Laboratory-Measured Optical Properties of Inorganic and Organic Aerosols at Relative Humidities up to 95%

Relative humidity (RH) affects the liquid water content of an aerosol, altering its scattering and absorption of visible light, which is important for aerosol effects on visibility and climate. Particle light extinction, light scattering, and light absorption coefficient values are reported here for laboratory-generated inorganic and organic carbon (OC) aerosols at RH values between 8% and 95%. Light scattering was measured with a nephelometer, light extinction was measured with an extinction cell, and light absorption was determined on the basis of the difference between those two values at three visible wavelengths (467, 530, and 660 nm). The instrumentation was benchmarked with nonabsorbing ammonium sulfate, absorbing polystyrene microspheres (PSMs) and nigrosin aerosol under controlled RH conditions. Agreement between dry measured scattering and extinction coefficients for ammonium sulfate was achieved within 3%. Optical closure with modeled scattering values based on measured ammonium sulfate particle size distributions was achieved within 7%. Measured single scattering albedo for dry absorbing PSMs agreed within 0.02 with the literature value. Light absorption by nigrosin increased by a factor of 1.24 +/- 0.06 at all wavelengths as RH increased from 38% to 95%. Light absorption of OC aerosol that was generated from wood pyrolysis demonstrated enhancements of 2.2 +/- 0.7 and 2.7 +/- 1.2 between 32% and 95% RH at the wavelengths of 467 and 530 nm, but no absorption was detected at 660 nm. A spectral dependence of light absorption by OC was observed with absorption increasing from 530 nm towards the 467 nm wavelength, consistent with previously reported ex situ measurements of filter extracts. The increase in OC light absorption with RH is currently not represented in radiative transfer models even though biomass burning produces most of the primary OC aerosol in the atmosphere.