Characterization of submicron particles influenced by mixed biogenic and anthropogenic emissions using high-resolution aerosol mass spectrometry: Results from CARES
An Aerodyne high resolution time-of-flight aerosol mass spectrometer\n(HR-ToF-AMS) was deployed during the Carbonaceous Aerosols and Radiative\nEffects Study (CARES) that took place in northern California in June\n2010. We present results obtained at Cool (denoted as the T1 site of the\nproject) in the foothills of the Sierra Nevada Mountains, where intense\nbiogenic emissions are periodically mixed with urban outflow transported\nby daytime southwesterly winds from the Sacramento metropolitan area.\nDuring this study, the average mass loading of submicrometer particles\n(PM1) was 3.0 mu g m(-3), dominated by organics (80 %) and sulfate (9.9\n%). The organic aerosol (OA) had a nominal formula of\nC1H1.38N0.004O0.44, thus an average organic mass-to-carbon (OM/OC) ratio\nof 1.70. Two distinct oxygenated OA factors were identified via Positive\nmatrix factorization (PMF) of the high-resolution mass spectra of\norganics. The more oxidized MO-OOA (O/C = 0.54) was interpreted as a\nsurrogate for secondary OA (SOA) influenced by biogenic emissions\nwhereas the less oxidized LO-OOA (O/C=0.42) was found to represent SOA\nformed in photochemically processed urban emissions. LO-OOA correlated\nstrongly with ozone and MO-OOA correlated well with two 1st generation\nisoprene oxidation products (methacrolein and methyl vinyl ketone),\nindicating that both SOAs were relatively fresh. A hydrocarbon like OA\n(HOA) factor was also identified, representing primary emissions mainly\ndue to local traffic. On average, SOA (= MO-OOA + LO-OOA) accounted for\n91% of the total OA mass and 72% of the PM1 mass observed at Cool.\nTwenty three periods of urban plumes from T0 (Sacramento) to T1 (Cool)\nwere identified using the Weather Research and Forecasting model coupled\nwith Chemistry (WRF-Chem). The average PM1 mass loading was considerably\nhigher in urban plumes than in air masses dominated by biogenic SOA. The\nchange in OA mass relative to CO (Delta OA/Delta CO) varied in the range\nof 5-196 mu g m(-3) ppm(-1), reflecting large variability in SOA\nproduction. The highest Delta OA/Delta CO was reached when air masses\nwere dominated by anthropogenic emissions in the presence of a high\nconcentration of biogenic volatile organic compounds (BVOCs). This\nratio, which was 97 mu g m(-3) ppm(-1) on average, was much higher than\nwhen urban plumes arrived in a low BVOC environment (similar to 36 mu g\nm(-3) ppm(-1)) or during other periods dominated by biogenic SOA (35 mu\ng m(-3) ppm(-1)). These results demonstrate that SOA formation is\nenhanced when anthropogenic emissions interact with biogenic precursors.