Extended analysis of the CARES aerosol chemistry data to characterize sources and processes of organic aerosol in the Sacramento Valley of California

This report explores the characteristics, sources, and processes of submicrometer particles in northern California via integrated analyses of atmospheric observation data obtained during the Carbonaceous Aerosols and Radiative Effects Study (CARES) that took place in northern California in June 2010. We focus on reporting aerosol chemistry, physical properties, and diurnal and temporal variations at Cool (denoted as the T1 site of the project) at the foothills of the Sierra Nevada Mountains, where intense biogenic emissions are periodically mixed with urban outflow transported by daytime southwesterly winds from the Sacramento metropolitan area. During CARES, the average mass loading of submicrometer particles (PM1) was 3.0 μg m , dominated by organics (80%) and sulfate (9.9%). The organic aerosol (OA) had a nominal formula of C1H1.38N0. 004O0.44, thus an average organic mass-to-carbon (OM/OC) ratio of 1.70. Three OA types were identified by PMF analysis of the high resolution mass spectra: two different OOA components (90% of total organics) and a HOA (10%). The more oxidized MOOOA (O/C = 0.54) was identified as biogenically influenced SOA, while the less oxidized LOOOA (O/C = 0.42) corresponded to anthropogenically influenced SOA (e.g., from the Sacramento area). The HOA factor corresponded mainly to primary emissions from local traffic. On average, SOA (= MO-OOA + LO-OOA) accounted for 91% of the total OA mass and 72% of the PM1 mass observed at Cool. Twenty three periods of urban plumes from T0 (Sacramento) to T1 (Cool) were identified using the Weather Research and Forecasting model coupled with Chemistry (WRF-Chem). The average PM1 mass loading was considerably higher in urban plumes than in air masses dominated by biogenic SOA. The change in OA mass relative to CO (OA/CO) varied in the range of 5-196 μg/m/ppm, reflecting large variability in SOA production. The highest OA/CO was reached when air masses were dominated by anthropogenic emissions in the presence of a high concentration of biogenic volatile organic compounds (BVOCs). This ratio, which was 97 μg/m/ppm on average, was much higher than when urban plumes arrived in a low BVOC environment (~36 μg/m/ppm) or during other periods dominated by biogenic SOA (35 μg/m/ppm). These results demonstrate that SOA formation is enhanced when anthropogenic emissions interact with biogenic precursors. During CARES, regional new particle events (NPE) were observed on most days over the Sacramento and western Sierra Foothills area. Simultaneous particle measurements at both the T0 (Sacramento, urban site) and the T1 (Cool, rural site located ~40 km northeast of Sacramento) sites of CARES indicate that the NPE usually occurred in the morning with the appearance of an ultrafine mode centered at ~15 nm (in mobility diameter, Dm, measured by a scanning mobility particle sizer operating in the range 10-858 nm) followed by the growth of this mode to ~50 nm in the afternoon. These events were generally associated with southwesterly winds bringing urban plumes from Sacramento to the T1 site. The growth rate was on average higher at T0 (7.1±2.7 nm/hr) than at T1 (6.2±2.5 nm/hr), likely due to stronger anthropogenic influences at T0. Using a high-resolution time-of-flight aerosol mass spectrometer (HR-ToFAMS), we investigated the evolution of the size-resolved chemical composition of new particles at T1. Our results indicate that the growth of new particles was driven primarily by the condensation of oxygenated organic species and, to a lesser extent, ammonium sulfate. New particles appear to be fully neutralized during growth, consistent with high NH3 concentration in the region. Nitrogen-containing organic ions (i.e., CHN, CH4N, C2H3N, and C2H4N) that are indicative of the presence of alkyl-amine species in submicrometer particles enhanced significantly during the NPE days, suggesting that amines might have played a role in these