Changes in ozone production and VOC reactivity in the atmosphere of the Mexico City Metropolitan Area
Graphical abstract
Introduction
Ozone (O3) concentrations in the Mexico City Metropolitan Area (MCMA) reduced substantially from the early 1990s up to about 2010, but since then the levels have stalled. Recent severe air pollution episodes suggest that the production of secondary pollutants may have started to rebound (Molina et al., 2019). Simultaneously, advancements in vehicle and emissions-control technologies and the introduction of cleaner fuels have led to drastic reductions of particle matter (PM), volatile organic compounds (VOCs), carbon monoxide (CO), and nitrogen oxides (NOx) emissions from mobile sources (e.g., Schifter et al., 2014; Koupal and Palacios, 2019; EPA, 2019). Current CO and VOC emission rates of new vehicles are orders of magnitude smaller than in the late 1990s (e.g., Bishop, 2019), albeit impacts on total ambient organic reactivity are still not fully documented. Aggressive emission controls for electricity generation along with changes in residential, commercial, and industrial activities due to population and economic growth have also affected the long-term variability of VOC and NOx emission sources. Nevertheless, after decades of substantial O3 reductions, there has been little additional progress in recent years in Mexico and many parts of the world, including China (Li et al., 2019), Europe (Yan et al., 2019) and the US (Yan et al., 2018). This stagnation indicates that there are new challenges to address for reducing O3 levels in urban areas.
The initial O3 reductions in the late 90s and beginning of this century in the MCMA were related to aggressive emission controls of O3 precursor species. These included improving fuels (e.g., removal of lead and reformulation of gasoline, reduction of sulfur content in gasoline and diesel), adopting catalytic converters, removing an oil refinery and heavy industries, shifting to natural gas in power plants and other industries, reformulating liquefied petroleum gas (LPG) for cooking and water heating, introducing a vehicle inspection and maintenance program, and a “no driving day” (Hoy no Circula) rule, (SEDEMA, 2019). The question of why O3 levels have not further decreased in recent years needs to be scientifically addressed to re-design and implement new effective emission control policies. Due to its meteorological conditions and emissions characteristics, the MCMA provides a unique opportunity to investigate the factors that drive changes in O3 levels in large urban areas, particularly of developing nations.
Formulation of successful emission control policies to reduce O3 levels requires knowledge of the chemical regime for O3 production P(O3). Photochemical P(O3) is a complex function of NOx and hydrogen oxide radicals (HOx = OH + HO2 + RO2) abundances and the partitioning of NOx between nitric oxide (NO) and nitrogen dioxide (NO2), and of HOx in OH, HO2, and RO2. In turn, ambient levels of HOx radicals and atmospheric reactivity depend strongly on the VOC abundance and chemical composition. In the MCMA, O3 formation and its spatial distribution are further impacted by air mass transport patterns that are locally induced by the complex topography, high altitude, and intense solar radiation.
The topography of the MCMA is characterized by a wide-open basin in the north, with high rim mountains at the west, south, and east, and a mountain passage at the southeast that drive the meteorological transport patterns in the city (Fig. 1). The wind circulation patterns associated with air pollution events have been studied in detail (e.g., Fast and Zhong, 1998; Doran and Zhong, 2000; de Foy et al., 2005, 2006; 2008; Fast et al., 2007). These studies show that although there is some interaction with synoptic-scale circulations, most of the day-to-day variability in surface wind speed and direction is affected by mesoscale, thermally driven mountain–valley flows. Thermal flows have significant vertical shear that leads to recirculation within the basin. The high elevation (2240 m.a.s.l) and low latitude (19°N) of the basin induce strong diurnal variation of the boundary layer that increases vertical mixing. However, near-surface convergence zones due to momentum down-mixing of thermally driven winds and light synoptic winds aloft can promote the accumulation of air pollutants in the basin. The studies also suggest that the basin has relatively effective venting through the southeast passage and open northern plateau, leading to short day-to-day accumulation of pollutants.
The interaction of synoptic and locally-driven circulations leads to high seasonal and spatial variability of O3 within the MCMA. The high O3 season spans from mid-February to mid-June, and the highest O3 concentrations are usually observed during the warm-dry period from March to May when high-pressure systems induce days with clear skies and weak winds (SEDEMA, 2018a). Anticyclonic westerly winds bring mostly dry air currents from November to April, whereas the rainy season from May to October is associated with moist easterly trade winds. In terms of spatial variability, since the beginning of O3 monitoring in the MCMA in the late 80's, the data showed that O3 concentrations are usually higher southwest of the city and lowest in the northeast due to dominant near-surface northerly winds (e.g., Bossert, 1997; Fast and Zhong, 1998). As shown in Fig. 1, high O3 levels are still significantly more prevalent in the southwest than in the northeast, further suggesting that air transport thus far has a primary role in determining the spatial distribution of O3 in the MCMA.
The most comprehensive studies on the P(O3) regimes in the MCMA to date are from the MCMA-2003 and MILAGRO 2006 field campaigns (Molina et al., 2007, 2010). The MCMA-2003 field campaign involved a multinational team of experts conducting measurements and modeling studies to better understand the air pollution problems in the MCMA (Molina et al., 2007). State-of-the-art instrumentation generated extensive measurements of oxidant precursors, radicals, photochemical products, VOC and PM intermediates, and meteorological parameters. A highly-instrumented mobile laboratory was deployed for vehicle-chase measurements and at multiple locations. An eddy covariance flux tower measured fluxes of VOCs and demonstrated to be a valuable tool for evaluating the emissions inventory (Velasco et al., 2005). Some of the key results included a better understanding of the meteorology and transport of the urban plume (de Foy et al., 2006); an improved speciated emissions inventory from on-road vehicles (Zavala et al., 2006); an improved characterization of the sources and atmospheric loadings of VOCs (Velasco et al., 2007) and the first spectroscopic detection of glyoxal in the atmosphere (Volkamer et al., 2005), which was subsequently used as a tracer to estimate the relative contributions of primary and secondary formaldehyde (HCHO) (Garcia et al., 2006; Lei et al., 2009); a more comprehensive characterization of NOx and VOCs precursors of O3 formation (Shirley et al., 2006; Sheehy et al., 2010; Lei et al., 2007); improved knowledge of the composition and mass loadings of primary and secondary fine PM (Salcedo et al., 2006), and the finding that the formation of secondary organic aerosol (SOA) was much more efficient than predicted by traditional models (Volkamer et al., 2006).
MILAGRO (Megacity Initiative: Local And Global Research Observations) was an international scientific collaboration to examine the emissions, transformations, and the export of atmospheric pollutants from a megacity using Mexico City as a case study (Molina et al., 2010). Measurements took place over 6-week period during 2006 and included a wide range of instruments at three supersites, on six research aircraft, and satellites. Additional platforms included mobile laboratories and stations located at urban and boundary sites. The suite of measurements expanded upon those of the MCMA-2003 campaign by including measurements of targeted emission sources, chemical, physical and optical properties of aerosols, boundary layer development, and other key meteorological parameters at multiple ground sites. The measurements provided much more comprehensive information on the characterization of meteorology, emissions, chemistry, and radiation specific for Mexico City and the surrounding region. Emission inventories were re-evaluated with observations, including area-averaged emission fluxes at the ground site, aircraft measurements, and source apportionment analysis (e.g., Aiken et al., 2009; Karl et al., 2009; Velasco et al., 2009; Zavala et al., 2009). Simultaneous measurements of NOx, VOCs, O3, HOx, as well as several radical precursors such as HONO, HCHO, and glyoxal, provided the observations needed to constraint modeling studies for improving our understanding of O3 formation and its sensitivity to precursors and SOA evolution (e.g., Li et al., 2011; Song et al., 2010). Urban plumes were detected at distances up to 1000 km downwind with continued O3 production for several days due to the formation of peroxyacetyl nitrates (PANs) and effectively increasing the NOx atmospheric lifetime (Mena-Carrasco et al., 2009).
Field observations and modeling exercises of radical production rates, as well as NOx termination reactions, VOC/NOx and CO/NOx ratios, and chemical tracers during both dedicated field campaigns, all indicated that P(O3) was highly VOC-limited (i.e., NOx-saturated) within the urban core, while the surrounding low-NOx areas were either NOx- or VOC-limited depending on the meteorological conditions (Lei et al., 2007, 2008; Song et al., 2010). However, the recent reversal from a downward O3 trend together with changes in VOC and NOx emissions indicates the need to re-evaluate the O3 formation regimes.
In this study, we investigate the reasons for the slow progress in O3 abatement for the MCMA in recent years by analyzing the changes in P(O3) regimes and VOC-OH reactivity. The study is based on VOC, CO, and NOx data collected from the 1990s to 2019 by the local air quality monitoring network and during the two dedicated air quality field campaigns described above. Changes in atmospheric reactivity are assessed by recent and past VOC measurements, while changes in P(O3) across the city are evaluated under a pseudo-stationary state (PSS) using as reference concurrent NO levels to understand potential impacts of current and future reduction emission policies. In addition, the contributions of locally and regionally produced O3 are investigated.
Section snippets
Methods
We first examine changes in VOCs, CO, and NOx ambient levels and their links to key emission sources. Second, we evaluate recent and past VOC-OH reactivity using VOC data from recent observations and previous measurements. Third, we estimate P(O3) under the PSS approximation. Finally, we contrast the VOC-OH reactivity and P(O3) in the context of observed changes of VOCs, CO, and NOx, and in concert with the analysis of the regional ozone's contribution to the total budget, to evaluate how the
Changes in CO and NOx ambient concentrations
The trends of air pollutants during the last three decades provide evidence of changes in the chemical composition of the MCMA's atmosphere. Fig. 2 shows that the large reduction from 1990 to about 2010 in daily maximum O3 concentrations was concurrent with large decreases in levels and variability of CO and NOx. Statistical significance F-tests (p ≤ 0.05) were performed for the linear regression of the last 5 years of data. The daily maximum O3 concentrations do not show a statistically
Conclusions
From an air quality management point of view, the understanding of the crossover between NOx-limited and NOx-saturated conditions is of the utmost importance because it determines the effects on P(O3) due to VOC and NOx emissions reduction strategies. In this study, the analyses of daytime O3 formation based on NO2/NO ratios, VOC-OH reactivity, VOCs, CO, and NO availability, are used to identify the possible causes of changes in P(O3) conditions from the early 2000s to present-day levels. The
Data availability
Ambient monitoring data used in this paper is available at www.aire.cdmx.gob.mx. Other pertinent data described in the paper is available upon request.
CRediT authorship contribution statement
Miguel Zavala: Conceptualization, Methodology, Data curation, Formal analysis, Writing - original draft. William H. Brune: Conceptualization, Writing - review & editing. Erik Velasco: Writing - review & editing. Armando Retama: Investigation, Writing - review & editing. Luis Adrian Cruz-Alavez: Investigation. Luisa T. Molina: Conceptualization, Writing - review & editing.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The authors acknowledge the staff of the Dirección de Monitoreo de la Calidad del Aire of Secretaría del Medio Ambiente of Mexico City for providing the air quality data used in this study. We also recognize the efforts of José Gabriel Elias-Castro, who heads the Chromatography Laboratory, for coordinating the field work for the measurement of VOCs, as well as the support of Olivia Rivera-Hernández, Miguel Sánchez-Rodríguez, Angélica Néria-Hernández, Oscar Hernández Castillo and María Luisa
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2023, Journal of Environmental Sciences (China)Citation Excerpt :The alternate patterns between PM2.5 and O3 pollution put the government into a dilemma and thus intensive focus has been placed to O3 in China. Volatile organic compounds (VOCs) and nitrogen oxides (NOX) are the key precursors for O3 formation, their emissions variations essentially drive the O3 change (Tan et al., 2018; Yu et al., 2021; Zavala et al., 2020). As a photochemical product, O3 concentration is modulated appreciably by direct and indirect impacts of the meteorological conditions (Liu and Wang, 2020; Ma et al., 2021a; Porter and Heald, 2019; Wang et al., 2017).
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