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Methyl chloride in the upper troposphere observed by the CARIBIC passenger aircraft observatory: Large‐scale distributions and Asian summer monsoon outflow
Author(s) -
Umezawa T.,
Baker A. K.,
Oram D.,
Sauvage C.,
O'Sullivan D.,
RautheSchöch A.,
Montzka S. A.,
Zahn A.,
Brenninkmeijer C. A. M.
Publication year - 2014
Publication title -
journal of geophysical research: atmospheres
Language(s) - English
Resource type - Journals
eISSN - 2169-8996
pISSN - 2169-897X
DOI - 10.1002/2013jd021396
Subject(s) - troposphere , anticyclone , atmospheric sciences , middle latitudes , zonal and meridional , mixing ratio , climatology , environmental science , southern hemisphere , outflow , latitude , northern hemisphere , atmosphere (unit) , monsoon , geography , geology , meteorology , geodesy
We present spatial and temporal variations of methyl chloride (CH 3 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 3 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 CH 3 Cl mixing ratios increase toward tropical latitudes, and the observed UT CH 3 Cl level over these two regions and the Atlantic was higher than that measured at remote surface sites. Strong emissions of CH 3 Cl 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 CH 3 Cl in the UT. We also observed elevated mixing ratios of CH 3 Cl and CO in air influenced by biomass burning in South America and Africa, and the enhancement ratios derived for CH 3 Cl to CO in those regions agree with previous observations. In contrast, correlations indicate a high CH 3 Cl to CO ratio of 2.9 ± 0.5 ppt ppb −1 in the Asian summer monsoon anticyclone and domestic biofuel emissions in South Asia are inferred to be responsible. We estimated the CH 3 Cl emission in South Asia to be 134 ± 23 Gg Cl yr −1 , which is higher than a previous estimate due to the higher CH 3 Cl to CO ratio observed in this study.