A study of internal and external mixing scenarios and its effect on aerosol optical properties and direct radiative forcing

Abstract

[1] The optical properties and hence the radiative forcing of atmospheric aerosols are determined, in part, by the way in which the various constituents are externally or internally mixed. The mixing state must be known to compute the effective refractive index, water activity, and size distribution of the aerosols. In this study we found that the percentage difference in the optical properties, including extinction, single scattering albedo, and asymmetry parameter, between an internal mixture and external mixture of black carbon and ammonium sulfate can be over 25% for the dry case and over 50% for the wet case for typical mass mixing ratios. The differences are a result of a complicated combination of nonlinear Mie theory on the refractive index, assumptions about the coagulated particle sizes for internal mixtures, and the role of water uptake and deliquescence as a function of relative humidity. The computed optical properties are used to estimate the globally average clear-sky direct radiative forcing for different mixing assumptions. The results are displayed as a function of relative humidity to conveniently see the mixing effects for dry aerosols at less than the crystallization point, for dry internal and wet external mixtures between the crystallization and deliquescence points, and for fully wet mixtures above the deliquescence point. For a 9:1 ammonium sulfate to black carbon mass ratio, nearly all the cooling effect predicted for an external mixture is lost for the internally mixed assumption, especially for relative humidities less than the deliquescence point.

The Canadian Fourth Generation Atmospheric Global Climate Model (CanAM4). Part I: Representation of Physical Processes

“…Under the assumption that ammonium sulphate ((NH 4 ) 2 SO 4 ) dominates the chemical composition of sulphate aerosols, their optical properties are parameterized using Li, Wong, Dobbie, and Chýlek (2001). Sea salt and mineral dust optical properties are parameterized using Lesins, Chýlek, and Lohmann (2002) while the optical properties of black and organic carbon are parameterized using Bäumer, Lohmann, Lesins, Li, and Croft (2007). Volcanic aerosols in the stratosphere are assumed to be distributed between a climatological mean tropopause and 10 hPa and to be composed of 25% water (H 2 O) and 75% sulphuric acid (H 2 SO 4 ).…”

Effects of Continentally Polluted Air Mass on Aerosol Optical Properties over the East China Sea

“…The Bruggeman mixing rule should be used for a mixture of waterinsoluble particles (Lesins et al 2002). Thus, this mixing rule was used to approximate the refractive index for a mixture of waterinsoluble components.…”

“…For a core-shell internal mixture (waterinsoluble components embedded within a matrix of water-soluble components), the refractive index was calculated by combining the Bruggeman mixing rule (Chylek et al 2000;Lesins et al 2002), the volume-weighted mixing rule (Ouimette and Flagan 1982;Hasan and Dzubay 1983), and the Maxwell-Garnet mixing rule (Lesins et al 2002). The Bruggeman mixing rule should be used for a mixture of waterinsoluble particles (Lesins et al 2002).…”

“…The volume-weighted mixing rule (method 1) was used to estimate the refractive indices at different RHs for a mixture of water-soluble components. After that, the Maxwell-Garnet mixing rule was used to estimate the refractive index for a mixture of water-soluble and waterinsoluble components (Lesins et al 2002):…”

Introducing multisensor satellite radiance-based evaluation for regional Earth System modeling

“…The G-SDSU approach, adapted from the R-STAR module, generally utilizes the refractive index derived from the OPAC [Hess et al, 1998] and other sources. It should be noted that there are some uncertainties introduced by defining chemical compositions and refractive index from the discrete aerosol categories, since aerosols are often characterized with multiple chemical compositions [Lesins et al, 2002].…”