Photochemical pollution control strategies require an understanding of photochemical oxidation precursors, making it important to distinguish between primary and secondary sources of HCHO. Estimates for the relative strengths of primary and secondary sources of formaldehyde (HCHO) were obtained using a statistical regression analysis with time series data of carbon monoxide (CO) and glyoxal (CHOCHO) measured in the Mexico City Metropolitan Area (MCMA) during the spring of 2003. Differences between Easter week and more typical weeks are evaluated. The use of CO-CHOCHO as HCHO tracers is more suitable for differentiating primary and secondary sources than CO-O₃. The application of the CO-O₃ tracer pair to mobile laboratory data suggests a potential in-city source of background HCHO. A significant amount of HCHO observed in the MCMA is associated with primary emissions.

We present a new numerical code, Mexican MAX-DOAS Fit (MMF), developed to retrieve profiles of different trace gases from the network of MAX-DOAS instruments operated in Mexico City. MMF uses differential slant column densities (dSCDs) retrieved with the QDOAS (Danckaert et al., 2013) software. The retrieval is comprised of two steps, an aerosol retrieval and a trace gas retrieval that uses the retrieved aerosol profile in the forward model for the trace gas. For forward model simulations, VLIDORT is used (e.g., Spurr et al., 2001; Spurr, 2006, 2013). Both steps use constrained least-square fitting, but the aerosol retrieval uses Tikhonov regularization and the trace gas retrieval optimal estimation. Aerosol optical depth and scattering properties from the AERONET database, averaged ceilometer data, WRF-Chem model data, and temperature and pressure sounding data are used for different steps in the retrieval chain. The MMF code was applied to retrieve NO₂ profiles with 2 degrees of freedom (DOF = 2) from spectra of the MAX-DOAS instrument located at the Universidad Nacional Autónoma de México (UNAM) campus. We describe the full error analysis of the retrievals and include a sensitivity exercise to quantify the contribution of the uncertainties in the aerosol extinction profiles to the total error. A data set comprised of measurements from January 2015 to July 2016 was processed and the results compared to independent surface measurements. We concentrate on the analysis of four single days and additionally present diurnal and annual variabilities from averaging the 1.5 years of data. The total error, depending on the exact counting, is 14 %–20 % and this work provides new and relevant information about NO₂ in the boundary layer of Mexico City.

A regional study of snow avalanche processes was undertaken in the SE Pyrenees. Dendrogeomorphology was used to date and reconstruct large-scale snow avalanche events that occurred in the last four decades. Dendrochronological analyses yielded the dates of nine winters when avalanches occurred in the recent past in six studied avalanche paths. Some of these avalanches were already known, but others had not been documented. In one case, the existing avalanche path map was improved with the dendrogeomorphological information of a larger past event. As a result of the dendrogeomorphological analyses, evidence for three regional-scale major avalanche years was identified in the SE Pyrenees from 1971 to 2004: 1971–1972, 1995–1996 and 2002–2003. The specific synoptic atmospheric situations and the most likely nivometeorological and snowpack conditions that released these major avalanches were determined using weather data for the seasons of major avalanche releases. In 1971–1972 the snow avalanche episode was characterized by a deep trough crossing the Pyrenees. In 1995–1996 a variety of meteorological situations produced several episodes of major avalanches. In 2002–2003 the more significant of two episodes was attributed to a north advection pumping an arctic air mass over the Pyrenees. The 1995–1996 avalanche season proved to be the most notable in the four past decades in the Pyrenees.

The Tropospheric Monitoring Instrument (TROPOMI) on the ESA Copernicus Sentinel-5 satellite (S5-P) measures carbon monoxide (CO) total column concentrations as one of its primary targets. In this study, we analyze TROPOMI observations over Mexico City in the period 14 November 2017 to 25 August 2019 by means of collocated CO simulations using the regional Weather Research and Forecasting coupled with Chemistry (WRF-Chem) model. We draw conclusions on the emissions from different urban districts in the region. Our WRF-Chem simulation distinguishes CO emissions from the districts Tula, Pachuca, Tulancingo, Toluca, Cuernavaca, Cuautla, Tlaxcala, Puebla, Mexico City, and Mexico City Arena by 10 separate tracers. For the data interpretation, we apply a source inversion approach determining per district the mean emissions and the temporal variability, the latter regularized to reduce the propagation of the instrument noise and forward-model errors in the inversion. In this way, the TROPOMI observations are used to evaluate the Inventario Nacional de Emisiones de Contaminantes Criterio (INEM) inventory that was adapted to the period 2017–2019 using in situ ground-based observations. For the Tula and Pachuca urban areas in the north of Mexico City, we obtain 0.10±0.004 and 0.09±0.005 Tg yr⁻¹ CO emissions, which exceeds significantly the INEM emissions of <0.008 Tg yr⁻¹ for both areas. On the other hand for Mexico City, TROPOMI estimates emissions of 0.14±0.006 Tg yr⁻¹ CO, which is about half of the INEM emissions of 0.25 Tg yr⁻¹, and for the adjacent district Mexico City Arena the emissions are 0.28±0.01 Tg yr⁻¹ according to TROPOMI observations versus 0.14 Tg yr⁻¹ as stated by the INEM inventory. Interestingly, the total emissions of both districts are similar (0.42±0.016 Tg yr⁻¹ TROPOMI versus 0.39 Tg yr⁻¹ adapted INEM emissions). Moreover, for both areas we found that the TROPOMI emission estimates follow a clear weekly cycle with a minimum during the weekend. This agrees well with ground-based in situ measurements from the Secretaría del Medio Ambiente (SEDEMA) and Fourier transform spectrometer column measurements in Mexico City that are operated by the Network for the Detection of Atmospheric Composition Change Infrared Working Group (NDACC-IRWG). Overall, our study demonstrates an approach to deploying the large number of TROPOMI CO data to draw conclusions on urban emissions on sub-city scales for metropolises like Mexico City. Moreover, for the exploitation of TROPOMI CO observations our analysis indicates the clear need for further improvements of regional models like WRF-Chem, in particular with respect to the prediction of the local wind fields.