Shortwave radiative impact of liquid–liquid phase separation in brown carbon aerosols

Abstract

Abstract. Atmospheric aerosol particles may undergo liquid–liquid phase separation (LLPS) when exposed to varying relative humidity. In this study, we model how the change in morphology affects the shortwave radiative forcing, in particular for particles containing organic carbon as a molecular absorber, often termed “brown carbon” (BrC). Preferentially, such an absorber will redistribute to the organic phase after LLPS. We limited our investigation to particle diameters between 0.04 and 0.5 µm, atmospherically relevant organic-to-inorganic mass ratios typical for aged aerosol (1:4<OIR<4:1) and absorptivities ranging from zero (purely scattering) to highly absorbing brown carbon. For atmospherically relevant O : C ratios, core-shell morphology is expected for phase-separated particles. We compute the scattering and absorption cross sections for realistic eccentric core-shell morphologies. For the size range of interest here, we show that assuming the core-shell morphology to be concentric is sufficiently accurate and numerically much more efficient than averaging over eccentric morphologies. In the UV region, where BrC absorbs strongest, phase-separated particles may exhibit a scattering cross section up to 50 % larger than those of homogenously mixed particles, while their absorption cross section is up to 20 % smaller. Integrating over the full solar spectrum, due to the strong wavelength dependence of BrC absorptivity, limits the shortwave radiative impact of LLPS in the thin aerosol layer approximation. For particles with very substantial BrC absorption there will be a radiative forcing enhancement of 4 %–11.8 % depending on the Ångström exponent (AAE) of BrC absorptivity for the case of small surface albedos and a decrease of up to 18 % for surfaces with high reflectivity. However, for those of moderate absorptivity, LLPS will have no significant shortwave radiative impact.