Characterization and deposition of aerosol organic matter in the eastern United States

Aerosol organic carbon (OC) was characterized in two eastern United States watersheds to investigate the potential importance of aerosol OC in watershed OC budgets arid cycling. Fluxes of 1.7 and 2.1 mg aerosol OC m· dwere measured for aerosol samples in Harcum, VA and Millbrook, NY, respectively. Scaled to the area of nearby watersheds (York River watershed, VA and Hudson River watershed, NY) these fluxes are similar in magnitude or greater than the magnitude of riverine OC exported by corresponding rivers indicating that aerosols must be taken into account when thinking about biogeochemical exchanges between the atmospheric, terrestrial, and aquatic realms. Fossilfuel and contemporary biomass emissions are the major sources of aerosol total OC (TOC) to the eastern United States, and radiocarbon signatures were used to estimate the relative contributions from these two sources. On average 33% of aerosol TOC could be attributed to fossil sources throughout the year with mean seasonal fossil TOC contributions (11% to 57% fossil) revealing significant heterogeneity in the relative magnitude of anthropogenic fossil and contemporary biomass TOC sources throughout the year. The 33% fossil aerosol TOC corresponds to a human-derived, 50% increase in aerosol TOC delivered to watersheds and aquatic systems above pre-industrial levels. The effects of such an increase in the delivery of TOC to watersheds are unknown and warrant further investigation. Further radiocarbon analyses on aerosol TOC sub-fractions showed the watersoluble component of aerosol OC (WSOC) to contain significantly more contemporaryaged OC than either bulk aerosol OC or its water-insoluble components. These differences represent a fundamental partitioning in the solubility of fossil and contemporary-derived aerosol OC, and its potential post-depositional fate in watersheds and soils. Fossil OC remains in the less bioavailable particulate phase and its transport is dependent on the erodibility of particles. Molecular characterization of WSOC revealed it to be a highly complex mixture of thousands of compounds including organosulfur compounds not previously recognized to be quantitatively important in the atmosphere. Several elemental formulas consistent with previously identified secondary organic aerosol compounds were present in high abundance providing evidence for the importance of the production of aerosol WSOC via atmospheric processing. Black carbon was present at levels within WSOC that could not explain the 12-14% fossil contributions to WSOC observed in radiocarbon analyses suggesting that some other water-soluble compounds must account for the fossil OC. Collectively, the characterization of the amounts and isotopic and molecular characteristics of aerosol OC presented here provide a foundation on which future studies of the inputs and fates of aerosol OC within watersheds and aquatic systems may be based. Significant quantities ofboth fossil and contemporary-derived OC are delivered to watersheds representing a potentially important allochthonous source of OC to aquatic systems that should be considered in future studies.