Articles | Volume 16, issue 17
Atmos. Chem. Phys., 16, 11207–11217, 2016
Atmos. Chem. Phys., 16, 11207–11217, 2016

Research article 12 Sep 2016

Research article | 12 Sep 2016

Aerosol optical extinction during the Front Range Air Pollution and Photochemistry Éxperiment (FRAPPÉ) 2014 summertime field campaign, Colorado, USA

Justin H. Dingle1, Kennedy Vu1, Roya Bahreini1,2, Eric C. Apel3, Teresa L. Campos3, Frank Flocke3, Alan Fried4, Scott Herndon5, Alan J. Hills3, Rebecca S. Hornbrook3, Greg Huey6, Lisa Kaser3, Denise D. Montzka3, John B. Nowak5, Mike Reeves3, Dirk Richter4, Joseph R. Roscioli5, Stephen Shertz3, Meghan Stell3, David Tanner6, Geoff Tyndall3, James Walega4, Petter Weibring4, and Andrew Weinheimer3 Justin H. Dingle et al.
  • 1Environmental Toxicology Graduate Program, University of California, Riverside, CA 92521, USA
  • 2Department of Environmental Sciences, University of California, Riverside, CA 92521, USA
  • 3National Center for Atmospheric Research, Boulder, CO 80301, USA
  • 4Institute for Arctic and Alpine Research, University of Colorado, Boulder, CO 80303, USA
  • 5Aerodyne Research, Inc., Billerica, MA 01821, USA
  • 6Department of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, GA 30033, USA

Abstract. Summertime aerosol optical extinction (βext) was measured in the Colorado Front Range and Denver metropolitan area as part of the Front Range Air Pollution and Photochemistry Éxperiment (FRAPPÉ) campaign during July–August 2014. An Aerodyne cavity attenuated phase shift particle light extinction monitor (CAPS-PMex) was deployed to measure βext (at average relative humidity of 20 ± 7 %) of submicron aerosols at λ = 632 nm at 1 Hz. Data from a suite of gas-phase instrumentation were used to interpret βext behavior in various categories of air masses and sources. Extinction enhancement ratios relative to CO (Δβext ∕ ΔCO) were higher in aged urban air masses compared to fresh air masses by  ∼  50 %. The resulting increase in Δβext ∕ ΔCO for highly aged air masses was accompanied by formation of secondary organic aerosols (SOAs). In addition, the impacts of aerosol composition on βext in air masses under the influence of urban, natural oil and gas operations (O&G), and agriculture and livestock operations were evaluated. Estimated non-refractory mass extinction efficiency (MEE) values for different air mass types ranged from 1.51 to 2.27 m2 g−1, with the minimum and maximum values observed in urban and agriculture-influenced air masses, respectively. The mass distribution for organic, nitrate, and sulfate aerosols presented distinct profiles in different air mass types. During 11–12 August, regional influence of a biomass burning event was observed, increasing the background βext and estimated MEE values in the Front Range.

Short summary
The focus of this paper was to use gas-phase tracers and aerosol composition to characterize the influence of the different sources on optical extinction (RH = 22 %) and summertime visibility in the Colorado Front Range. Our analysis indicates that aerosol nitrate contributed significantly to optical extinction in agriculturally influenced air masses, while in other plumes, organics could explain most of the observed variability in optical extinction.
Final-revised paper