Identifying chemical aerosol signatures using optical suborbital observations: how much can optical properties tell us about aerosol composition?
Authors: Kacenelenbogen Meloë S. F. , Tan Qian , Burton Sharon P. , Hasekamp Otto P. , Froyd Karl D. , Shinozuka Yohei , Beyersdorf Andreas J. , Ziemba Luke , Thornhill Kenneth L. , Dibb Jack E. , Shingler Taylor , Sorooshian Armin , Espinosa Reed W. , Martins Vanderlei , Jimenez Jose L. , Campuzano-Jost Pedro , Schwarz Joshua P. , Johnson Matthew S. , Redemann Jens , Schuster Gregory L. 
Published: March 2022

Abstract. Improvements in air quality and Earth's climate
predictions require improvements of the aerosol speciation in chemical
transport models, using observational constraints. Aerosol speciation (e.g.,
organic aerosols, black carbon, sulfate, nitrate, ammonium, dust or sea
salt) is typically determined using in situ instrumentation. Continuous, routine
aerosol composition measurements from ground-based networks are not
uniformly widespread over the globe. Satellites, on the other hand, can
provide a maximum coverage of the horizontal and vertical atmosphere but
observe aerosol optical properties (and not aerosol speciation) based on
remote sensing instrumentation. Combinations of satellite-derived aerosol
optical properties can inform on air mass aerosol types (AMTs). However,
these AMTs are subjectively defined, might often be misclassified and are
hard to relate to the critical parameters that need to be refined in models. In this paper, we derive AMTs that are more directly related to sources and
hence to speciation. They are defined, characterized and derived using
simultaneous in situ gas-phase, chemical and optical instruments on the same
aircraft during the Study of Emissions and Atmospheric Composition, Clouds,
and Climate Coupling by Regional Surveys (SEAC4RS, an airborne field
campaign carried out over the US during the summer of 2013). We find
distinct optical signatures for AMTs such as biomass burning (from
agricultural or wildfires), biogenic and polluted dust. We find that all
four AMTs, studied when prescribed using mostly airborne in situ gas measurements,
can be successfully extracted from a few combinations of airborne in situ aerosol
optical properties (e.g., extinction Ångström exponent, absorption Ångström
exponent and real refractive index). However, we find that the optically
based classifications for biomass burning from agricultural fires and
polluted dust include a large percentage of misclassifications that limit
the usefulness of results related to those classes. The technique and results presented in this study are suitable to develop a
representative, robust and diverse source-based AMT database. This database
could then be used for widespread retrievals of AMTs using existing and
future remote sensing suborbital instruments/networks. Ultimately, it has
the potential to provide a much broader observational aerosol dataset to
evaluate chemical transport and air quality models than is currently
available by direct in situ measurements. This study illustrates how essential it
is to explore existing airborne datasets to bridge chemical and optical
signatures of different AMTs, before the implementation of future spaceborne
missions (e.g., the next generation of Earth Observing System (EOS)
satellites addressing Aerosols, Cloud, Convection and Precipitation (ACCP)
designated observables).