2 Parameterization for the relative humidity dependence of light 3 extinction: Organic-ammonium sulfate aerosol

7 [1] Atmospheric particles are a complex mixture of inorganic and organic compounds. 8 This study uses laboratory generated particles to examine the connection between aerosol 9 light extinction, chemical composition, and hygroscopicity for particles composed of 10 internal mixtures of ammonium sulfate and water-soluble organic compounds. The 11 extinction coefficient (sep) at 532 nm was measured for size-selected particles at <10% 12 RH and 80% RH. The ratio of the extinction coefficients at 80% RH to <10% RH is 13 reported as fRHext(80% RH, dry). The fRHext(80% RH, dry) values were similar for 14 particles composed of various water-soluble organic compounds and different functional 15 groups. In addition, fRHext(80% RH, dry) values were relatively insensitive to the 16 composition of the organic fraction for internal mixtures of ammonium sulfate with 17 sugars, dicarboxylic acids and complex mixtures of water-soluble organic compounds. 18 Finally, fRHext(80% RH, dry) was found to vary linearly with the organic/inorganic 19 content, allowing for simple incorporation of organic properties into atmospheric models. 20 We derived a generalization of fRHext(80% RH, dry) = 2.90 – 0.015(wt% organic species) 21 for a particle size distribution with a dry mean optical diameter of 0.35 mm. This 22 parameterization for ammonium sulfate/water-soluble organic aerosol is applicable to the 23 fine particle mode fraction of atmospheric aerosol. Information necessary to 24 incorporate the variation in the size distribution is also included. This work suggests 25 that neglecting the water uptake by the organic fraction of atmospheric particles 26 could lead to significant underestimation of the cooling at the Earth’s 27 surface due to light scattering by aerosol. 28