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

机译：在美国东南部的自由对流层中氧化汞氧化

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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 aEuro parts per thousand aEuro-1aEuro-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: 0-1aEuro-km) and free-tropospheric (FT: 1-15aEuro-km) BrO from the ground. The average daytime tropospheric BrO vertical column density (VCD) of aEuro parts per thousand aEuro-2.3aEuro-aEuro parts per thousand x aEuro-10(13)aEuro-molecaEuro-cm(-2) (SZAaEuro-aEuro parts per thousand < aEuro-70 degrees) is consistent with our earlier reports on other days. The vertical profile locates essentially all tropospheric BrO above 4aEuro-km, and shows no evidence for BrO inside the MBL (detection limitaEuro-aEuro parts per thousand < aEuro-0.5aEuro-pptv). BrO increases to aEuro parts per thousand aEuro-3.5aEuro-pptv at 10-15aEuro-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 3.6aEuro-aEuro parts per thousand xaEuro-10(5)aEuro-molecaEuro-cm(-2)aEuro-s(-1) for bromine, while the contribution from ozone (O-3) is 0.8aEuro-aEuro parts per thousand x aEuro-10(5)aEuro-molecaEuro-cm(-2)aEuro-s(-1). Chlorine-induced oxidation is estimated to add < aEuro-5aEuro-% 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.
机译：目前尚未清楚地理解，在美国东南部大气汞的沉积升高。在这里，我们测量溴氧化物（兄弟）自由基的部分柱和垂直轮廓，汞氧化化学的关键组分，以更好地了解汞在大气中被氧化的过程和高度。我们使用位于沿海地区的沿海地区的基于地面MAX-DOAS仪器的数据从海湾微风中的墨西哥湾的Aeuro-1aeuro-km，我们之前检测到过博罗（Coburn等）。 2011）。我们的个人资料检索从WACCM化学传输模型（CTM）中同化了有关平流层兄弟的信息，并且仅在适度的低太阳能天顶角（SZAS）上的测量来估计参考光谱（SCDREF）中包含的兄弟级柱密度。该方法具有2.6度的自由度，避免高SZA出现的光谱并发症;关于SCDREF的知识进一步有助于最大化自由对流层（FT）的敏感性。具有低气溶胶载荷的无云案例研究日（2010年4月9日）为区分海洋边界层（MBL：0-1Aeuro-Km）和自由 - 从地面的自由散发（FT：1-15Aeuro-km）兄弟提供了最佳条件。 Aeuro-2.3aeuro-aeuro零件Aeuro-2.3aeuro-Aeuro零件Aeuro-2.3Aeuro-10（13）Aeuro-Sologaeuro-cm（szaaeuro-aeuro零件（Szaaeuro-aeuro零件）（Szaaeuro-aeuro零件‰ -70度）与我们的早期报告相一致。垂直轮廓基本上位于4Aeuro-Km以上的所有对流层兄弟，并且没有显示MBL内部的兄弟的证据（检测Limitaeuro-Aeuro零件‰，百万

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