On Refining an AERONET Opto-physical Aerosol Typology: Expressing Aerosol Optical Properties as Distribution Functions by Specific Aerosol Type

Abstract:

Development and deployment of a robust bulk columnar aerosol (BCA) typology for use in global aerosol studies, is an ongoing pursuit by many diligent researchers across differing scientific agencies. Utilizing NASA’s global sunphotometer network (AERONET) is compelling for its geographic expanse, and the tractability of analyzing its historic data archive. In this work pursue the notion of expressing the optical properties of specific aerosol types by distribution functions. In our prior development, we presented an aerosol classification based upon AERONET level 2.0 almucantar retrieval products. We identified five global types of BCA based upon intensive optical properties of spectral Single Scattering Albedo(SSA), spectral Indices of Refraction(real – RRI and imaginary - IRI), and two Angstrom Exponents(extinction – EAE and absorption - AAE). These BCA we classified as Maritime Aerosol, Dust Aerosol, Urban Industrial Aerosol, Biomass Burning Aerosol, and Mixed Aerosol.Classification of a particular aerosol observation as one of these aerosol types is by its five-dimensional Mahalanobis distance (MD) to each reference cluster. Studies using simulated degrees of polarization (SDLP) as a tool to further differentiate characterization of challenging aerosol populations enhanced this typology. Model output resulted in a global AERONET aerosol compendium and aerosol climatology maps. The model returns values of specific optical properties for specific geolocations. Aerosol classifications by this strategy are useful for interpreting aerosol retrievals from satellite borne instruments, and as input for climate models. Toward that end, we desired to express the typology scheme as not only a vectorized set (matrices) , but also in functional form. Aerosol types are further discriminated into sub-types by this same scheme. To refine the sub-space regions of the classification space, we developed analytic expressions for the basis optical properties, using density distribution functions (DDF) expressed mathematically. We present expressions for 5 aerosol optical properties for each of 8 aerosol types. A benefit to incorporating this approach in development of an aerosol typology is to offer a spectrum of values representing an optical property of a specific aerosol type, not merely a centroid value.