Mobile mini-DOAS measurements of the outflow of nitrogen dioxide from the Toluca Valley Metropolitan Area, Mexico

NO2 outflow fluxes were calculated from the City of Toluca, Mexico during January-February 2017 using a mobile zenith viewing mini-DOAS instrument. Measurements were performed in a cross-section of the outflow plume from the Toluca Valley Metropolitan Area with an instrument operating in the visible wavelength region (356 to 510 nm). NO2 retrievals were conducted in the 405 to 465 nm wavelength range. From these measurements, differential vertical columns of NO2 along the measurement route were derived. Using mass-averaged wind speed and wind direction from the Weather Research & Forecasting model, with a 1 km resolution, outflow fluxes of NO2 were calculated from each of the transect measurements. The average NO2 outflow flux for the entire campaign was 1.81 kg/s (156.24 ton/day). To our knowledge, this is the first time that mobile mini-DOAS measurements have been conducted around the City of Toluca and therefore the first time that NO2 outflow fluxes have been reported for this metropolitan area. Comparing our results with emissions inventories we found large variabilities of NOx emissions reported in emissions inventories and between emissions inventories and our measurements. From these discrepancies we infer that emissions inventories underestimate NOx emissions from the TVMA. This study contributes to our understanding of outflow fluxes from metropolitan areas and their possible exchange, being the Toluca Valley Metropolitan Area a few kilometers away from the Mexico City Metropolitan Area, one of the largest megacities of the world.