Document Type

Article

Publication Date

2-10-2016

Publication Title

Atmospheric Chemistry and Physics

ISSN

1680-7324

Volume

16

Issue

3

First Page

1511

Last Page

1530

DOI

http://dx.doi.org/10.5194/acp-16-1511-2016

Abstract

We collected mercury observations as part of the Nitrogen, Oxidants, Mercury, and Aerosol Distributions, Sources, and Sinks (NOMADSS) aircraft campaign over the southeastern US between 1 June and 15 July 2013. We use the GEOS-Chem chemical transport model to interpret these observations and place new constraints on bromine radical initiated mercury oxidation chemistry in the free troposphere. We find that the model reproduces the observed mean concentration of total atmospheric mercury (THg) (observations: 1.49 +/- 0.16 ngm(-3), model: 1.51 +/- 0.08 ngm(-3)), as well as the vertical profile of THg. The majority (65 %) of observations of oxidized mercury (Hg(II)) were below the instrument's detection limit (detection limit per flight: 58-228 pgm(-3)), consistent with model-calculated Hg(II) concentrations of 0-196 pgm(-3). However, for observations above the detection limit we find that modeled Hg(II) concentrations are a factor of 3 too low (observations: 212 +/- 112 pgm-3, model: 67 +/- 44 pgm(-3)). The high-est Hg(II) concentrations, 300-680 pgm(-3), were observed in dry (RH < 35 %) and clean air masses during two flights over Texas at 5-7 km altitude and off the North Carolina coast at 1-3 km. The GEOS-Chem model, back trajectories and observed chemical tracers for these air masses indicate subsidence and transport from the upper and middle troposphere of the subtropical anticyclones, where fast oxidation of elemental mercury (Hg(0)) to Hg(II) and lack of Hg(II) removal lead to efficient accumulation of Hg(II). We hypothesize that the most likely explanation for the model bias is a systematic underestimate of the Hg(0) + Br reaction rate. We find that sensitivity simulations with tripled bromine radical concentrations or a faster oxidation rate constant for Hg(0) + Br, result in 1.5-2 times higher modeled Hg(II) concentrations and improved agreement with the observations. The modeled tropospheric lifetime of Hg(0) against oxidation to Hg(II) decreases from 5 months in the base simulation to 2.8-1.2 months in our sensitivity simulations. In order to maintain the modeled global burden of THg, we need to increase the in-cloud reduction of Hg(II), thus leading to faster chemical cycling between Hg(0) and Hg(II). Observations and model results for the NOMADSS campaign suggest that the subtropical anticyclones are significant global sources of Hg(II).

Comments

AUTHORS

V. Shah (Department of Atmospheric Sciences, University of Washington, Seattle, WA, USA) L. Jaeglé (Department of Atmospheric Sciences, University of Washington, Seattle, WA, USA) L. E. Gratz (Environmental Program, Colorado College, Colorado Springs, CO, USA) J. L. Ambrose (School of Science, Technology, Engineering and Mathematics, University of Washington-Bothell, Bothell, WA, USA) J. L. Ambrose (now at: College of Engineering and Physical Sciences, University of New Hampshire, Durham, NH, USA) D. A. Jaffe (Department of Atmospheric Sciences, University of Washington, Seattle, WA, USA) D. A. Jaffe (School of Science, Technology, Engineering and Mathematics, University of Washington-Bothell, Bothell, WA, USA) N. E. Selin (Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA, USA) S. Song (Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA, USA) T. L. Campos (Earth Observing Laboratory, National Center for Atmospheric Research, Boulder, CO, USA) F. M. Flocke (Earth Observing Laboratory, National Center for Atmospheric Research, Boulder, CO, USA) M. Reeves (Earth Observing Laboratory, National Center for Atmospheric Research, Boulder, CO, USA) D. Stechman (Earth Observing Laboratory, National Center for Atmospheric Research, Boulder, CO, USA) M. Stell (Earth Observing Laboratory, National Center for Atmospheric Research, Boulder, CO, USA) J. Festa (Department of Atmospheric and Oceanic Sciences, University of California, Los Angeles, CA, USA) J. Stutz (Department of Atmospheric and Oceanic Sciences, University of California, Los Angeles, CA, USA) A. J. Weinheimer (Atmospheric Chemistry Observations and Modeling Laboratory, National Center for Atmospheric Research, Boulder, CO, USA) D. J. Knapp (Atmospheric Chemistry Observations and Modeling Laboratory, National Center for Atmospheric Research, Boulder, CO, USA) D. D. Montzka (Atmospheric Chemistry Observations and Modeling Laboratory, National Center for Atmospheric Research, Boulder, CO, USA) G. S. Tyndall (Atmospheric Chemistry Observations and Modeling Laboratory, National Center for Atmospheric Research, Boulder, CO, USA) E. C. Apel (Atmospheric Chemistry Observations and Modeling Laboratory, National Center for Atmospheric Research, Boulder, CO, USA) R. S. Hornbrook (Atmospheric Chemistry Observations and Modeling Laboratory, National Center for Atmospheric Research, Boulder, CO, USA) A. J. Hills (Atmospheric Chemistry Observations and Modeling Laboratory, National Center for Atmospheric Research, Boulder, CO, USA) D. D. Riemer (Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, FL, USA) N. J. Blake (Department of Chemistry, University of California, Irvine, CA, USA) C. A. Cantrell (Department of Atmospheric and Oceanic Sciences, University of Colorado, Boulder, CO, USA) R. L. Mauldin III (Department of Atmospheric and Oceanic Sciences, University of Colorado, Boulder, CO, USA) R. L. Mauldin III (Department of Physics, University of Helsinki, Helsinki, Finland)

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