Articles | Volume 18, issue 11
Atmos. Chem. Phys., 18, 8293–8312, 2018
Atmos. Chem. Phys., 18, 8293–8312, 2018

Research article 14 Jun 2018

Research article | 14 Jun 2018

Sources and characteristics of summertime organic aerosol in the Colorado Front Range: perspective from measurements and WRF-Chem modeling

Roya Bahreini1,2, Ravan Ahmadov3,4, Stu A. McKeen3,4, Kennedy T. Vu2, Justin H. Dingle2, Eric C. Apel5, Donald R. Blake6, Nicola Blake6, Teresa L. Campos5, Chris Cantrell7, Frank Flocke5, Alan Fried8, Jessica B. Gilman3, Alan J. Hills5, Rebecca S. Hornbrook5, Greg Huey9, Lisa Kaser5, Brian M. Lerner3,4,a, Roy L. Mauldin7, Simone Meinardi6, Denise D. Montzka5, Dirk Richter8, Jason R. Schroeder6,b, Meghan Stell5, David Tanner9, James Walega8, Peter Weibring8, and Andrew Weinheimer5 Roya Bahreini et al.
  • 1Department of Environmental Sciences, University of California, Riverside, CA 92521, USA
  • 2Environmental Toxicology Graduate Program, University of California, Riverside, CA 92521, USA
  • 3Earth System Research Laboratory, National Oceanic and Atmospheric Administration, Boulder, CO 80305, USA
  • 4Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, CO 80301, USA
  • 5Atmospheric Chemistry Observations and Modeling Laboratory, National Center for Atmospheric Research, Boulder, CO 80301, USA
  • 6Department of Chemistry, University of California, Irvine, CA 92697, USA
  • 7Department of Atmospheric and Oceanic Sciences, University of Colorado, Boulder, CO 80303, USA
  • 8Institute for Arctic and Alpine Research, University of Colorado, Boulder, CO 80303, USA
  • 9Department of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, GA 30033, USA
  • anow at: Aerodyne Research, Inc., Billerica, MA 01821, USA
  • bnow at: NASA Langley Research Center, Newport News, VA 23666, USA

Abstract. The evolution of organic aerosols (OAs) and their precursors in the boundary layer (BL) of the Colorado Front Range during the Front Range Air Pollution and Photochemistry Éxperiment (FRAPPÉ, July–August 2014) was analyzed by in situ measurements and chemical transport modeling. Measurements indicated significant production of secondary OA (SOA), with enhancement ratio of OA with respect to carbon monoxide (CO) reaching 0.085±0.003 µg m−3 ppbv−1. At background mixing ratios of CO, up to  ∼  1.8 µg m−3 background OA was observed, suggesting significant non-combustion contribution to OA in the Front Range. The mean concentration of OA in plumes with a high influence of oil and natural gas (O&G) emissions was  ∼  40 % higher than in urban-influenced plumes. Positive matrix factorization (PMF) confirmed a dominant contribution of secondary, oxygenated OA (OOA) in the boundary layer instead of fresh, hydrocarbon-like OA (HOA). Combinations of primary OA (POA) volatility assumptions, aging of semi-volatile species, and different emission estimates from the O&G sector were used in the Weather Research and Forecasting model coupled with Chemistry (WRF-Chem) simulation scenarios. The assumption of semi-volatile POA resulted in greater than a factor of 10 lower POA concentrations compared to PMF-resolved HOA. Including top-down modified O&G emissions resulted in substantially better agreements in modeled ethane, toluene, hydroxyl radical, and ozone compared to measurements in the high-O&G-influenced plumes. By including emissions from the O&G sector using the top-down approach, it was estimated that the O&G sector contributed to  <  5 % of total OA, but up to 38 % of anthropogenic SOA (aSOA) in the region. The best agreement between the measured and simulated median OA was achieved by limiting the extent of biogenic hydrocarbon aging and consequently biogenic SOA (bSOA) production. Despite a lower production of bSOA in this scenario, contribution of bSOA to total SOA remained high at 40–54 %. Future studies aiming at a better emissions characterization of POA and intermediate-volatility organic compounds (IVOCs) from the O&G sector are valuable.

Short summary
We measured organic aerosol (OA) and relevant trace gases during FRAPPÉ in the Colorado Front Range, with the goal of characterizing summertime OA formation. Our results indicate a significant production of secondary OA (SOA) in this region. About 2 μg m−3 of OA was present at background CO levels, suggesting contribution of non-combustion sources to SOA. Contribution of oil- and gas-related activities to anthropogenic SOA was modeled to be ~38 %. Biogenic SOA contributed to >40 % of OA.
Final-revised paper