A strong source of methyl chloride to the atmosphere from tropical coastal land

Abstract

Methyl chloride (CH3Cl), the most abundant halocarbon in the atmosphere, has received much attention as a natural source of chlorine atoms in the stratosphere. The annual global flux of CH3Cl has been estimated to be around 3.5 Tg on the grounds that this must balance the loss through reaction with OH radicals (which gives a lifetime for atmospheric CH3Cl of 1.5 yr). The most likely main source of methyl chloride has been thought to be oceanic emission, with biomass burning the second largest source. But recent seawater measurements indicate that oceanic fluxes cannot account for more than 12% of the estimated global flux of CH3Cl, raising the question of where the remainder comes from. Here we report evidence of significant CH3Cl emission from warm coastal land, particularly from tropical islands. This conclusion is based on a global monitoring study and spot measurements, which show enhancement of atmospheric CH3Cl in the tropics, a close correlation between CH3Cl concentrations and those of biogenic compounds emitted by terrestrial plants, and OH-linked seasonality of CH3Cl concentrations in middle and high latitudes. A strong, equatorially located source of this nature would explain why the distribution of CH3Cl is uniform between the Northern and Southern hemispheres, despite their differences in ocean and land area.

Distribution of methyl chloride, methyl bromide, and methyl iodide in the marine boundary air over the western Pacific and southeastern Indian Ocean.

“…6). Therefore, our finding on the enhancement of atmospheric CH 3 Cl in the present study strongly supports the idea that a significant amount of CH 3 Cl is emitted from tropical coastal lands (Yokouchi et al, 2000a). Although biomass burning could also be a cause of higher CH 3 Cl in the tropics (Crutzen et al, 1979;Talbot et al, 1996), our sam- pling period (December, 1996) was not the burning season in those regions.…”

“…The variations of the three methyl halides differed from one another and from that of C 2 Cl 4 , which decreased gradually from north to south, reflecting a significant industrial emission. For track A-B, a reference dataset from the 39th Japanese Antarctic Research Expedition cruise (R/V Shirase, which sailed from 31°N to 69°S between 14 November and 16 December 1997) (Yokouchi et al, 2000a), covering in part the voyage between lat 31°N and lat 50°S, whose results are added as a gray line in Figs. 2 and 3.…”

“…The ocean (Singh et al, 1983), biomass burning (Lobert et al, 1991) and fungal activity in rotten woods (Harper, 1985) are known sources for this compound. Recent studies have provided evidence that terrestrial systems, especially coastal areas can contribute significantly to CH 3 Cl in the global atmosphere (Yokouchi et al, 2000a; Rhew ing to be accounted for (Yvon and Butler, 1997). Methyl iodide (CH 3 I), being a major natural source of iodine in the atmosphere, is much more reactive than CH 3 Br or CH 3 Cl.…”

Quantum mechanical study of solvent effects in a prototype SN2 reaction in solution: Cl− attack on CH3Cl

“…[18][19][20][21][22] Having been so extensively studied, by both experimental [23][24][25][26][27] and computational [28][29][30][31][32][33][34][35][36] means, lends this class of reactions to be an outstanding starting position for benchmarking and refining new computational methods and techniques. In particular, the chloride/methyl chloride reaction is a strong candidate as a prototype reaction as it has been quite extensively studied in order to understand the exact nature of the transition state in the gas and condensed phases.…”

Determination of chloromethane and dichloromethane in a tropical terrestrial mangrove forest in Brazil by measurements and modelling

“…stratospheric ozone depletion). These halocarbons originate from both natural and anthropogenic sources such as biomass burning, incineration/industrial processes, oceanic emissions, coastal salt marshes and leaf litter (Khalil and Rasmussen, 1999;Lobert et al, 1999;Yokouchi et al, 2000b;Harper, 1985;Moore et al, 1996;Rhew et al, 2000;Blei et al, 2010;Kolusu et al, 2017). The quantification of halocarbons is uncertain.…”