Ambient aerosol properties in the remote atmosphere from global-scale in situ measurements
Authors: Brock Charles A. , Froyd Karl D. , Dollner Maximilian , Williamson Christina J. , Schill Gregory , Murphy Daniel M. , Wagner Nicholas J. , Kupc Agnieszka , Jimenez Jose L. , Campuzano-Jost Pedro , Nault Benjamin A. , Schroder Jason C. , Day Douglas A. , Price Derek J. , Weinzierl Bernadett , Schwarz Joshua P. , Katich Joseph M. , Wang Siyuan , Zeng Linghan , Weber Rodney , Dibb Jack , Scheuer Eric , Diskin Glenn S. , DiGangi Joshua P. , Bui ThaoPaul​​​​​​​ , Dean-Day Jonathan M. , Thompson Chelsea R. , Peischl Jeff , Ryerson Thomas B. , Bourgeois Ilann , Daube Bruce C. , Commane Róisín , Wofsy Steven C. 
Published: October 2021

Abstract. In situ measurements of aerosol microphysical, chemical,
and optical properties were made during global-scale flights from 2016–2018 as part of the Atmospheric Tomography Mission (ATom). The NASA DC-8 aircraft flew from ∼ 84∘ N to ∼ 86∘ S latitude over the Pacific, Atlantic, Arctic, and Southern oceans while
profiling nearly continuously between altitudes of ∼ 160 m and
∼ 12 km. These global circuits were made once each season.
Particle size distributions measured in the aircraft cabin at dry conditions
and with an underwing probe at ambient conditions were combined with bulk
and single-particle composition observations and measurements of water
vapor, pressure, and temperature to estimate aerosol hygroscopicity and
hygroscopic growth factors and calculate size distributions at ambient
relative humidity. These reconstructed, composition-resolved ambient size
distributions were used to estimate intensive and extensive aerosol
properties, including single-scatter albedo, the asymmetry parameter,
extinction, absorption, Ångström exponents, and aerosol optical
depth (AOD) at several wavelengths, as well as cloud
condensation nuclei (CCN) concentrations at fixed
supersaturations and lognormal fits to four modes. Dry extinction and
absorption were compared with direct in situ measurements, and AOD derived
from the extinction profiles was compared with remotely sensed AOD
measurements from the ground-based Aerosol Robotic Network (AERONET); this
comparison showed no substantial bias. The purpose of this work is to describe the methodology by which ambient
aerosol properties are estimated from the in situ measurements, provide
statistical descriptions of the aerosol characteristics of different remote
air mass types, examine the contributions to AOD from different aerosol
types in different air masses, and provide an entry point to the ATom
aerosol database. The contributions of different aerosol types (dust, sea
salt, biomass burning, etc.) to AOD generally align with expectations based
on location of the profiles relative to continental sources of aerosols,
with sea salt and aerosol water dominating the column extinction in most
remote environments and dust and biomass burning (BB) particles contributing
substantially to AOD, especially downwind of the African continent.
Contributions of dust and BB aerosols to AOD were also significant in the
free troposphere over the North Pacific. Comparisons of lognormally fitted size distribution parameters to values in
the Optical Properties of Aerosols and Clouds (OPAC) database commonly used
in global models show significant differences in the mean diameters and
standard deviations for accumulation-mode particles and coarse-mode dust. In
contrast, comparisons of lognormal parameters derived from the ATom data
with previously published shipborne measurements in the remote marine
boundary layer show general agreement. The dataset resulting from this work can be used to improve global-scale
representation of climate-relevant aerosol properties in remote air masses
through comparison with output from global models and assumptions used in
retrievals of aerosol properties from both ground-based and satellite remote sensing.