We have identified terrestrial sources of methyl iodide (CH3I) and assessed their importance in its atmospheric budget using a synthesis of field observations. Measurements include those from NASA DC-8 research flights over the United States and the North Atlantic, the AIRMAP long-term ground-observing network in New England, and a field campaign at Duke Forest, North Carolina. We found an average CH3I flux of ∼2,700 ng m-2 d-1 to the atmosphere from midlatitude vegetation and soils, a value similar in magnitude to previous estimates of the oceanic source strength. The large-scale aircraft measurements of vertical profiles over the continental U.S. showed CH3I-mixing ratios comparable to and greater than those observed over the North Atlantic. Overall, midlatitude terrestrial biomes appear to contribute 33 Gg yr-1 to the CH3I global budget. Copyright 2007 by the American Geophysical Union.

Gas phase elemental mercury (Hg°) was measured aboard the NASA DC-8 research aircraft during the Arctic Research of the Composition of the Troposphere from Aircraft and Satellites (ARCTAS) campaign conducted in spring 2008 primarily over the North American Arctic. We examined the vertical distributions of Hg° and ozone (O3) together with tetrachloroethylene (C2Cl4), ethyne (C2H 2), and alkanes when Hg°- and O3-depleted air masses were sampled near the surface (<1 km). This study suggests that Hg° and O3 depletions commonly occur over linear distances of ∼20-200 km. Horizontally there was a sharp decreasing gradient of ~100 ppqv in Hg° over <10 km in going from the bay near Ellesmere Island to the frozen open ocean. There was a distinct land-ocean difference in the vertical thickness of the Hg°-depleted layer - being variable but around a few 100 meters over the ocean whereas occurring only very near the surface over land. Data support that atmospheric mercury depletion events are driven by Hg° reactions with halogen atoms. Derived from data collected aboard the DC-8, we present mathematical expressions giving the rates of Hg° and O3 depletions as a function of the radical concentrations. These relationships can be a useful metric to evaluate models that attempt to reproduce springtime Arctic Hg° and/or O3 depletion events, and they can also be employed to obtain order-of-magnitude estimates of radical concentrations and the ratio [Br]/[Cl]. © 2011 The Author(s).

Atmospheric measurements of bromoform (CHBr3) and dibromomethane (CH2Br2) were conducted at two sites, Thompson Farm (TF) in Durham, New Hampshire (summer 2002-2004), and Appledore Island (AI), Maine (summer 2004). Elevated mixing ratios of CHBr3 were frequently observed at both sites, with maxima of 37.9 parts per trillion by volume (pptv) and 47.4 pptv for TF and AI, respectively. Average mixing ratios of CHBr3 and CH2Br2 at TF for all three summers ranged from 5.3-6.3 and 1.3-2.3 pptv, respectively. The average mixing ratios of both gases were higher at AI during 2004, consistent with AI's proximity to sources of these bromocarbons. Strong negative vertical gradients in the atmosphere corroborated local sources of these gases at the surface. At AI, CHBr3 and CH2Br2 mixing ratios increased with wind speed via sea-to-air transfer from supersaturated coastal waters. Large enhancements of CHBr3 and CH2Br2 were observed at both sites from 10 to 14 August 2004, coinciding with the passage of Tropical Storm Bonnie. During this period, fluxes of CHBr3 and CH2Br3 were 52.4 ± 21.0 and 9.1 ± 3.1 nmol m^-2 h^-1, respectively. The average fluxes of CHBr3 and CH2Br2 during nonevent periods were 18.9 ± 12.3 and 2.6 ± 1.9 nmol m^-2 h^-1, respectively. Additionally, CHBr3 and CH2Br2 were used as marine tracers in case studies to (1) evaluate the impact of tropical storms on emissions and distributions of marine-derived gases in the coastal region and (2) characterize the transport of air masses during pollution episodes in the northeastern United States. Copyright 2008 by the American Geophysical Union.