Articles | Volume 16, issue 6
Atmos. Chem. Phys., 16, 3743–3760, 2016

Special issue: Marine trace gases and aerosols over tropical oceans (AMT/ACP...

Atmos. Chem. Phys., 16, 3743–3760, 2016

Research article 21 Mar 2016

Research article | 21 Mar 2016

Mercury oxidation from bromine chemistry in the free troposphere over the southeastern US

Sean Coburn1,2, Barbara Dix1, Eric Edgerton3, Christopher D. Holmes4, Douglas Kinnison5, Qing Liang6, Arnout ter Schure7, Siyuan Wang1,2,a, and Rainer Volkamer1,2 Sean Coburn et al.
  • 1Department of Chemistry and Biochemistry, University of Colorado Boulder, Boulder, CO, USA
  • 2Cooperative Institute for Research in Environmental Sciences (CIRES), Boulder, CO, USA
  • 3Atmospheric Research and Analysis (ARA) Inc., Plano, TX, USA
  • 4Department of Earth, Ocean and Atmospheric Science, Florida State University, Tallahassee, FL, USA
  • 5National Center for Atmospheric Research (NCAR), Boulder, CO, USA
  • 6NASA Goddard Space Flight Center, Atmospheric Chemistry and Dynamics Branch, Greenbelt, MD 20771, USA
  • 7Electric Power Research Institute (EPRI), Palo Alto, CA, USA
  • anow at: Department of Chemistry, University of Michigan, Ann Arbor, MI, USA

Abstract. The elevated deposition of atmospheric mercury over the southeastern United States is currently not well understood. Here we measure partial columns and vertical profiles of bromine monoxide (BrO) radicals, a key component of mercury oxidation chemistry, to better understand the processes and altitudes at which mercury is being oxidized in the atmosphere. We use data from a ground-based MAX-DOAS instrument located at a coastal site ∼  1 km from the Gulf of Mexico in Gulf Breeze, FL, where we had previously detected tropospheric BrO (Coburn et al., 2011). Our profile retrieval assimilates information about stratospheric BrO from the WACCM chemical transport model (CTM), and uses only measurements at moderately low solar zenith angles (SZAs) to estimate the BrO slant column density contained in the reference spectrum (SCDRef). The approach has 2.6 degrees of freedom, and avoids spectroscopic complications that arise at high SZA; knowledge about SCDRef further helps to maximize sensitivity in the free troposphere (FT). A cloud-free case study day with low aerosol load (9 April 2010) provided optimal conditions for distinguishing marine boundary layer (MBL: 0–1 km) and free-tropospheric (FT: 1–15 km) BrO from the ground. The average daytime tropospheric BrO vertical column density (VCD) of ∼  2.3  ×  1013 molec cm−2 (SZA  <  70°) is consistent with our earlier reports on other days. The vertical profile locates essentially all tropospheric BrO above 4 km, and shows no evidence for BrO inside the MBL (detection limit  <  0.5 pptv). BrO increases to  ∼  3.5 pptv at 10–15 km altitude, consistent with recent aircraft observations. Our case study day is consistent with recent aircraft studies, in that the oxidation of gaseous elemental mercury (GEM) by bromine radicals to form gaseous oxidized mercury (GOM) is the dominant pathway for GEM oxidation throughout the troposphere above Gulf Breeze. The column integral oxidation rates are about 3.6  × 105 molec cm−2 s−1 for bromine, while the contribution from ozone (O3) is 0.8  ×  105 molec cm−2 s−1. Chlorine-induced oxidation is estimated to add  <  5 % to these mercury oxidation rates. The GOM formation rate is sensitive to recently proposed atmospheric scavenging reactions of the HgBr adduct by nitrogen dioxide (NO2), and to a lesser extent also HO2 radicals. Using a 3-D CTM, we find that surface GOM variations are also typical of other days, and are mainly derived from the FT. Bromine chemistry is active in the FT over Gulf Breeze, where it forms water-soluble GOM that is subsequently available for wet scavenging by thunderstorms or transport to the boundary layer.

Short summary
Here we present a day of case study measurements of the vertical distribution of bromine monoxide over the coastal region of the Gulf of Mexico. These measurements are used to assess the contribution of bromine radicals to the oxidation of elemental mercury in the troposphere. We find that the measured levels of bromine in the troposphere are sufficient to quickly oxidize mercury, which has significant implications for our understanding of atmospheric mercury processes.
Final-revised paper