Methyl chloride in the upper troposphere observed by the CARIBIC passenger aircraft observatory: Large-scale distributions and Asian summer monsoon outflow

We present spatial and temporal variations of methyl chloride (CH Cl) in the upper troposphere (UT) observed mainly by the Civil Aircraft for Regular Investigation of the atmosphere Based on an Instrument Container (CARIBIC) passenger aircraft for the years 2005-2011. The CH Cl mixing ratio in the UT over Europe was higher than that observed at a European surface baseline station throughout the year, indicative of a persistent positive vertical gradient at Northern Hemisphere midlatitudes. A series of flights over Africa and South Asia show that CHCl mixing ratios increase toward tropical latitudes, and the observed UT CHCl level over these two regions and the Atlantic was higher than that measured at remote surface sites. Strong emissions of CHCl in the tropics combined with meridional air transport through the UT may explain such vertical and latitudinal gradients. Comparisons with carbon monoxide (CO) data indicate that noncombustion sources in the tropics dominantly contribute to forming the latitudinal gradient of CHCl in the UT. We also observed elevated mixing ratios of CHCl and CO in air influenced by biomass burning in South America and Africa, and the enhancement ratios derived for CH Cl to CO in those regions agree with previous observations. In contrast, correlations indicate a high CHCl to CO ratio of 2.9 ± 0.5 ppt ppb in the Asian summer monsoon anticyclone and domestic biofuel emissions in South Asia are inferred to be responsible. We estimated the CHCl emission in South Asia to be 134 ± 23 Gg Cl yr, which is higher than a previous estimate due to the higher CHCl to CO ratio observed in this study.