[A-AS22_1AM2] Atmospheric Chemistry

[AAS22-17] Methyl chloride in the upper troposphere observed by CARIBIC: large-scale distributions and Asian summer monsoon outflow

*Taku UMEZAWA¹, Angela K. BAKER¹, David ORAM², Carina SAUVAGE¹, Debbie O'SULLIVAN², Armin RAUTHE-SCHOECH¹, Stephen A. MONTZKA³, Andreas ZAHN⁴, Carl A.M. BRENNINKMEIJER¹ (1.Max Planck Institute for Chemistry, 2.National Centre for Atmospheric Science, School of Environmental Sciences, University of East Anglia, 3.Earth System Research Laboratory, NOAA, 4.Institute for Meteorology and Climate Research, Karlsruhe Institute of Technology)

Keywords:CARIBIC, aircraft observation, methyl chloride, upper troposphere

CARIBIC is a flying observatory onboard a Lufthansa A340-600 aircraft that observes various atmospheric compounds at almost monthly intervals. In this study, we present spatial and temporal variations of methyl chloride (CH₃Cl) in the upper troposphere (UT) observed mainly by CARIBIC for the years 2005-2011. The CH₃Cl 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 NH mid latitudes. A series of flights over Africa and South Asia show that CH₃Cl mixing ratios increase toward tropical latitudes, and the observed UT CH₃Cl level over these two regions and the Atlantic was higher than that measured at remote surface sites. Strong emissions of CH₃Cl 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 non-combustion sources in the tropics dominantly contribute to forming the latitudinal gradient of CH₃Cl in the UT. We also observed elevated mixing ratios of CH₃Cl and CO in air influenced by biomass burning in South America and Africa, and the emission ratios derived for CH₃Cl to CO in those regions agree with previous observations. In contrast, correlations indicate a high CH₃Cl 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 CH₃Cl emission in South Asia to be 134±23 Gg Cl yr^-1, which is higher than a previous estimate due to the higher CH₃Cl to CO ratio observed in this study.