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

Abstract

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 particl...