Aerosol water content and soot concentrations are important components of aerosol forcing. Aerosols contain varying amounts of water depending upon their aerosol hygroscopicity, and anthropogenic aerosols are among the most hygroscopic aerosols; hence, it is important to properly model aerosol hygroscopic effects when computing the effect of anthropogenic aerosols upon the climate system. Soot is the dominant absorbing particulate, and atmospheric soot originates exclusively from fossil fuel burning and biomass burning.
Present-day satellite retrievals have not discerned the aerosol fraction associated with water or soot. Satellite retrievals of these two aerosol species are important for checking and constraining global model outputs (since satellite measurements are the only practical way of obtaining continuous global coverage).
FORTRAN code was developed for inferring the volume fraction of water and soot in atmospheric aerosols. This code is applicable to satellite and surface remote sensing of aerosols, and can be used to determine the aerosol water content and particulate black carbon concentration in the atmospheric column. Consequently, this code enables the comparison of modeled aerosol water content and black carbon concentrations to retrievals of the same for the first time.
The technique uses the aerosol real refractive index to infer the aerosol water fraction. The refractive index of the most common hygroscopic aerosols (i.e., ammonium sulfate, ammonium nitrate, sea salt) has similar refractive indices for similar solute/water mixing ratios, which enables a mapping of the refractive index to the water concentration. The presence of non-soluble aerosols alters this mapping, so the insoluble/soluble ratio is adjusted with an empirical equation that is a function of the refractive index. It is estimated that the present version of the retrieval can provide instantaneous geometric hygroscopic growth factors to within 0.3, which is accurate enough to differentiate “less hygroscopic” and “more hygroscopic” aerosols.
The technique uses the aerosol imaginary refractive index to infer the soot fraction. The imaginary refractive index of most atmospheric aerosols is two orders of magnitude smaller than the absorption index for soot, so soot is responsible for nearly all of the atmospheric aerosol absorption in many regions of the world. It is estimated that the soot fraction can be determined to within ±50% by assuming that all aerosol absorption at the red and near infrared wavelengths is associated with soot, which is an improvement upon the soot concentrations that can be determined using global transport models.
Ultimately, inferring the aerosol water content and soot concentration from satellite retrievals is highly desirable for constraining and validating global aerosol models, and thereby improving assessment of the anthropogenic aerosol radiative forcing. This package utilizes the aerosol refractive index that is only now becoming available in the satellite datasets. This software package is also useful for developing new and improved satellite retrievals of aerosol composition (without the intermediate step of retrieving the aerosol refractive index).