PNNL

Abstract

The CARES campaign was conducted during June, 2010 in the vicinity of Sacramento, California to study aerosol formation and aging in a region where anthropogenic and biogenic emissions regularly mix. Here, we describe measurements from an Aerodyne High Resolution Aerosol Mass Spectrometer (AMS), an Ionicon Proton Transfer Reaction Mass Spectrometer (PTR-MS), and trace gas detectors (CO, NO, NOx) deployed on the G-1 research aircraft to investigate ambient gas- and particle-phase chemical composition. AMS measurements showed that the particle phase is dominated by organic aerosol (OA) (85% on average) with smaller concentrations of sulfate (5%), nitrate (6%) and ammonium (3%) observed. PTR-MS data showed that isoprene dominated the biogenic volatile organic compound concentrations (BVOCs), with monoterpene concentrations generally below the detection limit. Using two different metrics, median OA concentrations and the slope of plots of OA vs. CO concentrations (i.e., ?OA/?CO), we contrast organic aerosol evolution on flight days with different prevailing meteorological conditions to elucidate the role of anthropogenic and biogenic emissions on OA formation. Airmasses influenced predominantly by biogenic emissions had median OA concentrations of 2.9 µg/m3 and near zero ?OA/?CO. Those influenced predominantly by anthropogenic emissions had median OA concentrations of 4.7 µg/m3 and ?OA/?CO ratios of 35 - 44 µg/m3ppmv. When biogenic and anthropogenic emissions mix, OA levels are dramatically enhanced with median OA concentrations of 11.4 µg/m3 and ?OA/?CO ratios of 77 - 157 µg/m3ppmv. Taken together, our observations show that production of OA is enhanced when anthropogenic emissions from Sacramento mix with isoprene-rich air from the foothills. A strong, non-linear dependence of SOA yield from isoprene is the mechanistic explanation for this enhancement most consistent with both the gas- and particle-phase data. If these observations are found to be robust in other seasons and in areas outside of Sacramento, regional and global aerosol modules will need to incorporate NOx-dependent SOA yields into their algorithms. Regardless of the mechanism, accurately predicting OA mass concentrations and their effect on radiation balance will require an accurate accounting of the interactions of biogenic and anthropogenic emissions.