Effect of relative humidity on the aerosol backscattering coefficient at 0694- and 106-μm wavelengths

The effect of relative humidity on the backscattering of 0.694- and 10.6-μm radiation by aerosol particles in the lower troposphere is modeled. Two models of particle composition are considered: (1) all particles are composed of a uniform mixture of water-soluble material, dustlike material, and soot (uniform internal mixture) and (2) pure soot particles coexist with particles which are mixtures of water-soluble and dustlike materials (external mixture of soot). The amount of soot ranges from 1% to 20% of the volume of the aerosol. Changes in relative humidity have a greater effect on the backscattering coefficient, βπ, at 0.694 μm than at 10.6 μm. If soluble material accounts for 30% of the volume of mixed particles and if an urban type aerosol size distribution is assumed, an increase in relative humidity from 0% to 99% results in an increase in βπ at 0.694 μm ranging from a factor of 5.7 for an external mixture containing 20% soot by volume to a factor of 15.6 in the case of a uniform internal mixture containing 20% soot. At 10.6 μm the increase in βπ ranges from a factor of 2.1 to a factor of 2.8. The backscatter-to-extinction relation for 0.694-μm radiation propagating through a region of varying relative humidity is also investigated.