Radial Growth and Optical Properties of Hygroscopic Absorbing Aerosols at High Relative Humidity

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Description

The hygroscopic growth of aerosol particles at high relative humidities, up to and including 100% RH, is important in a number of situations: at cloud base just before particles activate, in the twilight zone at the outflow of clouds, in fog, and in the human lung. Therefore, this hygroscopic regime has relevance to cloud formation, satellite retrieval of optical depth, visibility, and human health. This regime has been little studied theoretically, and experimental work is extremely challenging due to strong dependence of the vapor pressure of water on temperature and thus the stringent requirements on temperature control. Here an approximation is presented for the radial growth of hygroscopic aerosols covering a wide range of dry diameters that is accurate over the range 90-100% RH and remains finite at 100% RH. This approximation readily illustrates the dependences of radius on dry solute mass and relative humidity, and can readily be applied in regional and global models that treat aerosol lifecycle. This RH regime is also an important but significantly understudied one for optical properties of absorbing aerosols. Here calculations are presented for the optical properties of absorbing aerosol particles in this region of RH, specifically scattering and absorption cross sections and other properties such as single scattering albedo, for particles consisting of black carbon with a non-absorbing coating, black carbon with an absorbing coating, and particles that are mildly absorbing. Implications for the aging of biomass burn plumes are discussed, with specific reference to the LASIC field campaign and to ORACLES.