Abstract
Abstract. An Aerodyne high resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS) was deployed during the Carbonaceous Aerosols and Radiative Effects Study (CARES) that took place in northern California in June 2010. We present results obtained at Cool (denoted as the T1 site of the project) in 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 this study, the average mass loading of submicrometer particles (PM1) was 3.0 μg m−3, dominated by organics (80%) and sulfate (9.9%). The organic aerosol (OA) had a nominal formula of C1H1.38N0.004OM0.44, thus an average organic mass-to-carbon (OM/OC) ratio of 1.70. Two distinct oxygenated OA factors were identified via Positive matrix factorization (PMF) of the high-resolution mass spectra of organics. The more oxidized MO-OOA (O/C = 0.54) was interpreted as a surrogate for secondary OA (SOA) influenced by biogenic emissions whereas the less oxidized LO-OOA (O/C = 0.42) was found to represent SOA formed in photochemically processed urban emissions. LO-OOA correlated strongly with ozone and MO-OOA correlated well with two 1st generation isoprene oxidation products (methacrolein and methyl vinyl ketone), indicating that both SOAs were relatively fresh. A hydrocarbon like OA (HOA) factor was also identified, representing primary emissions mainly due to 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 (ΔOA/ΔCO) varied in the range of 5-196 μg m−3 ppm−1, reflecting large variability in SOA production. The highest ΔOA/ΔCO 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−3 ppm−1 on average, was much higher than when urban plumes arrived in a low BVOC environment (~36 μg m−3 ppm−1) or during other periods dominated by biogenic SOA (35 μg m−3 ppm−1). These results demonstrate that SOA formation is enhanced when anthropogenic emissions interact with biogenic precursors.
Highlights
Atmospheric aerosols significantly affect the Earth’s climate (IPCC, 2007), human health (Pope et al, 2009), the ecological balance (Mahowald, 2011), and visibility (Watson, 2002)
While forest fires may occur in the region during summer (Worton et al, 2011) we found no indication of wildfires based on the time series of Org 60 and Org 73 (C3H5O+2 ), two key tracers of biomass burning in AMS mass spectra (Alfarra et al, 2007; Aiken et al, 2010)
The average mass loading during the entire campaign was 3.0 μg m−3, with organics (80 % of the total PM1 mass) being the dominant component followed by sulfate (9.9 %), ammonium (4.5 %), nitrate (3.6 %), black carbon (1.6 %), and chloride (0.1 %)
Summary
Atmospheric aerosols significantly affect the Earth’s climate (IPCC, 2007), human health (Pope et al, 2009), the ecological balance (Mahowald, 2011), and visibility (Watson, 2002). Aerosols consist of a wide range of chemical compounds. Analyses of many datasets from around the world showed that organic species generally represent the dominant fraction and account for 20–90 % of the mass in submicron particles (Zhang et al, 2007a). Organic aerosols (OA) are classified into primary (POA) or secondary (SOA). POA refers to aerosols directly emitted by a source, such as fossil fuel combustion, biomass burning or food cooking, while SOA are generated by reactions of gaseous precursors. OA can be classified depending on their sources, e.g., biogenic or anthropogenic
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