Diagnosing the Transport of Pollution to the Upper Troposphere / Lower Stratosphere using Aura Microwave Limb Sounder Measurements of Methyl Chloride and Other Trace Gases

Abstract:

Methyl chloride (CH₃Cl) is the largest natural source of chlorine in the stratosphere, and its relative importance in stratospheric ozone chemistry is expected to increase in the coming years as anthropogenic sources of chlorine decline following strict emissions controls. Biomass burning is the second largest source of CH₃Cl (the first being biogenic production by vegetation), and measurements from the Aura Microwave Limb Sounder (MLS) suggest that the seasonal and interannual variability of CH₃Cl in the upper troposphere / lower stratosphere (UTLS) largely reflect variability in biomass burning and vertical transport from source regions by convective activity. We compare the spatial and temporal variability of UTLS CH₃Cl in simulations from the Whole Atmosphere Chemistry Climate Model (WACCM) nudged to the Modern Era Retrospective Reanalysis (MERRA) wind fields to the MLS measurements for 2005-2012. Using sensitivity simulations in which we perturb emissions from various CH₃Cl sources and source regions, we examine what the MLS measurements of CH₃Cl and other trace gases such as CO can tell us about the sources and transport of pollution to the radiatively critical UTLS region.