Methanol-CO correlations in Mexico City pollution outflow from aircraft and satellite during MILAGRO

Abstract. The correlation between methanol (CH₃OH) and carbon monoxide (CO) is of particular interest for characterizing biogenic and anthropogenic emission sources of CH₃OH and other chemical species. Here, the CH₃OH/CO enhancement ratio (ΔCH₃OH/ΔCO) in the lower to middle troposphere is examined using coincident CH₃OH and CO observations from aircraft (NCAR C-130 and NASA DC-8) and from the Tropospheric Emission Spectrometer (TES) satellite during the MegaCity Initiative: Local and Global Research Observations (MILAGRO) in the Mexico City region in March 2006. ΔCH₃OH/ΔCO ratios from the two in-situ aircraft measurements are far higher than previously reported CH₃OH emission ratios relative to CO from US cities. This may reflect combustion of different fuel types in this area, and possibly photochemical production of CH₃OH in Mexico City outflow. TES CH₃OH and CO retrievals over the MILAGRO domain show relatively high sensitivity in the 600–800 hPa range, associated with Mexico City pollution outflow. The TES derived ΔCH₃OH/ΔCO ratios during MILAGRO are 18–24 ppt ppb⁻¹, which are similar to those observed from the DC-8 (26–39 ppt ppb⁻¹), but lower than the C-130 observations (41–55 ppt ppb⁻¹). Differences between the ΔCH₃OH/ΔCO ratios measured aboard the two aircraft preclude an absolute validation of the TES-derived ratios for this dataset. The ΔCH₃OH/ΔCO ratios observed from TES over this domain reflect bulk enhancements of CH₃OH and CO in Mexico City outflow. Although the TES measurements are not expected to resolve small-scale variability in the ΔCH₃OH/ΔCO ratio downwind of the strong source region of Mexico City, it is demonstrated that TES can clearly distinguish differences in the ΔCH₃OH/ΔCO ratio due to different source categories of CH₃OH. An example of this is shown by contrasting measurements over Mexico City (strong anthropogenic emissions) with those over the Amazon Basin (strong biogenic emissions). The results from this case study show the potential to gain insight into global sources of CH₃OH and related species from satellite observations, especially for regions and time periods where no in situ measurements are available.