Document Type


Publication Date


Publication Title

Atmospheric Chemistry and Physics









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.


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

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