Optical Properties and Associated Hygroscopicity of Clay Aerosols

Airborne mineral dust particles contribute a significant fraction to the total aerosol mass, thus they make a substantial contribution to the Earth's radiative budget by direct scattering and absorption of radiation. Quantifying their contribution is complicated by the variability of optical properties as a function of water uptake. To improve understanding, we directly measured the relative humidity (RH) dependence of extinction [fRH_ext(RH, Dry)] for three key silicate clay components (illite, kaolinite, and montmorillonite) of mineral dust aerosols through cavity ring-down spectroscopy at 532 nm. The three clays studied show significant differences in fRH_ext(RH, Dry) at three RH values, and reasons for this are explored. With 68% RH as an example, we used the fRH_ext(RH, Dry) and Mie theory to calculate a growth factor for comparison with other measurement techniques. Humidified tandem differential mobility analyzer and quartz crystal microbalance growth factors from the literature are larger than our optical measurements indicate. An apparent decrease in particle size calculated from optical measurements for illite and kaolinite was further investigated by determining the aerosol electrical mobility size distribution of 68% RH and dry clay particles at that indicated shrinkage of approximately 10% at elevated humidity. Direct optical measurement has advantages because the effects of irregular shape and internal voids are observed. Our calculated growth factors provide a lower limit and can be incorporated into climate models in conjunction with other results to reduce the uncertainty associated with the optical response to water uptake on clay aerosols.