Mobile mini-DOAS measurement of the outflow of NO₂ and HCHO from Mexico City

Abstract. We here present the results from mobile measurements using two ground-based zenith viewing Differential Optical Absorption Spectroscopy (DOAS) instruments. The measurement was performed in a cross-section of the plume from the Mexico City Metropolitan Area (MCMA) on 10 March 2006 as part of the MILAGRO field campaign. The two instruments operated in the UV and the visible wavelength region respectively and have been used to derive the differential vertical columns of HCHO and NO₂ above the measurement route. This is the first time the mobile mini-DOAS instrument has been able to measure HCHO, one of the chemically most important and interesting gases in the polluted urban atmosphere.

Using a mass-averaged wind speed and wind direction from the WRF model the instantaneous flux of HCHO and NO₂ has been calculated from the measurements and the results are compared to the CAMx chemical model. The calculated flux through the measured cross-section was 1.9 (1.5–2.2) kg/s of HCHO and 4.4 (4.0–5.0) kg/s of NO₂ using the UV instrument and 3.66 (3.63–3.73) kg/s of NO₂ using the visible light instrument. The modeled values from CAMx for the outflow of both NO₂ and HCHO, 1.1 and 3.6 kg/s, respectively, show a reasonable agreement with the measurement derived fluxes.