Total column of carbon monoxide above Mexico City

Atmospheric Chemistry and Physics Discussions This discussion paper is/has been under review for the journal Atmospheric Chemistry and Physics (ACP). Please refer to the corresponding final paper in ACP if available. Abstract Carbon monoxide (CO) is a main pollutant in urban agglomerations. Quantifying the total burden of this pollutant in a megacity is challaging because not only its surface concentration but also its vertical dispersion present different behaviours and high variability. The diurnal trend of columnar CO in the boundary layer of Mexico City has been 5 measured during various days with ground-based infrared absorption spectroscopy. Daytime CO total columns are retrieved from solar spectra and for the first time, nocturnal CO total columns using moonlight have been retrieved within a megacity. The measurements were taken at the Universidad Nacional Autónoma de M ´ exico (UNAM) campus located in Mexico City (19.33 until February 2008 with a Fourier-transform infrared spectrometer at 0.5 cm −1 resolution. The atmospheric CO background column was measured from the high altitude site Altzomoni (19.12 • N, 98.65 • W, 4010 m a.s.l.) located 60 km southeast of Mexico City. The total CO column within the city presents large variations that are caused mainly by fresh CO emissions at the surface, but also the transport of cleaner or more polluted 15 air masses within the field-of-view of the instrument and other processes contribute to its variability. The mean background value above the boundary mixing layer was found to be around (1.2±0.2)×10 18 molecules/cm 2 , while inside the city, the late morning mean on weekdays and Sundays was found to be (3.2±0.3)×10 18 molecules/cm 2 and (2.1±0.4)×10 18 molecules/cm 2 , respectively. Continuous CO column retrieval dur-20 ing the day and night (when available), in conjunction with surface CO measurements, allow for a reconstruction of the effective mixing layer height. The limitations from this simplified approach, as well as the potential of using continuous column measurements in order to derive top-down CO emissions from a large urban area, are discussed. Also, further monitoring will provide more insight in daily and weekly emission patterns and 25 a usable database for the quantitative validation of CO from satellite observations in a megacity.