**Abstract**

Air pollution can be produced from anthropogenic or natural sources. Most of the policies enacted to improve air quality focus on reducing anthropogenic sources of pollution, but if natural sources increase, then air quality does not improve with these policies. In this chapter, we first define the diurnal and monthly cycle of particulate matter and ozone concentration, depending on the weather, using data from air quality monitoring stations from Greater Mexico City. We then look at a mayor public policy intervention during the COVID-19 pandemic that drastically reduced anthropogenic sources of PM but did not reduce natural sources by doing robust trend analysis on air quality station data. We evaluate the effect of these interventions by looking at national air quality standards and the number of days air pollutants have been within recommended levels. The results show that during lockdown, air quality improved because less anthropogenic sources of PM were active. However, natural sources contributed to air pollution during that time.

**Keywords:** particulate matter, ozone, policy intervention, Mann-Kendall trend analysis, air quality
