Airborne Vertical Profiling of Mercury Speciation near Tullahoma, TN, USA

Abstract

Atmospheric transport and in situ oxidation are important factors influencing mercury concentrations at the surface and wet and dry deposition rates. Contributions of both natural and anthropogenic processes can significantly impact burdens of mercury on local, regional and global scales. To address these key issues in atmospheric mercury research, airborne measurements of mercury speciation and ancillary parameters were conducted over a region near Tullahoma, Tennessee, USA, from August 2012 to June 2013. Here, for the first time, we present vertical profiles of Hg speciation from aircraft for an annual cycle over the same location. These airborne measurements included gaseous elemental mercury (GEM), gaseous oxidized mercury (GOM) and particulate bound mercury (PBM), as well as ozone (O 3 ), sulfur dioxide (SO 2 ), condensation nuclei (CN) and meteorological parameters. The flights, each lasting ~3 h, were conducted typically one ) were largely associated with air masses coming from west/northwest, while events with low GOM (<20 pg·m ) were generally associated with winds from a wider range of wind directions. This is the first set of speciated mercury vertical profiles collected in a single location over the course of a year. Even though there are current concerns that the KCl denuders used in this study may under-collect GOM, especially in the presence of elevated ozone, the collected data in this region shows the strong seasonality of oxidized mercury concentrations throughout the low to middle free troposphere.

Multi-model study of mercury dispersion in the atmosphere: vertical and interhemispheric distribution of mercury species

“…A combination of ETMEP and CARIBIC observations over Germany resulted in a uniform TM and GEM distribution in the free troposphere during summer and TM concentrations close to those measured at ground level were found on six overflights of the CARIBIC aircraft in April, June, and September. A similar vertical dis- tribution was found in North America during winter (Brooks et al, 2014) and summer Gratz et al, 2015;Shah et al, 2016). In none of these cases a substantial TM gradient was found inside the free troposphere and the GEM / TM ratio was in the range of 0.95-0.99 in the upper free troposphere, which is a ratio typically found inside the PBL.…”

“…In none of these cases a substantial TM gradient was found inside the free troposphere and the GEM / TM ratio was in the range of 0.95-0.99 in the upper free troposphere, which is a ratio typically found inside the PBL. During spring (14 April to 4 June) Brooks et al (2014) consistently found low TM concentrations above 5000 m which indicates a stratospheric intrusion of air masses with low mercury concentrations. Here, the GEM / TM ratio in the upper troposphere decreased to 0.88 to 0.92.…”

“…We created similar average vertical mercury profiles for North America based on 185 hourly samples from three profile flights at Tullahoma, TN, between 18 January and 14 April 2013 (Brooks et al, 2014) (Fig. 4) and 898 hourly samples from seven NOMADSS flights between 20 June and 12 July 2013 (Fig.…”

“…Finally, we created a third profile for spring from three profile flights at Tullahoma, TN, on 15 April, 10 May, and 4 June 2013 (Brooks et al, 2014) (Fig. 6).…”

“…While gaseous elemental mercury (GEM) makes up the vast majority of total atmospheric mercury near the surface (Sprovieri et al, 2016 this issue), recent aircraft-based observations have indicated that there is significant oxidation of mercury occurring in the free troposphere (FT) (Brooks et al, 2014;Lyman and Jaffe, 2012;Jaffe et al, 2014;Gratz et al, 2015;Shah et al, 2016). However, apart from GEM no individual mercury compound has been identified so far and the atmospheric oxidized mercury is an unknown mixture of mercury bound to Br, Cl, OH, O, and NO 2 compounds .…”