Centroid values of aerosol optical properties for 8 sub-types based in AERONET inversion data (1993–2018)

In this project, we adapted our previously defined 5 aerosol optical
typology scheme (Hamill et al. 2016) to result in a more discriminating 8
aerosol typology scheme (Giordano 2019). Previously we presented an
aerosol classification based upon AERONET level 2.0 almucantar retrieval
products from the period 1993 to 2012. In the initial phases of this
research, we opto-physically identified five major types of Bulk Columnar
Aerosol (BCA) based solely 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 were classified as Maritime Aerosol, Dust
Aerosol, Urban Industrial Aerosol, Biomass Burning Aerosol, and Mixed
Aerosol. The classification of a particular observation as one of these
aerosol types is determined by its five-dimensional Mahalanobis distance
(MD) to the centroid of each reference cluster (itself a 5-D
hyperellipsoid). To retain a greater number of AERONET sites in the study
(200+), we kept the variable space to 5-D. To generate reference clusters,
we only retained data points that were found to lie within 2 MD from the
data centroid. Our typology is based on AERONET retrieved quantities,
which do not include low optical depth values (AOD440nm <
0.4 as per AERONET criteria for almucantar scan inversion).  The
classifications obtained are made available to be used in interpreting
aerosol retrievals from satellite-borne instruments and as input for
regional climate models. A major result of this aerosol typology is a
dataset describing the types of aerosol particles that are distinct from
one another in optical properties and a geographic distribution of those
aerosol types. We used the typology scheme upon the qualifying AERONET
data archive and produced seasonal aerosol climatologies by aerosol type
for each of the AERONET sites included in the study, regional aerosol
climatology maps, and a time-integrated global aerosol climatology map
based entirely upon ground-based photometric data (Giordano 2022). An
internally hyperlinked compendium of the individual AERONET site aerosol
climatologies was produced to contain the results of the first phase of
this work [available at
https://ars.els-cdn.com/content/image/1-s2.0-S1352231016304265-mmc1.pdf].
Each of these original five aerosol types (Hamill et al. 2016, Giordano
2019) was further discriminated into specific sub-types by this same
scheme to achieve an 8-aerosol typology (Giordano 2019 Chapter 2). For
example, optical discrimination into specific sub-types of Biomass Burning
aerosol may provide insight into sources exhibiting spectrally distinct
smoke properties. Here we segmented the Biomass Burning Aerosol type into
the sub-types of Flaming (BMF) and Smoldering (BMS) using the centroid
separation method and the MD criteria for in-class inclusion was adjusted
to 1.5 MD. Similarly, we found great confidence in discriminating the
MIXED aerosol type into two distinct regimes which we simply labeled as
MIXEDtype1 (MIXED1) and MIXEDtype2 (MIXED2). These can be visually
verified by examining any one of many possible renditions of 3-D optical
spaces noting their 5-D centroids are separated by a distance of 3.47-3.85
MD [Giordano 2019 Chapter 2]. Likewise, the Urban Industrial Aerosol class
was further discriminated into European Urban Industrial (EURO UI) and
North American (NA UI), whose 5-D centroids are separated by a distance of
2.60–3.08 MD. We then used the previously employed mathematical strategies
to sort the global AERONET data retrievals into the aerosol types
classified against their reference standards. We believe the strategies
regarding aerosol differentiation using polarization data (Hamill, Piedra
and Giordano 2020)  are  an additional
method useful for analysis of the newer AERONET version 3 data
retrievals, and data collected from the deployment of newer CIMEL
sun-photometers (with enhanced polarization measurement capabilities) to
the network. The resulting AERONET-based 8-aerosol optical typology, in a
5-D basis is useful for applications in aerosol optics, including direct
forward modeling of radiative transfer to determine the effects of aerosol
absorption and/or scattering on vertical heating profiles and ground
received irradiance quantities, for input into more complicated remote
sensing algorithms, used as calibration/validation values for in-situ and
laboratory experimental studies, and evaluating radiative forcing
calculations in atmospheric models. [Work related to an 8-aerosol typology
in 6-D, 8-D, 9-D and 10-D optical property bases, and their
files, are to be published subsequently as a different database project in
2023.]