Climatology and Variability of Methyl Chloride in the UTLS from Aura Microwave Limb Sounder Measurements

Methyl chloride (CH3Cl) is the most abundant chlorine compound in the troposphere and is by far the largest natural source of stratospheric chlorine. Biogenic production by tropical and subtropical plants and biofuel and biomass burning account for most of its emissions. The Microwave Limb Sounder (MLS), launched as part of NASA's Aura mission in July 2004, provides the first daily global observations of CH3Cl. We exploit the unparalleled scope of the nearly nine-year MLS data set to investigate spatial and temporal variability in the distribution of CH3Cl in the upper troposphere / lower stratosphere (UTLS). Seasonal variations in regional fire activity and other sources as well as in the frequency, strength, and location of convection, which can loft pollution from the surface to the UTLS very rapidly, lead to climatological CH3Cl "hotspots" in the MLS record. Apart from these annually recurring patterns, anomalously strong enhancements (or depressions) in tropical upper tropospheric CH3Cl can be linked to specific events in some years, such as large-scale forest fires associated with El Nino conditions. In addition to its utility for tracking the lofting of pollution by deep convection, CH3Cl is valuable as a tracer of large-scale transport and mixing processes in the UTLS.