**1. Introduction**

Greater Mexico City (GMC) is one of the largest cities in the world, with more than 22 million inhabitants [1]. The population density in the capital of the country is 6163.3 inhabitants per square kilometer. Municipalities located to the east and north of GMC (e.g., Iztapalapa and Gustavo A. Madero) are more populated than those in the south (e.g., Milpa Alta). The combined activity of vehicles and industries consumes more than 45 million liters of petroleum fuel per day, generating thousands of tons of pollutants.

The labor dynamics of the Mexico City metropolitan area include long daily commutes in search of better salaries and working conditions. The cause of the mobility toward the central metropolis is explained by the diversity and specialization of the labor market, as well as the better salaries offered there. Mobility requires the use of public or private transport, whose emissions are difficult to control. Emissions from industry or vehicular traffic, when combined with the climatic and topographic characteristics of the region, lead to the production of pollutants [2]. Ozone (O3) and particulate matter (PM10 and PM2.5) are two criteria pollutants whose concentrations have remained elevated over time, creating environmental contingencies in GMC.

The formation of O3 from pollutants has been studied, based on physics, chemistry, and statistics [3–5]. Ozone is formed by the photochemical reaction of volatile organic compounds (VOCs) and nitrogen oxides (NOx). Previous studies have reported that the formation of O3 is sensitive to VOCs in the GMC, explaining the decrease in the rate of titration of O3 by NO and the decrease in NOx in a VOC-limited environment. Control strategies to reduce VOC emissions will decrease ozone concentrations in VOC-limited regimes but increase their formation and concentration in NOx-limited areas [6].

Velasco et al. [7] found that in GMC, the release of heat stored in the urban surface forms a shallow stable layer (~200 m) near the ground, which favors the stagnation of nocturnal emissions. Strong inversion layers occur in the atmosphere of GMC during the night and early morning hours. After sunrise, surface heating favors convection and layer mixing, then the O3 balance depends on the photochemical production of the pollutant, the entrainment from the upper residual layer, and the destruction by titration with nitric oxide.

No reduction in ozone concentrations above GMC is observed on weekends when the number of cars on the roads is lower than during the week. Enforcement of traffic rules that restrict car circulation (with the goal of NOx reduction) during environmental contingencies does not necessarily reduce ozone production [8]. Improving air quality in the GMC requires the implementation of comprehensive measurements at the regional scale.

Lei et al. [5] explain that the O3 formation characteristics and sensitivity to emission changes were found to be weakly dependent on the meteorological conditions for GMC. The O3 formation is sensitive to NOx and VOC levels and to the photochemical plume transport pathway.

During the COVID-19 pandemic shutdown, emissions of primary criteria pollutants at GMC were significantly reduced; however, the daily mean ozone concentration profile was similar to that of previous non-pandemic years. The reductions in NOx were so drastic that ozone formation quickly shifted from a VOC-sensitive regime to a NOx-sensitive regime. A VOC-sensitive regime means that an increase in VOC leads to an increase in O3, while an increase in NOx leads to a decrease of O3; this regime is typical of densely populated urban atmospheres such as GMC [9].

Dust particles contribute to PM10 concentrations in GMC, particularly in the northeastern part of the city, where geologic material from the dry Lake of Texcoco is a dust source [10, 11]. This source alone generated about 80% of the total coarse particles measured in the northeastern GMC during exceptional dust events [12]. Another source of dust is the agricultural areas of Tenango del Aire and Chalco. These regions affected the central, southern, and southeastern parts of the city and contributed about 75% of the total coarse particles [12]. PM2.5 and PM10 affect the air quality during the rainy season when rain removes pollutants from the air. Bare soil and traffic-related conditions from February to May also contribute to the increasing concentration of PM2.5 and PM10 [13].

In this chapter, we evaluate the impact of policy interventions on anthropogenic pollution sources in the air quality of Mexico City. First, ground-based particulate matter (PM10, PM2.5) and ozone (O3) concentration data are used to define the diurnal and annual cycle of air pollution concentrations in Mexico City. Then, the effect of

*Air Quality in Mexico City after Mayor Public Policy Intervention DOI: http://dx.doi.org/10.5772/intechopen.111558*

the mobility restrictions enacted in March 2020 to stop the spread of the COVID-19 pandemic is evaluated through trend analysis of the time series of the pollutants from 2012 to 2022. Finally, the number of days exceeding the National Air Quality Standard is presented to evaluate the effect of reducing anthropogenic sources of pollution during March-April 2020.
