Methyl chloride is the largest natural source of ozone-depleting chlorine compounds, and accounts for about 15 per cent of the present atmospheric chlorine content. This contribution was likely to have been relatively greater in pre-industrial times, when additional anthropogenic sources-such as chlorofluorocarbons-were absent. Although it has been shown that there are large emissions of methyl chloride from coastal lands in the tropics, there remains a substantial shortfall in the overall methyl chloride budget. Here we present observations of large emissions of methyl chloride from some common tropical plants (certain types of ferns and Dipterocarpaceae), ranging from 0.1 to 3.7 microg per gram of dry leaf per hour. On the basis of these preliminary measurements, the methyl chloride flux from Dipterocarpaceae in southeast Asia alone is estimated at 0.91 Tg yr-1, which could explain a large portion of missing methyl chloride sources. With continuing tropical deforestation, natural sources of chlorine compounds may accordingly decrease in the future. Conversely, the abundance of massive ferns in the Carboniferous period may have created an atmosphere rich in methyl chloride.

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.

We investigated seasonal variations in atmospheric CH3I at high, middle, and low latitudes in both hemispheres: at Alert (82.5°N, 62.5°W), over the northern and western Pacific Ocean (shipboard measurements); Cape Ochiishi (43.2°N, 145.5°E); Tsukuba (36.0°N, 140.1°E); Happo Ridge (36.7°N, 137.8°E); Hateruma Island (24.1°N, 123.8°E); San Cristobal Island (1.0°S, 89.4°W); Cape Grim (40.4°S, 144.6°E); and Syowa Station in Antarctica (68.5°S, 41.3°E) throughout 2004 and over an extended period of years at some sites. The highest median CH3I concentration was observed at San Cristobal Island, on the equator in the eastern Pacific, although no concentration elevation was found on or near the equator over the western Pacific. The lowest concentration was found in the Arctic and Antarctica, where they showed very similar seasonal variations that were clearly inversely correlated with variations in incident solar radiation. In the marine boundary air at midlatitude, the CH3I concentration was well correlated with surface seawater temperature (SST). Global atmospheric CH3I in wintertime was rather uniformly distributed (around 0.5 ppt) at middle and high latitudes.

It has been suggested that the emissions of volatile organic compounds (VOCs) from the ocean could be affected by global warming, with feedback effects on the climate. In order to detect changes in their emissions as a response to global environmental change, long‐term observations are required. Here we report for the first time long‐term variations of atmospheric methyl iodide (CH3I), the most abundant iodine‐containing compound predominantly emitted from the ocean. We monitored its concentration periodically at five remote sites covering 82.5°N–40.4°S and over the western and northern Pacific Ocean from the late 1990s to 2011. At most observation sites, CH3I increased from 2003/2004 to 2009/2010 by several tens of per cent, with a decreasing trend before 2003. The inter‐annual variation pattern is well approximated by a sine curve with a period of 11 years and showed a good correlation with the Pacific Decadal Oscillation (PDO), suggesting that CH3I emissions are affected by global‐scale, sea surface temperature (SST)‐related, decadal anomalies. The influence of natural oscillations or environmental change on trace gas emissions from the ocean may be greater than has been thought previously, and these emissions may thus be sensitive to future climate change.

Abstract.The global distribution of atmospheric methyl bromide (CH3Br) obtained from extensive new measurements of atmospheric CH3Br from latitude 82.5øN to 69.1øS, showed a small decrease from mid-to high-latitudes, a gradient between the northern and southern hemispheres with a ratio of 1.2 to 1.3, and occasional high concentrations in the tropics.The observed data and modeled distributions of industrial CH3Br were used to apportion CH3Br between natural and industrial components for both hemisphere. We obtained an estimated man-made contribution of 4.3 pptv and 2.3 pptv in the northern and southern hemispheres, respectively and a natural (non-industrial) background concentration in both hemispheres of 6 pptv with a slight increase in the tropics.