Investigating the sources and atmospheric processing of fine particles from Asia and the Northwestern United States measured during INTEX B

Abstract. During the National Aeronautics and Space Administration (NASA) Intercontinental Chemical Transport Experiment, Phase B (INTEX-B), in the spring of 2006, airborne measurements were made in the United States Pacific Northwest of the major inorganic ions and the water-soluble organic carbon (WSOC) of submicron (PM1.0) aerosol. An atmospheric trajectory (Hysplit) and a Lagrangian particle dispersion model (Flexpart) quantifying source contributions for carbon monoxide (CO) was used to segregate air masses into those of primarily Asian influence (>75% Asian CO) or North American influence (>75% North American CO). Of the measured compounds, fine particle mass mostly consisted of water-soluble organic carbon and sulfate, with highest median WSOC and sulfate concentrations in North American air masses. The fraction of WSOC to sulfate was significantly lower than one at altitudes above 3 km, opposite to what has been observed closer to Asia and in the northeastern United States, where organic components were at higher concentrations than sulfate in the free troposphere. The observations could be explained by loss of sulfate and organic aerosol due to precipitation scavenging, with reformation of mainly sulfate during advection from Asia to North America. WSOC sources were investigated by multivariate linear regression analyses of WSOC and volatile organic compounds (VOCs). In Asian air masses, of the WSOC variability that could be explained (49%), most were related to fossil fuel combustion VOCs, compared to North American air masses, where 75% of the WSOC variability was explained through a nearly equal combination of fossil fuel combustion and biogenic VOCs. Distinct WSOC plumes encountered during the experiment were also studied. A plume observed near the California Central Valley at 0.6 km altitude was related to both fossil fuel combustion and biogenic VOCs. Another Central Valley plume observed over Nevada at 3 to 5 km, in a region of cloud detrainment, was mostly related to biogenic VOCs.