## 3 Results  
Comparison of the Ambient Distribution of Meteorological Variables and PM\({}_{2.5}\) Air Pollution within Each Borough between Day and Night  
Annual average RH and TEMP are shown in Supplementary Materials Figure S1. Relative humidity (RH) as well as TEMP obeyed seasonal variations during the year. Relative humidity showed minor variations throughout Mexico City's territory; however, it was lower during the dry season, rising up to 99% during the rainy season. The TEMP had similar behavior, with temperatures relatively constant along the year with small variations (14 to 16 \({}^{\circ}\)C); however, for some days during the winter, temperatures could drop to 0 \({}^{\circ}\)C or rise up to 31 \({}^{\circ}\)C during the spring. Regarding the wind, its predominant direction (WDR) was from north to south/southeast. Yet, another pattern was identified from north and northwest where the winds seemed to blow towards the northeast. In the south, the wind's direction was also from north to south. Average wind speed (WSP) along the year was relatively constant, ranging from 0.0019 to 3.72 m/s depending on the season (graphical information on wind direction is provided in Supplementary Materials Figure S2). Throughout the period of this study, fewer variations were identified apart from the obvious differences in the weather conditions between day and night. In general, nights were more humid (RH 71.1% (65.5-79.9%)) and cooler (TEMP 16.3 \({}^{\circ}\)C (14.8-16.8 \({}^{\circ}\)C)) than days (RH 54.1% (45.4-61.3%) and TEMP 19.7 \({}^{\circ}\)C (18.3-21.1 \({}^{\circ}\)C)). The WSP varied more during the night (1.5 m/s (0.95-2.3 m/s)) than during the day (1.2 m/s (1.1-1.5 m/s)) (detailed information on the weather conditions by geographical distribution between days and nights is provided in Supplementary Materials Table S4). No meaningful differences were observed in relation to TEMP and RH by geographical location; however, the most humid area during both day and night was the southeast (day: 62.4% (51.9-63.4%), night: (76.7% (68.5-81%) (\(p=\) NS)). The WSP showed no significant differences, neither geographically nor between day and night (detailed information on the weather conditions by geographical distribution between days and nights is provided in Supplementary Materials Table S4). The PM\({}_{2.5}\) personal measurements obtained in this study are shown in Figure 2. During daytime, concentrations of PM\({}_{2.5}\) were above national standards for the year 2002 (65 \(\upmu\)g/m\({}^{3}\))--the current national standard is 45 \(\upmu\)g/m\({}^{3}\) in the 13 boroughs [8]. The highest levels of PM\({}_{2.5}\) were identified in Cuauhtemoc (180 \(\upmu\)g/m\({}^{3}\)), Benito Juarez (108 \(\upmu\)g/m\({}^{3}\) (69-160 \(\upmu\)g/m\({}^{3}\))), and Miguel Hidalgo (118 \(\upmu\)g/m\({}^{3}\)) in the center of the city, and Azcapotzalco (112 \(\upmu\)g/m\({}^{3}\)), which is located in the north part of the city. Incidentally, there was a gap between the city center and the eastern area, with low concentrations that incremented in the center of Iztapalapa (133 \(\upmu\)g/m\({}^{3}\)), which is a borough in the southeast part of the city. The lowest concentrations were observed in the boroughs of Coyoacan (66 (63-69 \(\upmu\)g/m\({}^{3}\))) and Magdalena Contreras (77 \(\upmu\)g/m\({}^{3}\)), located in the southwest (Figure 2A). During the night, PM\({}_{2.5}\) median concentrations throughout Mexico City were lower than during the day; the highest median concentrations were detected in Tlalpan (96 \(\upmu\)g/m\({}^{3}\) (64-118 \(\upmu\)g/m\({}^{3}\))) in the south of the city, and in Iztacalco (87 \(\upmu\)g/m\({}^{3}\)) in the center; followed by Coyoacan (63 \(\upmu\)g/m\({}^{3}\) (27-74 \(\upmu\)g/m\({}^{3}\))) in the southwest, and Cuauhtemoc (59 \(\upmu\)g/m\({}^{3}\) (55-62 \(\upmu\)g/m\({}^{3}\))) in the center.