Optical Extinction and Aerosol Hygroscopicity in the Southeastern United States

Abstract:

Most aerosol particles take up water and grow as relative humidity increases, leading to increased optical extinction, reduced visibility, greater aerosol optical depths (AODs), and altered radiative forcing, even while dry particulate mass remains constant. Relative humidity varies greatly temporally, horizontally, and especially vertically. Thus hygroscopicity is a confounding factor when attempting to link satellite-based observations of AOD to surface measurements of particulate mass or to model predictions of aerosol mass concentrations.

Airborne observations of aerosol optical, chemical, and microphysical properties were made in the southeastern United States in the daytime in summer 2013 during the NOAA SENEX and NASA SEAC⁴RS projects. Applying κ-Köhler theory for hygroscopic growth to these data, the inferred hygroscopicity parameter κ for the organic fraction of the aerosol was <0.11. This κ for organics is toward the lower end of values found from laboratory studies of the aerosol formed from oxidation of biogenic precursors and from several field studies in rural environments.

The gamma (γ) parameterization is commonly used to describe the change in aerosol extinction as a function of relative humidity. Because this formulation did not fit the airborne data well, a new parameterization was developed that better describes the observations. This new single-parameter κ_ext formulation is physically based and relies upon the well-known approximately linear relationship between particle volume and optical extinction. The fitted parameter, κ_ext, is nonlinearly related to the chemically derived κ parameter used in κ-Köhler theory. The values of κ_ext determined from the airborne measurements are consistent with independent observations at a nearby ground site.