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Discussion papers
https://doi.org/10.5194/acp-2019-773
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/acp-2019-773
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: research article 20 Sep 2019

Submitted as: research article | 20 Sep 2019

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This discussion paper is a preprint. It is a manuscript under review for the journal Atmospheric Chemistry and Physics (ACP).

Characterization of Organic Aerosol across the Global Remote Troposphere: A comparison of ATom measurements and global chemistry models

Alma Hodzic1, Pedro Campuzano-Jost2,3, Huisheng Bian4, Mian Chin4, Peter R. Colarco4, Douglas A. Day2,3, Karl D. Froyd2,8, Bernd Heinold6, Duseong S. Jo2,3, Joseph M. Katich2,8, Jack K. Kodros5, Benjamin A. Nault2,3, Jeffrey R. Pierce5, Eric Ray2,8, Jacob Schacht6, Gregory P. Schill2,8, Jason C. Schroder2,3, Joshua P. Schwarz2,8, Dianna T. Sueper2,3, Ina Tegen6, Simone Tilmes1, Kostas Tsigaridis7,8, Pengfei Yu9,10, and Jose L. Jimenez2,3 Alma Hodzic et al.
  • 1National Center for Atmospheric Research, Boulder, CO, USA
  • 2Cooperative Institute for Research in Environmental Sciences (CIRES), University of Colorado, Boulder, CO, USA
  • 3Department of Chemistry, University of Colorado, Boulder, CO, USA
  • 4NASA Goddard Space Flight Center, Greenbelt, MD, USA
  • 5Department of Atmospheric Science, Colorado State University, Fort Collins, CO, USA
  • 6Leibniz Institute for Tropospheric Research, Leipzig, Germany
  • 7Center for Climate Systems Research, Columbia University, New York, NY, USA
  • 8NASA Goddard Institute for Space Studies, New York, NY, USA
  • 9NOAA Earth System Research Laboratory (ESRL), Chemical Sciences Division, Boulder, CO, USA
  • 10Institute for Environmental and Climate Research, Jinan University, Guangzhou, Guangdong, China

Abstract. The spatial distribution and properties of submicron organic aerosols (OA) are among the key sources of uncertainty in our understanding of aerosol effects on climate. Uncertainties are particularly large over remote regions of the free troposphere and Southern Ocean, where very little data has been available, and where OA predictions from AeroCom Phase II global models span a factor of 400–1000, greatly exceeding the model spread over source regions. The (nearly) pole-to-pole vertical distribution of non-refractory aerosols was measured with an aerosol mass spectrometer onboard the NASA DC8 aircraft as part of the Atmospheric Tomography (ATom) mission during the northern hemisphere summer (August 2016) and winter (February 2017). This study presents the first extensive characterization of OA mass concentrations and their level of oxidation in the remote atmosphere. OA and sulfate are the major contributors by mass to submicron aerosols in the remote troposphere, together with sea salt in the marine boundary layer. Sulfate was dominant in the lower stratosphere. OA concentrations have a strong seasonal and zonal variability, with the highest levels measured in the summer and over the regions influenced by the biomass burning from Africa (up to 10 μg sm−3). Lower concentrations (~ 0.1–0.3 μg sm−3) are observed in the northern mid- and high-latitudes and very low concentrations (< 0.1 μg sm−3) in the southern mid- and high-latitudes. The ATom dataset is used to evaluate predictions of eight current global chemistry models that implement a variety of commonly used representations of OA sources and chemistry, as well as of the AeroCom-II ensemble. The current model ensemble captures the average vertical and spatial distribution of measured OA concentrations, and the spread of the individual models remains within a factor of 5. These results are significantly improved over the AeroCom-II model ensemble, which shows large overestimations over these regions. However, some of the improved agreement with observations occurs for the wrong reasons, as models have the tendency to greatly overestimate the primary OA fraction, and underestimate the secondary fraction. Measured OA in the remote free troposphere are highly oxygenated with organic aerosol to organic carbon (OA / OC) ratios of ~ 2.2–2.8 and are 30–60 % more oxygenated than in current models, which can lead to significant errors in OA concentrations. The model/measurement comparisons presented here support the concept of a more dynamic OA system as proposed by Hodzic et al. (2016), with enhanced removal of primary OA, and a stronger production of secondary OA in global models needed to provide a better agreement with observations.

Alma Hodzic et al.
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Short summary
Organic aerosols (OA) are a key source of uncertainty in aerosol climate effects. We present the first pole-to-pole OA characterization during the NASA Atmospheric Tomography aircraft mission. OA have a strong seasonal & zonal variability, with the highest levels in the summer and over fire-influenced regions, and lowest ones in the southern high-latitudes. We show that global models predict well the OA distribution, but not the relative contribution of OA emissions vs. chemical production.
Organic aerosols (OA) are a key source of uncertainty in aerosol climate effects. We present the...
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