Novel approaches to improve estimates of short-lived halocarbon emissions during summer from the Southern Ocean using airborne observations

• Main Authors: E. Asher, R. S. Hornbrook, B. B. Stephens, D. Kinnison, E. J. Morgan, R. F. Keeling, E. L. Atlas, S. M. Schauffler, S. Tilmes, E. A. Kort, M. S. Hoecker-Martínez, M. C. Long, J.-F. Lamarque, A. Saiz-Lopez, K. McKain, C. Sweeney, A. J. Hills, E. C. Apel
• Format: Article
• Language: English
• Published: Copernicus Publications 2019-11-01
• Series: Atmospheric Chemistry and Physics
• Online Access: https://www.atmos-chem-phys.net/19/14071/2019/acp-19-14071-2019.pdf

<p>Fluxes of halogenated volatile organic compounds (VOCs) over the Southern Ocean remain poorly understood, and few atmospheric measurements exist to constrain modeled emissions of these compounds. We present observations of <span class="inline-formula">CHBr₃</span>, <span class="inline-formula">CH₂Br₂</span>, <span class="inline-formula">CH₃I</span>, <span class="inline-formula">CHClBr₂</span>, <span class="inline-formula">CHBrCl₂</span>, and <span class="inline-formula">CH₃Br</span> during the <span class="inline-formula">O₂∕N₂</span> Ratio and <span class="inline-formula">CO₂</span> Airborne Southern Ocean (ORCAS) study and the second Atmospheric Tomography mission (ATom-2) in January and February of 2016 and 2017. Good model–measurement correlations were obtained between these observations and simulations from the Community Earth System Model (CESM) atmospheric component with chemistry (CAM-Chem) for <span class="inline-formula">CHBr₃</span>, <span class="inline-formula">CH₂Br₂</span>, <span class="inline-formula">CH₃I</span>, and <span class="inline-formula">CHClBr₂</span> but all showed significant differences in model&thinsp;:&thinsp;measurement ratios. The model&thinsp;:&thinsp;measurement comparison for <span class="inline-formula">CH₃Br</span> was satisfactory and for <span class="inline-formula">CHBrCl₂</span> the low levels present precluded us from making a complete assessment. Thereafter, we demonstrate two novel approaches to estimate halogenated VOC fluxes; the first approach takes advantage of the robust relationships that were found between airborne observations of <span class="inline-formula">O₂</span> and <span class="inline-formula">CHBr₃</span>, <span class="inline-formula">CH₂Br₂</span>, and <span class="inline-formula">CHClBr₂</span>. We use these linear regressions with <span class="inline-formula">O₂</span> and modeled <span class="inline-formula">O₂</span> distributions to infer a biological flux of halogenated VOCs. The second approach uses the Stochastic Time-Inverted Lagrangian Transport (STILT) particle dispersion model to explore the relationships between observed mixing ratios and the product of the upstream surface influence of sea ice, chl <span class="inline-formula"><i>a</i></span>, absorption due to detritus, and downward shortwave radiation at the surface, which in turn relate to various regional hypothesized sources of halogenated VOCs such as marine phytoplankton, phytoplankton in sea-ice brines, and decomposing organic matter in surface seawater. These relationships can help evaluate the likelihood of particular halogenated VOC sources and in the case of statistically significant correlations, such as was found for <span class="inline-formula">CH₃I</span>, may be used to derive an estimated flux field. Our results are consistent with a biogenic regional source of <span class="inline-formula">CHBr₃</span> and both nonbiological and biological sources of <span class="inline-formula">CH₃I</span> over these regions.</p>