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

During the National Aeronautics and Space Ad- ministration (NASA) Intercontinental Chemical Transport Experiment, Phase B (INTEX-B), in the spring of 2006, air- borne measurements were made in the United States Pacific Northwest of the major inorganic ions and the water-soluble organic carbon (WSOC) of submicron (PM 1 . 0 ) aerosol. An atmospheric trajectory (HYSPLIT) and a Lagrangian particle dispersion model (Flexpart) quantifying source contributions for carbon monoxide (CO) were used to segregate air masses into those of primarily Asian influence ( \u3e 75% Asian CO) or North American influence ( \u3e 75% North American CO). Of the measured compounds, fine particle mass mostly con- sisted of water-soluble organic carbon and sulfate, with me- dian sulfate and WSOC concentrations in two to four times higher, respectively, in North American air masses versus transported Asian air masses. The fraction of WSOC to sul- fate in transported Asian air masses was significantly lower than one at altitudes above 3 km due to depleted organic aerosol, opposite to what has been observed closer to Asia and in the northeastern United States, where organic compo- nents were at higher concentrations than sulfate in the free troposphere. The observations could be explained by loss of sulfate and organic aerosol by precipitation scavenging, with reformation of mainly sulfate during advection from Asia to North America. In contrast to free tropospheric mea- surements, for all air masses below approximately 2 km altitude median WSOC-sulfate ratios were consistently between one and two. WSOC sources were investigated by multivari- ate linear regression analyses of WSOC and volatile organic compounds (VOCs). In Asian air masses, of the WSOC variability that could be explained (49%), most was related to fossil fuel combustion VOCs, compared to North Amer- ican 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 en- countered 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