Enhanced SOA formation from mixed anthropogenic and biogenic emissions during the CARES campaign

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, NO_x) 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., ΔOA/ΔCO), 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 2.2 μg m⁻³ and near zero ΔOA/ΔCO. Those influenced predominantly by anthropogenic emissions had median OA concentrations of 4.7 μg m⁻³ and ΔOA/ΔCO ratios of 35–44 μg m⁻³ ppmv. But, when biogenic and anthropogenic emissions mixed, OA levels were enhanced, with median OA concentrations of 11.4 μg m⁻³ and ΔOA/ΔCO ratios of 77–157 μg m⁻³ ppmv. Taken together, our observations show that production of OA was enhanced when anthropogenic emissions from Sacramento mixed with isoprene-rich air from the foothills. After considering several anthropogenic/biogenic interaction mechanisms, we conclude that NO_x concentrations play a strong role in enhancing SOA formation from isoprene, though the chemical mechanism for the enhancement remains unclear. 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 more complex representations of NO_x-dependent SOA mechanisms and yields into their algorithms. Ultimately, accurately predicting OA mass concentrations and their effect on radiation balance will require a mechanistically-based treatment of the interactions of biogenic and anthropogenic emissions.