Abstract
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.
Highlights
Atmospheric aerosols are important components of the atmospheric system, interacting chemically and physically with gas-phase components and affecting climate processes through aerosol–radiation and aerosol–cloud interactions (IPCC, 2013)
The overarching goal of this paper is to describe how the in situ measurements are combined into a single consistent description of the aerosol microphysical, chemical, hygroscopic, and optical properties listed in Table 1, to present a summary of aerosol properties in different air masses encountered during Atmospheric Tomography Mission (ATom), and to provide an entry point to the ATom dataset for use in modeling and remote sensing investigations of atmospheric composition and climate
We compare modal fits to the ATom size distributions with model assumptions and a marine dataset, discuss the ATom observations in the context of an existing comprehensive airborne dataset, and evaluate the mass absorption cross sections (MACs) values we calculate for the coated refractory black carbon (rBC) relative to some recently published analyses
Summary
Atmospheric aerosols are important components of the atmospheric system, interacting chemically and physically with gas-phase components and affecting climate processes through aerosol–radiation and aerosol–cloud interactions (IPCC, 2013). To effectively simulate the role of atmospheric aerosol in climate processes, models must adequately represent the mass, composition, and phase of different aerosol types and their distribution amongst particle sizes, the spatial and temporal distribution of the components, their mixing state (existing as external mixtures with different compositions or internal mixtures with blended compositions), their optical properties (often a function of particle size), and their hygroscopic properties and suitability to serve as cloud condensation nuclei (CCN) Underlying these properties are the physical and chemical processes being represented in the simulations, including emissions of particles and gas-phase precursors, atmospheric transport, gas-phase, heterogeneous, and aqueous chemistry, cloud processing, evaporation, wet and dry deposition, and transformations such as condensation and coagulation. There is an imperative for improving aerosol representation in global models: the largest source of uncertainty in understanding climate sensitivity remains aerosol–radiation and aerosol–cloud interactions (the direct and indirect effects, respectively)
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