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We have used an AGCM (atmospheric general circulation model)-based Chemistry Transport Model (ACTM) for the simulation of methane (CH 4 ) in the height range of earth's surface to about 90 km. The model simulations are compared with measurements at hourly, daily, monthly and interannual time scales by filtering or averaging all the timeseries appropriately. From this model-observation comparison, we conclude that the recent (1990)(1991)(1992)(1993)(1994)(1995)(1996)(1997)(1998)(1999)(2000)(2001)(2002)(2003)(2004)(2005)(2006) trends in growth rate and seasonal cycle at most measurement sites can be fairly successfully modeled by using existing knowledge of CH 4 flux trends and seasonality. A large part of the interannual variability (IAV) in CH 4 growth rate is apparently controlled by IAV in atmospheric dynamics at the tropical sites and forest fires in the high latitude sites. The flux amplitudes are optimized with respect to the available hydroxyl radical (OH) distribution and model transport for successful reproduction of latitudinal and longitudinal distribution of observed CH 4 mixing ratio at the earth's surface. Estimated atmospheric CH 4 lifetime in this setup is 8.6 years. We found a small impact (less than 0.5 ppb integrated over 1 year) of OH diurnal variation, due to temperature dependence of reaction rate coe‰cient, on CH 4 simulation compared to the transport related variability (order of G15 ppb at interannual timescales). Model-observation comparisons of seasonal cycles, synoptic variations and diurnal cycles are shown to be useful for validating regional flux distribution patterns and strengths. Our results, based on two emission scenarios, suggest reduced emissions from temperate and tropical Asia region (by 13, 5, 3 Tg-CH 4 for India, China and Indonesia, respectively), and compensating increase (by 9, 9, 3 Tg-CH 4 for Russia, United States and Canada, respectively) in the boreal Northern Hemisphere (NH) are required for improved model-observation agreement.

Abstract. Ambient concentrations of four marine-derived halocarbons (methyl iodide, ethyl iodide, bromoform and dibromomethane) and two man-made halocarbons (trichloroethylene and tetrachloroethylene) were measured during western Pacific cruises and east and southeast Asian cruises. Ethyl iodide was detected in the atmosphere for the first time and was identified as an atmospheric iodine source compound. Bromoform concentrations were positively correlated with those of dibromomethane, and methyl iodide showed variations similar to those of ethyl iodide. However, there was no correlation between the bromocarbons and the iodocarbons. The concentrations of methyl iodide and ethyl iodide changed more markedly, possibly owing to higher rates of photodecomposition of iodocarbons.

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.

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