Articles | Volume 16, issue 8
Atmos. Chem. Phys., 16, 4987–5007, 2016
Atmos. Chem. Phys., 16, 4987–5007, 2016

Research article 22 Apr 2016

Research article | 22 Apr 2016

Aerosol optical properties in the southeastern United States in summer – Part 1: Hygroscopic growth

Charles A. Brock1, Nicholas L. Wagner1,2, Bruce E. Anderson3, Alexis R. Attwood1,2,a, Andreas Beyersdorf3, Pedro Campuzano-Jost2,4, Annmarie G. Carlton5, Douglas A. Day2,4, Glenn S. Diskin3, Timothy D. Gordon1,2,b, Jose L. Jimenez2,4, Daniel A. Lack1,2,c, Jin Liao1,2,d, Milos Z. Markovic1,2,e, Ann M. Middlebrook1, Nga L. Ng6,7, Anne E. Perring1,2, Matthews S. Richardson1,2, Joshua P. Schwarz1, Rebecca A. Washenfelder1,2, Andre Welti1,2,f, Lu Xu7, Luke D. Ziemba3, and Daniel M. Murphy1 Charles A. Brock et al.
  • 1NOAA Earth System Research Laboratory, Boulder, Colorado, USA
  • 2Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, Colorado, USA
  • 3NASA Langley Research Center, Hampton, Virginia, USA
  • 4Department of Chemistry and Biochemistry, University of Colorado, Boulder, Colorado, USA
  • 5Department of Environmental Sciences, Rutgers University, New Brunswick, New Jersey, USA
  • 6School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, Georgia, USA
  • 7School of Chemical and Biomolecular Engineering, Georgia Institute of Technology, Atlanta, Georgia, USA
  • anow at: Droplet Measurement Technologies, Boulder, Colorado, USA
  • bnow at: Handix Scientific, Boulder, Colorado, USA
  • cnow at: TEAC Consulting, Brisbane, Australia
  • dnow at: NASA Goddard Space Flight Center, Greenbelt, Maryland, USA
  • enow at: Air Quality Research Division, Environment Canada, Toronto, Ontario, Canada
  • fnow at: Department of Physics, Leibniz Institute for Tropospheric Research, Leipzig, Germany

Abstract. Aircraft observations of meteorological, trace gas, and aerosol properties were made during May–September 2013 in the southeastern United States (US) under fair-weather, afternoon conditions with well-defined planetary boundary layer structure. Optical extinction at 532 nm was directly measured at relative humidities (RHs) of  ∼  15,  ∼  70, and  ∼  90 % and compared with extinction calculated from measurements of aerosol composition and size distribution using the κ-Köhler approximation for hygroscopic growth. The calculated enhancement in hydrated aerosol extinction with relative humidity, f(RH), calculated by this method agreed well with the observed f(RH) at  ∼  90 % RH. The dominance of organic aerosol, which comprised 65 ± 10 % of particulate matter with aerodynamic diameter  <  1 µm in the planetary boundary layer, resulted in relatively low f(RH) values of 1.43 ± 0.67 at 70 % RH and 2.28 ± 1.05 at 90 % RH. The subsaturated κ-Köhler hygroscopicity parameter κ for the organic fraction of the aerosol must have been  <  0.10 to be consistent with 75 % of the observations within uncertainties, with a best estimate of κ  =  0.05. This subsaturated κ value for the organic aerosol in the southeastern US is broadly consistent with field studies in rural environments. A new, physically based, single-parameter representation was developed that better described f(RH) than did the widely used gamma power-law approximation.

Short summary
Microscopic pollution particles make the atmosphere look hazy and also cool the earth by sending sunlight back to space. When the air is moist, these particles swell with water and scatter even more sunlight. We showed that particles formed from organic material – which dominates particulate pollution in the southeastern U.S. – does not take up water very effectively, toward the low end of most previous studies. We also found a better way to mathematically describe this swelling process.
Final-revised paper