Chemical characteristics of water-soluble organic compounds (WSOC) in PM2.5 in Beijing, China: 2011–2012

• Nitrate were higher in summer and autumn than in winter.
• Higher WSOC/OC ratio in summer suggests that OC was more aged.
• Negative correlation between WSOC and RH when humidity was higher than 80% in summer.
• SNA contributed more to PM2.5 in summer and OM contributed more in other seasons.
• The OC/EC minimum ratio method was not suitable for estimating SOC in winter.

PM2.5 filter sampling was conducted on a daily basis for one year from 2011 to 2012 at an urban site in Beijing. One-third of the samples were subjected to chemical analysis, including water-soluble inorganic ions, organic carbon (OC), elemental carbon (EC) and water-soluble organic carbon (WSOC). The results show that OM (Organic Matter = OC × 1.6; average 50.8% of PM2.5) was the most abundant species, followed by SNA (SO42 − + NO3− + NH4+; average 35%); however, SNA contributed more to PM2.5 in summer and OM contributed more in other seasons. The concentrations of nitrate were higher in summer and autumn than that in winter, suggesting that nitrate had formed in acidic and ammonium-poor environments in summer. WSOC accounted for 26.6% of OC and 8.1% of PM2.5 and exhibited distinct seasonal variations: the lowest concentration occurred in spring (3.90 ± 1.78 μg/m3) and the highest in autumn (5.82 ± 3.73 μg/m3). The higher WSOC/OC ratio in summer suggests that OC was more aged, oxidized and hygroscopic during this season. The seasonal trend of SOC (Secondary Organic Carbon) was the opposite of WSOC/OC, which may indicate that the OC/EC minimum ratio method was not suitable for estimating SOC in this study. Correlations between WSOC and K+, EC and inorganic ions indicated that WSOC was dominated by secondary formation, except that biomass burning was an important source in autumn. Aqueous chemical processes may play an important role in the formation of WSOC in winter. Meteorological conditions had an important influence on WSOC: positive correlations were observed between WSOC and relative humidity, but there was a negative correlation when humidity was higher than 80% in summer.