Measurement report: Evolution and distribution of NH$_3$ over Mexico City from ground-based and satellite infrared spectroscopic measurements

Abstract

Ammonia (NH$_3$) is the most abundant alkaline compound in the atmosphere, with consequences for the environment, human health, and radiative forcing. In urban environments, it is known to play a key role in the formation of secondary aerosols through its reactions with nitric and sulfuric acids. However, there are only a few studies about NH$_3$ in Mexico City. In this work, atmospheric NH$_3$ was measured over Mexico City between 2012 and 2020 by means of ground based solar absorption spectroscopy using Fourier transform infrared (FTIR) spectrometers at two sites (urban and remote). Total columns of NH$_3$ were retrieved from the FTIR spectra and compared with data obtained from the Infrared Atmospheric Sounding Interferometer (IASI) satellite instrument. The diurnal variability of NH$_3$ differs between the two FTIR stations and is strongly influenced by the urban sources. Most of the NH$_3$ measured at the urban station is from local sources, while the NH$_3$ observed at the remote site is most likely transported from the city and surrounding areas. The evolution of the boundary layer and the temperature play a significant role in the recorded seasonal and diurnal patterns of NH$_3$. Although the vertical columns of NH$_3$ are much larger at the urban station, the observed annual cycles are similar for both stations, with the largest values in the warm months, such as April and May. The IASI measurements underestimate the FTIR NH$_3$ total columns by an average of 32.2 ± 27.5 % but exhibit similar temporal variability. The NH$_3$ spatial distribution from IASI shows the largest columns in the northeast part of the city. In general, NH$_3$ total columns over Mexico City measured at the FTIR stations exhibited an average annual increase of 92 ± 3.9 × 10$^{13}$ molecules cm$^{−2}$ yr$^{−1}$ (urban, from 2012 to 2019) and 8.4 ± 1.4 × 10$^{13}$ molecules cm$^{−2}$ yr$^{−1}$ (re- mote, from 2012 to 2020), while IASI data within 20 km of the urban station exhibited an average annual increase of 38 ± 7.6 × 10$^{13}$ molecules cm$^{−2}$ yr$^{−1}$ from 2008 to 2018