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

Abstract

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 (SCD Ref ). The approach has 2.6 degrees of freedom, and avoids spectroscopic complications that arise at high SZA; knowledge about SCD Ref 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 × 10 13 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 × 10 5 molec cm −2 s −1 for bromine, while the contribution from ozone (O 3 ) is 0.8 × 10 5 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 (NO 2 ), and to a lesser extent also HO 2 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.

BrO and inferred Br&lt;sub&gt;&lt;i&gt;y&lt;/i&gt;&lt;/sub&gt; profiles over the western Pacific: relevance of inorganic bromine sources and a Br&lt;sub&gt;&lt;i&gt;y&lt;/i&gt;&lt;/sub&gt; minimum in the aged tropical tropopause layer

“…Elevated concentrations of BrO (up to 3 ppt) in pristine air masses over the tEPO Wang et al, 2015), and above Florida (Coburn et al, 2016) most likely reflect a variable influence of bromine sources from the lower atmosphere (e.g., SSA) downwind of marine convection (Long et al, 2014;Schmidt et al, 2016). Interestingly, the source region of the TORERO flights observed deep convective outflow that originated over the tWPO warm pool in the area south of the CONTRAST study area, where we observe significant BrO in the MBL consistent with SSA bromine sources.…”

“…Bromine is important for the oxidation of atmospheric mercury (Coburn et al, 2016;Goodsite et al, 2004;Holmes et al, 2006;Parrella et al, 2012), the destruction of ozone (von Glasow et al, 2004;Read et al, 2008;Saiz-Lopez and von Glasow, 2012;Schmidt et al, 2016;Wofsy et al, 1975), and oxidative capacity. Recent first simultaneous measurements of BrO and IO over the entire height of the tropical troposphere suggest that bromine and iodine together account regionally for 34 % of column-integrated ozone loss over the tEPO .…”

“…In particular, the lower ozone leads to a lower production of OH radicals from ozone photolysis, lower NO x leads to a decrease in the OH / HO 2 ratio, and HOBr photolysis increases the OH / HO 2 ratio. Fourth, BrO x is thought to be the primary oxidant of mercury in the atmosphere (Coburn et al, 2016;Goodsite et al, 2004;Holmes et al, 2006) and an important loss mechanism for dimethyl sulfide . Fifth, the net-effect of BrO x impacts on O 3 , NO x , and HO x leads to an increase in the lifetime of CO, hydrocarbons, and climate-active gases such as methane (Lelieveld et al, 1998;Parrella et al, 2012;Saiz-Lopez and von Glasow, 2012).…”

Global impacts of tropospheric halogens (Cl, Br, I) on oxidants and composition in GEOS-Chem

“…This can cause significant O 3 formation through increased RO 2 concentrations (Knipping and Dabdub, 2003), notably in regions with elevated ClNO 2 (Sarwar et al, 2014). Halogens also play an important role in determining the chemistry of mercury (Holmes et al, 2009;Parrella et al, 2012;Wang et al, 2015;Coburn et al, 2016). The literature on tropospheric halogens has been the topic of several recent reviews, which cover the background in more detail Saiz-Lopez et al, 2012b).…”

Subtropical subsidence and surface deposition of oxidized mercury produced in the free troposphere

“…In another study (Myers et al, 2013), Hg present at the upper boundary (5.4 km) of the regional CMAQ model was found to contribute about 40 % to dry deposition and about 80 % to wet deposition in July over the US. Coburn et al (2016) estimated that most of the surface Hg(II) over Florida in April 2010 was produced above 700 hPa. However, these model estimates are limited to specific regions and seasons.…”