Articles | Volume 16, issue 8
Atmos. Chem. Phys., 16, 5009–5019, 2016
Atmos. Chem. Phys., 16, 5009–5019, 2016

Research article 22 Apr 2016

Research article | 22 Apr 2016

Aerosol optical properties in the southeastern United States in summer – Part 2: Sensitivity of aerosol optical depth to relative humidity and aerosol parameters

Charles A. Brock1, Nicholas L. Wagner1,2, Bruce E. Anderson3, Andreas Beyersdorf3, Pedro Campuzano-Jost2,4, Douglas A. Day2,4, Glenn S. Diskin3, Timothy D. Gordon1,2,a, Jose L. Jimenez2,4, Daniel A. Lack1,2,b, Jin Liao1,2,c, Milos Z. Markovic1,2,d, Ann M. Middlebrook1, Anne E. Perring1,2, Matthews S. Richardson1,2, Joshua P. Schwarz1, Andre Welti1,2,e, 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
  • anow at: Handix Scientific LLC, Boulder, Colorado, USA
  • bnow at: TEAC Consulting, Brisbane, Australia
  • cnow at: NASA Goddard Space Flight Center, Greenbelt, Maryland, USA
  • dnow at: Air Quality Research Division, Environment Canada, Toronto, Ontario, Canada
  • enow at: Leibniz Institute for Tropospheric Research, Department of Physics, Leipzig, Germany

Abstract. Aircraft observations of meteorological, trace gas, and aerosol properties were made between May and September 2013 in the southeastern United States (US). Regionally representative aggregate vertical profiles of median and interdecile ranges of the measured parameters were constructed from 37 individual aircraft profiles made in the afternoon when a well-mixed boundary layer with typical fair-weather cumulus was present (Wagner et al., 2015). We use these 0–4 km aggregate profiles and a simple model to calculate the sensitivity of aerosol optical depth (AOD) to changes in dry aerosol mass, relative humidity, mixed-layer height, the central diameter and width of the particle size distribution, hygroscopicity, and dry and wet refractive index, while holding the other parameters constant. The calculated sensitivity is a result of both the intrinsic sensitivity and the observed range of variation in these parameters. These observationally based sensitivity studies indicate that the relationship between AOD and dry aerosol mass in these conditions in the southeastern US can be highly variable and is especially sensitive to relative humidity (RH). For example, calculated AOD ranged from 0.137 to 0.305 as the RH was varied between the 10th and 90th percentile profiles with dry aerosol mass held constant. Calculated AOD was somewhat less sensitive to aerosol hygroscopicity, mean size, and geometric standard deviation, σg. However, some chemistry–climate models prescribe values of σg substantially larger than we or others observe, leading to potential high biases in model-calculated AOD of  ∼  25 %. Finally, AOD was least sensitive to observed variations in dry and wet aerosol refractive index and to changes in the height of the well-mixed surface layer. We expect these findings to be applicable to other moderately polluted and background continental air masses in which an accumulation mode between 0.1–0.5 µm diameter dominates aerosol extinction.

Short summary
Two research aircraft made dozens of vertical profiles over rural areas in the southeastern US in summer 2013. These measurements show that, in addition to how much pollution was present and how moist the atmosphere was, the size of the pollutant particles affected how much sunlight was reflected back to space. These measurements will help climate modelers determine which characteristics of pollution are important to predict with accuracy.
Final-revised paper