Semi-continuous aerosol carbon measurements: Addressing atmospheric processes of local and global concern

Time-resolved ambient particulate organic (OC) and elemental carbon (EC) data measured in Atlanta, GA during the 1999 Atlanta Supersite Experiment and over the Northwestern Pacific Ocean during ACE-Asia were investigated to determine the temporal and spatial trends of atmospheric carbonaceous aerosol affecting local to global-scale processes, to examine the relative contributions of primary and secondary OC to measured particulate OC, and to evaluate the state-of-science for time-resolved aerosol carbon measurements.; Mean 1-hr average concentrations (ranges in parentheses) of PM2.5 OC, EC, and total carbon in the summertime urban polluted area of Atlanta during the Atlanta study were 8.3 (3.6–5.8), 2.3 (0.3–9.6), and 10.6 (4.6–24.6) μgC m−3, respectively, based on Rutgers University/OGI in-situ thermal-optical carbon analyzer measurements. In contrast, background OC and EC concentrations over the Pacific Ocean during ACE-Asia were 0.21 and 0.09 μgC m−3, respectively. The highest total carbon concentration during ACE-Asia (5.3 μgC m−3) was measured over the East Sea (Sea of Japan) and was comparable to the lowest summertime concentrations in Atlanta. The low OC/EC ratio of 2.5 over the background Pacific Ocean relative to 3.7 in polluted marine environments near East Asia coupled with backward trajectory results suggests a longer lifetime for EC relative to OC.; Carbonaceous matter (organic material 40%; EC 8%) comprised about 48% of PM2.5 mass in Atlanta. Secondary OC was estimated to contribute approximately 46% of measured particulate OC with 1-hr average contributions between 24 and 88%. Distinctive diurnal and weekly patterns of PM2.5 EC concentrations in Atlanta that are similar to traffic patterns in the area and the close tracking of EC and estimated primary OC concentrations to changes in CO concentrations indicate that vehicle emissions were the dominant contributor to measured EC and primary OC concentrations. Secondary OC concentrations in Atlanta were influenced by “fresh” secondary organic aerosol formed by photochemical reactions locally in the early afternoons as seen in the Los Angeles air basin and “aged” secondary organic aerosol transported from upwind regions. The vertical transport of upper air to ground level and the favorable partitioning of semi-volatile organic compounds to the particulate phase driven by temperature decreases and relative humidity increases at night are the most probable explanation for observed nocturnal jumps of secondary OC and ozone concentrations.; The good performance of the in-situ thermal-optical carbon analyzer in urban and remote environments and the much better agreement among several semi-continuous methods in Atlanta than among integrated methods suggest that time-resolved particulate OC and EC measurements provide more reliable particulate OC and EC data in the atmosphere than integrated filter sampling.