Resolving sources of water-soluble organic carbon in fine particulate matter measured at an urban site during winter

Abstract

Measurements of daily PM_2.5 were carried out during winter between January 11 and February 27, 2010 in an urban area of Korea, in order to better understand the influence of sources and atmospheric processing of organic aerosols. The aerosol samples were analyzed for organic carbon and elemental carbon (OC and EC), water-soluble OC (WSOC), eight ionic species, and oxalate. The water-soluble fraction of OC was between 33 and 58% with an average of 45%. Strong correlations among WSOC, sulfate (SO₄²⁻) (R² = 0.69), and oxalate (R² = 0.82) concentrations, and between potassium (K⁺) and WSOC concentrations (R² = 0.81) suggest that the observed WSOC could originate from similar oxidation processes to those for SO₄²⁻ and oxalate, as well as biomass burning. Also moderate correlations of the WSOC with EC and carbon monoxide (CO) indicate that there was some contribution to WSOC from primary fossil fuel combustion. Results from a principle component analysis (PCA) indicate that in addition to the biomass burning and primary non-biomass burning emissions, the observed WSOC could be formed through production pathways similar to secondary organic carbon (SOC), SO₄²⁻, and oxalate. Sources of WSOC inferred, based on the correlations, were confirmed by source categories identified by the PCA. Over the study period, three haze episodes exceeding a 24 h PM_2.5 concentration of 50 μg m⁻³ were identified. Of the major components in PM_2.5, EC concentrations were elevated during episode I (18–19 January), while the secondary SO₄²⁻ concentrations were enhanced during episodes II (30–31 January) and III (22–23 February). However, little difference in OC concentrations among the episodes was observed. It is suggested that the aerosol particles collected during episodes II and III were more aged than those during episode I. Estimates of fossil fuel combustion, biomass burning, and SOC contributions to WSOC indicate that the fossil fuel combustion provided the highest contribution (62.3%) to WSOC in episode I, while the greatest contribution (60.6%) to WSOC from SOC was observed in episode II. The results demonstrate that the sampled aerosol particles were more aged or further processed during episodes II and III than during episode I.