Solar absorption infrared spectroscopic measurements over Mexico City: Methane enhancements

In this contribution, the experiment for performing solar-absorption infrared measurements from UNAM's Atmospheric Observatory (Universidad Nacional Autononal de Mexico) located at the in the University campus in Mexico City is described. The Fourier Transform InfRared (FTIR) spectrometer and solar-tracker system are operating since June 2010 and from the recorded spectra, the total column amounts of several atmospheric gases can be derived. The current study presents the results obtained for methane (CH 4 ), an important pollutant involved in ozone production and a rapid increasing greenhouse gas. The total column densities retrieved with high temporal resolution, present a large dispersion and day-to-day variability. A mean value of 2.92×10 19 molecules/cm 2 (1.825 ppm), with a 95% confidence interval between 2.66 and 3.19×10 19 molecules/cm 2 , have been obtained for the period 06/2010 to 12/2011. No clear annual cycle can be determined from the monthly means due to the large variability in the measurements, suggesting a significant effect of the local emissions to the natural background concentrations. Some days with extraordinary enhancements are presented and a simple back trajectory analysis points to a predominant source direction from the Northeast of the measurement site. The methane contaminated air masses passing over the UNAM Atmospheric Observatory, however, originate presumably not from one but several dispersed sources. A more detailed analysis with modeling of the dynamics of these air masses is required.