Tropospheric levels of O₃ have historically exceeded the official annual Mexican standards within the Monterrey Metropolitan Area (MMA) in NE Mexico. High-frequency and high-precision measurements of tropospheric O₃, NO_y, NO₂, NO, CO, SO₂, PM₁₀ and PM_2.5 were made at the Obispado monitoring site near the downtown MMA from September 2012 to August 2013. The seasonal cycles of O₃ and NO_y are driven by changes in meteorology and to a lesser extent by variations in primary emissions. The NO_y levels were positively correlated with O₃ precursors and inversely correlated with O₃ and wind speed. Recorded data were used to assess the O₃-Volatile Organic Compounds (VOC)-NO_x system’s sensitivity through an observational-based approach. The photochemical indicator O₃/NO_y was derived from measured data during the enhanced O₃ production period (12:00–18:00 Central Daylight Time (CDT), GMT-0500). The O₃/NO_y ratios calculated for this time period showed that the O₃ production within the MMA is VOC sensitive. A box model simulation of production rates of HNO₃ (P_HNO3) and total peroxides (P_perox) carried out for O₃ episodes in fall and spring confirmed the VOC sensitivity within the MMA environment. No significant differences were observed in O₃/NO_y from weekdays to weekends or for P_HNO3/P_perox ratios, confirming the limiting role of VOCs in O₃ production within the MMA. The ratified photochemical regime observed may allow the environmental authorities to revise and verify the current policies for air quality control within the MMA.

Meteorology and long-term trends in air pollutant concentrations may obscure the results from short-term policies implemented to improve air quality. This study presents changes in CO, NO₂, O₃, SO₂, PM₁₀, and PM_2.5 based on their anomalies during the COVID-19 partial (Phase 2) and total (Phase 3) lockdowns in Mexico City (MCMA). To minimise the impact of the air pollutant long-term trends, pollutant anomalies were calculated using as baseline truncated Fourier series, fitted with data from 2016 to 2019, and then compared with those from the lockdown. Additionally, days with stagnant conditions and heavy rain were excluded to reduce the impact of extreme weather changes. Satellite observations for NO₂ and CO were used to contrast the ground-based derived results. During the lockdown Phase 2, only NO₂ exhibited significant decreases (p < 0.05) of between 10 and 23% due to reductions in motor vehicle emissions. By contrast, O₃ increased (p < 0.05) between 16 and 40% at the same sites where NO₂ decreased. During Phase 3, significant decreases (p < 0.05) were observed for NO₂ (43%), PM₁₀ (20%), and PM_2.5 (32%) in response to the total lockdown. Although O₃ concentrations were lower in Phase 3 than during Phase 2, those did not decrease (p < 0.05) from the baseline at any site despite the total lockdown. SO₂ decreased only during Phase 3 in a near-road environment. Satellite observations confirmed that NO₂ decreased and CO stabilised during the total lockdown. Air pollutant changes during the lockdown could be overestimated between 2 and 10-fold without accounting for the influences of meteorology and long-term trends in pollutant concentrations. Air quality improved significantly during the lockdown driven by reduced NO₂ and PM_2.5 emissions despite increases in O₃, resulting in health benefits for the MCMA population. A health assessment conducted suggested that around 588 deaths related to air pollution exposure were averted during the lockdown. Our results show that to reduce O₃ within the MCMA, policies must focus on reducing VOCs emissions from non-mobile sources. The measures implemented during the COVID-19 lockdowns provide valuable information to reduce air pollution through a range of abatement strategies for emissions other than from motor vehicles.

Waste stabilization ponds (WSPs) are a cost-efficient method to treat municipal and non-toxic industrial effluents. Numerous studies have shown that WSPs are a source of greenhouse gas (GHG). However, most reports concerned anaerobic ponds (AP) and few have addressed GHG emissions from facultative (FP) and aerobic/maturation ponds (MPs). In this paper, GHG emissions from three WSP in series are presented. These WSPs were designed as anaerobic, facultative and aerobic/maturation and were treating agricultural wastewater. CH4 fluxes from 0.6 +/- 0.4 g CH4 m(-2) d(-1) in the MP, to 7.0 +/- 1.0 g CH4 m(-2) d(-1) in the (AP), were measured. A linear correlation was found between the loading rates of the ponds and CH4 emissions. Relatively low CO2 fluxes (0.2 +/- 0.1 to 1.0 +/- 0.8 g CO2 m(-2) d(-1)) were found, which suggest that carbonate/bicarbonate formation is caused by alkaline pH. A mass balance performed showed that 30% of the total chemical oxygen demand removed was converted to CH4. It has been concluded that the WSP system studied emits at least three times more GHG than aerobic activated sludge systems and that the surface loading rate is the most important design parameter for CH4 emissions.

Here, we present an assessment of long-term trends in O₃ and odd oxygen (O₃ + NO₂) at the industrial Monterrey metropolitan area (MMA) in NE Mexico. Diurnal amplitudes in O_x (AV_d) are used as a proxy for net O₃ production, which is influenced by the NO₂ photolysis rate. No significant differences in the AV_d are observed between weekends and weekdays, although the largest AV_d values are observed at sites downwind of industrial areas. The highest O₃ mixing ratios are observed in spring, with minimum values in winter. The largest annual variations in O₃ are typically observed downwind of the MMA, with the lowest variations generally recorded in highly populated areas and close to industrial areas. A wind sector analysis of mixing ratios of O₃ precursors revealed that the dominant sources of emissions are located in the industrial regions within the MMA and surrounding area. Significant increasing trends in O₃ in spring, summer, and autumn are observed depending on site location, with trends in annual averages ranging between 0.19 and 0.33 ppb yr⁻¹. Overall, from 1993 to 2014, within the MMA, O₃ has increased at an average rate of 0.22 ppb yr⁻¹ (p < 0. 01), which is in marked contrast with the decline of 1.15 ppb yr⁻¹ (p < 0. 001) observed in the Mexico City metropolitan area (MCMA) for the same period. No clear trend is observed from 1996 to 2014 within the Guadalajara metropolitan area (GMA).