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

#### **Figure 5.**

*Daily PM10 and PM2.5 concentrations and hourly O3 concentrations in urban, rural, and semi-urban areas of Mexico City.*


**Table 2.**

*Results of Chow test (p < 0.05).*

GMC, the Chow test is performed. This test allows us to detect a structural change at a given time. In this case, the mobility restrictions started on March 23rd, 2020, or day of the year (DOY) 83. Therefore, the test is performed for this day. The results of the Chow test for DOY 83 are shown in **Table 2**. The PM2.5 diurnal cycle is also influenced by working hours since many PM2.5 anthropogenic sources are related to transportation and industrial activities. Finally, the ozone diurnal cycle is also influenced by the diurnal cycle of solar radiation.

At the 95% confidence level, a significant change in PM2.5 concentrations was found in the semi-urban areas. Ozone concentrations also changed significantly in the three areas on that day.

#### **3.5 National standard exceedances**

The final part of the assessment of the effect of lockdown on air quality is done by looking at the number of exceedances of the National Standard (NOM) for air quality (NOM-025-SSA1-2014 until 2022 and NOM-025-SSA1-2021 after 2022). The NOM-025-SSA1-2014 stipulates that daily PM2.5 concentrations should not exceed 45 μg/m3 and PM10 concentrations should not exceed 75 μg/m3 . In addition, the hourly average of O3 should not be greater than 0.090 parts per million (ppm) in order to be considered "good" air quality.

The number of exceedances of the NOM for PM and ozone concentrations during the wet and dry seasons from 2012 to 2022 at urban, rural, and semi-urban stations in Mexico City is shown in **Figure 6**.

In the 2020 dry season (when the mobility restrictions were enacted), the recommended PM10 concentration levels were exceeded five times in urban areas and once in rural areas. Fewer NOM exceedances were reported that year than in previous years. The recommended levels of PM2.5 concentration were exceeded only once in semi-urban areas during the dry season. In terms of PM2.5 concentrations during the dry season, air quality improved in urban and rural areas due to major policy

#### **Figure 6.**

*(a) Number of exceedances of NOM for PM10, (b) number of exceedances of NOM's limits for PM2.5, and (c) number of exceedances of NOM for O3. In all cases, the wet and dry seasons are shown for urban, rural, and semi-urban areas in GMC.*

intervention. In contrast, the recommended levels of ozone were exceeded more often than in previous years, especially in semi-urban areas.
