Enhanced SOA formation from mixed anthropogenic and biogenic emissions during the CARES campaign J. E. Shilling1, R. A. Zaveri1, J. D. Fast1, L. Kleinman2, M. L. Alexander3, M. R. Canagaratna4, E. Fortner4, J. M. Hubbe1, J. T. Jayne4, A. Sedlacek2, A. Setyan5, S. Springston2, D. R. Worsnop4, and Q. Zhang5 1Atmospheric Sciences and Global Change Division, Pacific Northwest National Laboratory Richland, WA 99352, USA 2Atmospheric Sciences Division, Brookhaven National Laboratory, Upton, NY 11973, USA 3Environmental and Molecular Sciences Laboratory, Pacific Northwest National Laboratory Richland, WA 99352, USA 4Aerodyne Research, Inc., Billerica, MA 08121, USA 5Department of Environmental Toxicology, University of California, Davis, CA 95616, USA
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., 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 NOx 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
NOx-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.
Citation: Shilling, J. E., Zaveri, R. A., Fast, J. D., Kleinman, L., Alexander, M. L., Canagaratna, M. R., Fortner, E., Hubbe, J. M., Jayne, J. T., Sedlacek, A., Setyan, A., Springston, S., Worsnop, D. R., and Zhang, Q.: Enhanced SOA formation from mixed anthropogenic and biogenic emissions during the CARES campaign, Atmos. Chem. Phys., 13, 2091-2113, doi:10.5194/acp-13-2091-2013, 2013.