molecular markers (OMMs) was measured using a gas chromatography-mass spectrometer. Fine PM with an aerodynamic diameter <2.1 μm (PM) dominated Abs (79.8 ± 10.4%) of the total size ranges with maxima and minima in winter and summer, respectively. The distributions of Abs shifted to larger PM sizes from winter to spring and summer due to lower primary emissions and increased BrC chromophores in dust. Except for low-volatility (p* < 10 atm) polycyclic aromatic hydrocarbons (PAHs), the non-polar OMMs, including n-alkanes, PAHs, oxygenated PAHs, and steranes, showed a bimodal distribution pattern. Secondary products of biogenic precursors and biomass burning tracers presented a unimodal distribution peaking at 0.4-0.7 μm, while sugar alcohols and saccharides were enriched in coarse PM. Their seasonal variations in average concentrations reflected intense photochemical reactions in summer, more biomass burning emissions in winter, and stronger microbial activity in spring and summer. Positive matrix factorization was used for the source apportionment of Abs in fine and coarse PM samples. Biomass burning contributed an average of 53.9% to the Abs of PM extracts. The Abs of coarse PM extracts was associated with various dust-related sources where the aging processes of aerosol organics could occur.