10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
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Distribution of Organic Aerosols (OA) during the NASA ATmospheric Tomography (ATom) Campaigns: Chemical Removal and Aging as a Function of Photochemical Age
JOSE-LUIS JIMENEZ, Pedro Campuzano-Jost, Benjamin A. Nault, Jason Schroder, Douglas Day, Joseph Katich, Joshua P. Schwarz, Nicola Blake, Donald Blake, Bruce Daube, Roisin Commane, Steven Wofsy, Eric Ray, Katherine Travis, Colette Heald, Simone Tilmes, Alma Hodzic, Huisheng Bian, Peter Colarco, Mian Chin, Anna Hodshire, Jack Kodros, Jeffrey R. Pierce, University of Colorado-Boulder
Abstract Number: 1007 Working Group: Aerosol Chemistry
Abstract Submicron aerosol mass in the remote free troposphere (FT) originates mostly from long-range transport from distant biogenic, anthropogenic, and biomass burning sources. Very limited local mass production in this region increases the sensitivity of aerosol concentrations to slow removal processes. As yet, few studies with an advanced aerosol payload have targeted the remote FT. Current global models exhibit a very large diversity in predicting aerosol concentrations in these regions of the atmosphere, particularly when trying to model organic aerosol (OA), which, together with sulfate, is the most prevalent type of submicron aerosols in the remote FT.
As part of NASA’s Atmospheric Tomography (ATom) aircraft mission, we have acquired a global dataset of organic aerosol (OA) concentration and composition over the remote Atlantic and Pacific Oceans from 0 to 12 km and from 65 S to 80 N during four deployments encompassing the seasonal cycle for both hemispheres. This dataset provides unique new constraints on the spatial distribution of OA and its contribution to the global aerosol background; of particular interest are the OA/Sulfate ratio and OA oxidation state that are critical for estimating the activity of cloud condensation nuclei (CCN) in the remote troposphere. We find that, except for the cleanest of the ATom-sampled airmasses (less than 0.25 ug sm-3), OA concentrations are comparable and often larger than sulfate.
OA was highly oxidized, significantly more than over the continental FT, with O:C ratios often in excess of 1. This translates into average OA/OC ratios in excess of 2.5, about 80% larger than what is assumed in most global OA models (1.4). Incorporating this correction into current models, as well as more accurate, revised SOA parametrizations leads to a large overprediction of OA in the FT for most models evaluated, suggesting that additional constrains on the OA lifetime in these models is needed.
Using several different hydrocarbon-ratio based photochemical clocks in combination with back-trajectories to infer the age of the airmasses sampled during ATom, we estimate that the lifetime of OA in the remote FT is on the order of 10 days. This is significantly shorter than the FT lifetime assuming just wet and dry deposition as the primary loss mechanisms (~50 days in GEOS-Chem v10), and suggests a chemical removal mechanism, likely heterogeneous OH oxidation, possibly with a contribution from photolysis. This provides a key constraint for modeling of OA in the FT, based solely on measurements. The likelihood of different chemical removal mechanisms and their implementation and validation in a global model (GEOSChem v11) will be discussed. The contributions of methanesulfonic acid (MSA) and particulate organic nitrates (pRONO2) to total OA in the remote troposphere will be discussed as well.