Terrestrial and Marine Sources of Methyl Iodide in the Rural Southeast

Methyl iodide (CH3I) is the most abundant organo-iodine compound in the atmosphere and is influential in aerosol formation and tropospheric ozone depletion. It is used in the production of pesticides and pharmaceuticals and is also produced biogenically by marine algae and terrestrial vegetation and soils. Short and long-term methyl iodide exposure via inhalation can be harmful to humans. In order to investigate the sources and distributions of methyl iodide in the rural southeastern U.S, a two-week long campaign was conducted at the Appalachian Atmospheric Interdisciplinary Research (AppalAIR) site in Boone, NC, from June 15 through July 2, 2013. Whole air samples were collected hourly and analyzed for a comprehensive suite of volatile organic compounds (VOCs), including methyl iodide. In this paper, relative levels of methyl iodide are compared to other terrestrial regions in the U.S. and anthropogenic and biogenic tracers are used to elucidate the summertime sources of methyl iodide observed in the rural southeast. 1.0 Introduction Methyl iodide (CH3I) is an important intermediate for aerosol formation and also plays a key role in tropospheric ozone depletion. It is produced predominately by marine phytoplankton, at levels around 2 Tg/year [1]. Methyl iodide and bromoform (CHBr3) are sources of the halogen oxide radicals IO and BrO. These radicals influence oxidation processes in the troposphere, as their formation consumes ozone both in the troposphere and the stratosphere. During strong atmospheric uplift in the equatorial regions, methyl iodide and bromoform can reach the stratosphere and contribute to ozone depletion, via the formation of halogen oxide radicals. Most previous studies have focused on marine sources of methyl iodide, with levels measured in coastal regions and marine environments. Sive et al. (2007) [2] investigated terrestrial sources of methyl iodide at Thompson Farm in Durham, New Hampshire and Duke Forest in Durham, North Carolina, and found terrestrial fluxes of methyl iodide of 2,655 ng m-2 d-1 and 1,790 ng m-2 d-1, respectively. These values are comparable to the estimated oceanic flux of ~2,600 ng m-2 d-1 [2]. In an effort to expand this investigation, several key tracers are used to deduce the source of methyl iodide observed in the sampling area; these gases include trichloroethene (C2HCl3) and tetrachloroethene (C2Cl4), isoprene (C5H8), bromoform (CHBr3), and ethene (C2H4). Tetrachloroethene has a longer lifetime in the atmosphere, with a lifetime of ~96 days as opposed to trichloroethene, which has an atmospheric lifetime of 4-15 days [3]. The atmospheric ratio of trichloroethene to tetrachloroethene can indicate the proximity of the anthropogenic source. Trichloroethene and tetrachloroethene are both used as industrial solvents and are produced predominately in mid-northern latitudes. If the ratio is small, then trichloroethylene has had time to be removed from the air mass being sampled, which indicates a long travel time and distance for the solvents. If the ratio is high, then the source is in close proximity, as trichloroethylene has not had time to be removed from the air