Optical properties of light absorbing organic carbon aerosol and its mixtures at high sub-saturated humidities

Concern over the anthropogenic influence on climate has drawn attention to the role of aerosol in the earth’s climate system, because aerosols counteract the warming effect by greenhouse gases. The aerosol effect on climate is dependent on the balance between absorption and scattering of light by the particles, which are affected by ambient relative humidity (RH). Although absorption and scattering have been measured under dry conditions, their measurement at high RH requires laboratory measurements with a high degree of control. While numerous studies have investigated how RH affects light scattering and light extinction, few have explored RH effects on light absorption. Light absorbing organic carbon aerosol (LAOC) constitute a large fraction of biomass burning emissions and biomass burning is the largest source of primary organic aerosol in the atmosphere. This laboratory study investigated light extinction, light scattering and light absorption under controlled RH conditions (up to 95% RH) of LAOC and its mixtures with ammonium sulfate and sodium chloride, two common inorganic components of atmospheric aerosol. Measured scattering and extinction coefficients at visible wavelengths were used to determine light absorption. Extensive instrumentation development and benchmarking with independently measured and modeled values were required. The measured absorption of LAOC aerosol increased by a factor of 2.1 ± 0.7 and 2.3 ± 1.2 between 32 and 95% RH at blue and green wavelengths. When the LAOC aerosol were mixed with ammonium sulfate and sodium chloride, extinction and scattering growth occurred with increasing RH and inorganic mass fraction, but the same absorption increase as for pure LAOC was observed. Using a simple radiative transfer model, the treatment of LAOC aerosol optical properties with increasing RH was observed to be important over surfaces with an albedo of 0.85, characteristic of fresh snow. The modeling predicted a top of the atmosphere simple forcing efficiency of 145 W g-1 and a radiative forcing of 144 mW m-2 at 95% RH compared to 58 W g-1 of LAOC and 57 mW m-2 under dry conditions. If the LAOC were treated as non-absorbing OC, the simple forcing efficiency and radiative forcing would be -0.33 W g-1 and -0.32 mW m-2, respectively, under dry conditions and -0.57 W g-1 and -0.56 mW m-2 at 95% RH. These findings indicate that it is important to represent the increase in light absorption with RH in climate models. Most global climate models that include the optical properties of organic matter currently do not account for absorption and the absorption increase by organic carbon emitted from biomass combustion or pyrolysis. Additionally, a change in LAOC absorption with RH might have further implications for semi-direct and indirect climate effects and tropospheric chemistry