Abstract
Abstract. Formaldehyde (HCHO) total column densities over the Mexico City metropolitan area (MCMA) were retrieved using two independent measurement techniques: multi-axis differential optical absorption spectroscopy (MAX-DOAS) and Fourier transform infrared (FTIR) spectroscopy. For the MAX-DOAS measurements, the software QDOAS was used to calculate differential slant column densities (dSCDs) from the measured spectra and subsequently the Mexican MAX-DOAS fit (MMF) retrieval code to convert from dSCDs to vertical column densities (VCDs). The direct solar-absorption spectra measured with FTIR were analyzed using the PROFFIT (PROFile FIT) retrieval code. Typically the MAX-DOAS instrument reports higher VCDs than those measured with FTIR, in part due to differences found in the ground-level sensitivities as revealed from the retrieval diagnostics from both instruments, as the FTIR and the MAX-DOAS information do not refer exactly to the same altitudes of the atmosphere. Three MAX-DOAS datasets using measurements conducted towards the east, west or both sides of the measurement plane were evaluated with respect to the FTIR results. The retrieved MAX-DOAS HCHO VCDs where 6 %, 8 % and 28 % larger than the FTIR measurements which, supported with satellite data, indicates a large horizontal inhomogeneity in the HCHO abundances. The temporal change in the vertical distribution of this pollutant, guided by the evolution of the mixing-layer height, affects the comparison of the two retrievals with different sensitivities (total column averaging kernels). In addition to the reported seasonal and diurnal variability of HCHO columns within the urban site, background data from measurements at a high-altitude station, located only 60 km away, are presented.
Highlights
Megacities are in constant evolution, exhibiting continuous changes in territorial extension, population size and spatial redistribution, as well as in the types of socio-economic activities performed every day
The Universidad Nacional Autónoma de México (UNAM) station is equipped with a Fourier transform infrared spectrometer (FTIR) from Bruker Optics that measures solar-absorption spectra at different spectral regions with mercury–cadmium–telluride (MCT) and indium–gallium–arsenide (InGaAs) detectors and five band-pass filters placed on a rotating wheel
HCHO vertical column densities (VCDs) from a highresolution FTIR instrument were measured from the remote site at Altzomoni, providing relevant information about the variability of the background concentrations
Summary
Megacities are in constant evolution, exhibiting continuous changes in territorial extension, population size and spatial redistribution, as well as in the types of socio-economic activities performed every day. In many cases the spatial growth is uneven, resulting in areas of the city being more prone to emissions or accumulation of pollutants due to chemical transformations or transport patterns influenced by meteorological conditions. HCHO VCDs measured at Altzomoni are in the same order of magnitude as HCHO VCDs reported by Vigouroux et al (2018) for several “clean” sites stations belonging to the NDACC network, such as Zugspitze, and other mountain sites ( at a latitude of 47◦ and an altitude of 3 km), as well as for Mauna Loa, at a latitude of 20◦ and an altitude of 3.4 km. Further analysis should be conducted regarding the diurnal HCHO cycle at Altzomoni; the detected maximum at late afternoon could be attributed to upslope transport or to secondary HCHO production that has reached a maximum at a certain hour of the day
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