Formation of secondary organic carbon and cloud impact on carbonaceous aerosols at Mount Tai, North China

Carbonaceous aerosols measured at Mount Tai in north China in 2007 were further examined to study the formation of secondary organic carbon (SOC) and the impact of clouds on carbonaceous species. A constrained EC-tracer method and a multiple regression model showed excellent agreement in estimating SOC concentration. The average concentrations of non-volatile and semi-volatile SOC (SOCNV and SOCSV) were 2.61, 5.58 μg m−3 in spring and 2.81, 10.44 μg m−3 in summer. The total SOC accounted for 57.3% and 71.2% of total organic carbon in spring and summer, respectively, indicating the presence of high loading of secondary organic aerosol (SOA) in the North China Plain. The fraction of SOCNV increased with photochemical age (as indicated by NOx/NOy ratios) of air mass, whereas SOCSV was also influenced by the dynamic equilibrium between formation and sink. Significant scavenging by clouds of non-volatile organic carbon (OCNV) and elemental carbon (EC) was observed, whereas semi-volatile organic carbon (OCSV) concentrations increased during clouds, suggesting substantial SOA formation through aqueous-phase reactions in clouds. A mass balance model was proposed to quantify the scavenging coefficients for OCNV, EC and formation rates of OCSV in clouds. The scavenging coefficient constant of EC (KEC) varied from 0.11 to 0.90 h−1, and was higher than that of OCNV (KNV-OC: 0.07–0.55 h−1), implying internal mixing of EC with more hygroscopic species. The formation rate constant (JSV-OC) and sink constant (SSV-OC) of OCSV ranged from 0.09 to 1.39 h−1 and 0.001 to 1.07 h−1, respectively. These field derived parameters could be incorporated into atmospheric models to help close the gap between predicted and observed SOA loadings in the atmosphere.