Methyl chloride in the UT/LS observed by CARIBIC: global distribution, Asian summer monsoon outflow, and use as a tracer for tropical air

Abstract:

We present spatiotemporal variations of methyl chloride (CH₃Cl) in the UT/LS observed mainly by the CARIBIC passenger aircraft 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 year-round, 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 transport through the UT may explain such vertical and latitudinal gradients. Comparisons with 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 CH₃Cl and CO in air influenced by biomass burning in South America and Africa, and the enhancement 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 CH₃Cl emissions from 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.

We also examine the use of CH₃Cl as a tracer of tropical tropospheric air in the LMS, where we identified air masses with elevated CH₃Cl that were however stratospheric in terms of N₂O. Back trajectories suggest recent low-latitude origins of such air masses in early summer. In this season, high CH₃Cl LMS air shows a clear branch connecting stratospheric and tropical tropospheric air on N₂O-CH₃Cl scatterplots. This distinct feature vanishes in late summer when the LMS is ventilated by tropospheric air.