Secondary Aerosol Formation under Variable Relative Humidity and OH Exposure: Field Observations Using an Oxidation Flow Reactor in Bakersfield, California
YING ZHOU, Alexander B. MacDonald, Xuanlin Du, Roya Bahreini, Don Collins, University of California, Riverside
Abstract Number: 259
Working Group: Urban Aerosols
Abstract
Secondary aerosol (SA) contributes significantly to ambient PM2.5, yet its formation and aging mechanisms remain poorly understood, particularly via aqueous phase chemistry. This study used an oxidation flow reactor (OFR) and a mini aerosol mass spectrometer (mAMS) to investigate SA formation from filtered ambient air under varying OH exposure (OHexp) and relative humidity (RH) in Bakersfield, California. The OHexp ranged from 3.7 × 1010 to 8.9 × 1010 molec cm-3 s, equivalent to 0.29–0.69 days photochemical aging for a low OHexp condition, and 5.0–6.9 days for a high OHexp condition. RH was stepped between 40%, 85%, and 100% to create environments in the reactor with dry seed aerosol, aqueous seed aerosol, and cloud droplets, respectively. Relative to the study-average ambient secondary organic aerosol (SOA) concentration of 5.59 ± 4.56 µg m-3, we observed significant SOA production in the OFR, with average increases of 4.60 ± 3.71, 7.15 ± 4.76, 7.29 ± 7.86, 16.20 ± 10.60, 6.90 ± 5.05, and 22.59 ± 9.65 µg m-3 across six conditions of dry seed aerosol at low and high OHexp, aqueous seed aerosol at low and high OHexp, and cloud droplets at low and high OHexp, respectively. The SOA enhancement for each RH level was greater under the high OHexp condition, accompanied by a decrease in f43 and an increase in f44. These effects were most pronounced in the presence of aqueous seed aerosol and cloud droplets, underscoring the role of oxidative aging and aqueous phase processes in SOA formation. A persistent diurnal pattern in SOA formation was observed, especially for the high OHexp condition, with peaks in the early morning and late afternoon. These findings offer critical insight for improving chemical transport models and developing region-specific PM2.5 mitigation strategies, especially in agriculturally influenced and meteorologically complex regions like California’s Central Valley.