Carbon aerosol is a major fraction of the fine particulate matter in the atmosphere, contributing to air quality degradation and affecting climate change. Carbon aerosols are classified in two main fractions, organic carbon (OC) and elemental carbon (EC). Combustion of fossil fuels and biomass are the main sources of directly emitted carbon aerosols. In addition, OC can be produced in the atmosphere from the condensation of volatile organic compounds (VOCs). The dominant removal pathway of carbon aerosol from the atmosphere is wet deposition. The wet removal of OC is expected to vary seasonally, due to the variation of secondary organic aerosol (SOA) and its variation with temperature and emissions sources. The wet removal of EC is limited by its hydrophobic character; EC needs to undergo an “aging” process before removal by precipitation.

The concentration of OC and EC in precipitation was monitored from 2011 to 2013 at Bondville, IL. OC in precipitation was fractionated into soluble (DOC) and insoluble (WIOC) material. The concentration of carbonaceous aerosol in precipitation was complemented with the concentration of ions measured by the National Atmospheric Deposition Program (NADP) and the concentration of aerosol species in air measured by the Interagency Monitoring of Protected Visual Environments (IMPROVE). Results indicated that 95% of the total OC in precipitation was dissolved. The concentration of DOC was 1.9 times higher than the total molar concentration of sulfate, nitrate, and ammonium. DOC was positively correlated (R²=0.54) with the deficit of measured anions in precipitation, suggesting that measured DOC is predominantly represented by organic acids. DOC was positively correlated with all ions in precipitation but not with EC, indicating that non-combustion sources are mostly contributing to the DOC fraction. WIOC was positively correlated with all ions and carbon fractions in precipitation. EC in precipitation was only correlated with ammonium, sulfate, and WIOC.

The WIOC/EC ratio in precipitation was 14.6, about four times higher than the OC/EC ratio in air (3.8), suggesting than EC is poorly removed by precipitation as compared with OC. However, the DOC/EC ratio in precipitation was 470, two orders of magnitude higher than the ratio in air which can be caused by the scavenging of VOCs. The scavenging ratio (SR), defined as the ratio of the concentration in rain to that in air, was used as an indicator of the effectiveness of the wet removal process. The SR was found to vary seasonally and by constituent.