**4. Discussion**

The results of the analysis show that wind is the main environmental variable affecting PM and O3 concentrations; therefore, long-range transport of pollutants can be a limitation of policies reducing anthropogenic sources of pollutants. A refinery complex is located around 80 km to the northeast of the city center in Tula, Hidalgo. It contributes to air pollution in GMC *via* long-range transport of pollutants, mainly PM10 [13, 24]. Also, high winds cause erosion of dry soils, especially in rural areas during the dry season [12]. Therefore, there is a positive significant relation between PM concentrations and wind speed in rural areas.

The diurnal cycles of PM10 concentration show that in the early morning, concentrations in semi-urban areas are higher than in rural and urban areas. As the working hours begin, PM10 concentrations increase in urban and rural areas. This has to do with transportation, industrial production, long-range transport of pollutants, soil erosion during the morning hours, and the boundary layer height cycle [25].

The ozone diurnal cycle is affected by solar radiation and economic activity since ozone precursors are related to vehicle emissions and solar radiation contributes to the formation of surface ozone [26].

The monthly averages show that the rainy season washes out pollutants, thus causing an improvement in air quality. Also, wet soils are less likely to be affected by wind erosion. Thus, some natural sources of PM and O3 are reduced.

The trend analysis shows that in the long run, there was an effect of mobility restrictions (and halting or reduction of nonessential businesses and industries) in PM concentrations. However, ozone increased during this period as previous studies in the same region have shown [9]. The short-term analysis done with the Chow test showed that at the start of the restrictions, there was a decrease in PM2.5 concentrations in semi-urban areas and ozone in the whole GMC. The effect on the other areas was not statistically significant. Finally, in 2020, the NOM of air quality was exceeded a few times in PM2.5 and PM10 during the lockdown; however, the limits of ozone were exceeded more times than in the 10 previous years.
