Abstract. 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.

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Long-term time series of volcanic plumes composition constitute valuable indicators of the evolution of the magmatic and volcanic systems. We present here a 4-years long time series of molecular ratios of HF/HCl, HCl/SO 2 , SiF 4 /SO 2 , HF/SiF 4 measured in the Popocatépetl ′ s volcanic plume using ground-based solar absorption FTIR spectroscopy. The instrument, based in the Altzomoni NDACC (Network for the Detection of Atmospheric Composition Change) station, facing the Popocatépetl volcano, provides an unrivaled precision. The computed mean and standard deviation of the HF/HCl and HCl/SO 2 ratios for this period were found to be 0.24 ± 0.03 and 0.11 ± 0.03, respectively. SiF 4 was detected in three occasions and the SiF 4 /SO 2 ratios ranged between (1.9 ± 0.5) × 10 −3 and (9.9 ± 0.4) × 10 −3 . The HBr/HCl and HBr/SO 2 ratios remained below their detection limits (1.25 × 10 −4 and 1.25 × 10 −5 , respectively), given that a part of the HBr has already been converted to other bromine species (e.g., BrO, Br 2 ) a few kilometers downwind of the crater. Combining our time series with satellite SO 2 fluxes and seismic data, we explain the significant long-term HCl/SO 2 variations by changes in the conduit and edifice permeabilities, impacting the deep and shallow degassing processes. The high temporal resolution of the data also allows capturing the variation of the volcanic plume composition preceding and induced by a common moderate explosion at Popocatépetl volcano. We interpret the observed variations of the HCl/SO 2 ratio during the explosion in terms of changes in the contribution of the deep/shallow degassing. We additionally report the detection of an increase of SiF 4 after the explosion, likely explained by in-plume HF-ash interaction. During this event, SiF 4 /HCl vs. HF/HCl was found to have a linear relation with a slope of −1/4, which implies a conservation of fluorine.

Continuous carbon monoxide (CO) total column densities above the Universidad Nacional Autónoma de México (UNAM) campus in Mexico City have been derived from solar absorption infrared spectroscopic measurements since October 2007. Its diurnal evolution is used in the present study in conjunction with other ground-based and satellite data to develop a top-down emission estimate of the annual CO emission of the Mexico City Metropolitan Area (MCMA). The growth-rate of the total column around noon under low ventilation conditions is calculated and allows us to derive the average surface emission-flux at UNAM, while similar measurements taken at the edge of the MCMA in Tecámac provide information on background CO levels in the Mexico basin. Based on 3 yr of measurements, CO column measurements from the Infrared Atmospheric Sounding Interferometer (IASI) satellite instrument are used to reconstruct the spatial distribution of this anthropogenic pollutant over the MCMA. The agreement between the measured columns of the satellite and ground-based measurements is excellent, particularly when a comparison strategy based on time-displaced air masses is used. The annual emission of the Mexico Megacity is estimated to be (2.15 ± 0.5) Tg yr⁻¹ for the year 2008, while the official inventory for that year reported 1.6 Tg yr⁻¹. The difference is slightly higher than the conservative uncertainty estimated in this work suggesting that the emission might be underestimated by the conventional bottom-up method. A larger discrepancy is found in the spatial distribution of the emissions, when comparing the emission flux over UNAM (derived from the ground-based measurement) with that of the inventory integrated over a representative area. The methodology presented here represents a new and useful strategy to evaluate the contribution of megacities to the global anthropogenic gas emissions. Additionally, three different strategies to compare ground and space-based measurements above an inhomogeneous and strongly contaminated area like Mexico City are presented and discussed

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