Atmospheric Changes and Ozone Increase in Mexico City during 2020 Lockdown Period

Sakthi Selvalakshmi Jeyakumar Instituto Politecnico Nacional Jonathan Muthuswamy Ponniah Instituto Politecnico Nacional Gopalakrishnan Gnanachandrasamy Sun Yat-Sen University Sandra Soledad Morales-Garcia (  ssmoralesg@hotmail.com ) Instituto Politecnico Nacional https://orcid.org/0000-0003-0647-8489 Pedro Francisco Rodríguez-Espinosa Instituto Politecnico Nacional Gowrappan Muthusankar French Institute of Pondicherry: Institut Francais de Pondichery Diana Cecilia Escobedo-Urias Instituto Politecnico Nacional


Introduction
The COVID-19 is a worldwide ongoing pandemic, initially chronicled in Wuhan, capital of Hubei province in China (Raibhandari et al., 2020) during late December 2019. This contagious disease created chaos across the globe affecting 208 countries including the developed countries like USA, UK, Australia, Europe, Japan, Germany, Africa, Mexico, India, Singapore, Malaysia, Canada and several other Latin American countries with a total affected human cases of 44 million (as of 30th October, 2020).
Without any exception, Mexico also reported its rst case during mid-January, 2020 in the state of Nayarit and Tabasco and gradually all over the country totaling 1,957,889 cases and deaths were upto 167,760 persons (as of 10th February, 2020). Among the different states in Mexico, the most affected cases was detected in Mexico City (as of 10th February, 2020). The Mexican government initiated the lockdown slowly during mid-March 2020 by ceasing all high activities and closing schools, movie theaters, restaurants, public malls to avoid large people gatherings for social distancing. Recently, air pollution is the most important environmental problem labelled in most of megacities posing threats for human health due to the presence of primary and secondary pollutants resulted from anthropogenic activities and in-situ atmospheric reactions. Mexico City, renowned as one of the ve largest populated cities in the world is bustled with heavy human activities and residents > 22 million habitants (United Nations, 2018). On this context, Mexico City always pinpointed for the air pollution since 1940, the era of industrial activities and gradual rise in pollution (Lezama, 2000). There are numerous studies which reported the strangling air problems in Mexico City Metropolitan Area (MCMA) especially the high ozone (O 3 ) and PM 2.5 concentrations making this the most polluted City in North America (Molina et al. 2007). Despite the huge population, the Mexico City host a large vehicular eet available as taxi cab, passenger car, ride hauling services, light duty vehicles and heavy vehicles for cargo system. After a very serious episodes of severe air pollution for several years, the Mexican government introduced Bus Rapid Transit (BRT) system as an initiative to minimize the air pollution. Since the epoch of industrialization, there was a regular increase in vehicles for freight systems considered as an important source for emitting pollutants in interurban areas and general vehicle eet in urban areas (Bel, 2018).
Most of the megacities faces air pollution risking human health (Holgate, 2017), in recent decades, Mexico City has reported major air pollution calamity due to the high concentrations of air pollutants such as NO 2 , SO 2 , CO, O 3, PM 2.5 which was high compared to the permissible Air Quality Standards guidelines (WHO, 2019). The majority of these pollutants are emitted through urban road transport containing massive vehicle eet (metro, metro bus, microbus) and simultaneous tra c congestion, which is generally used by 12 million people (INEGI, 2018). Besides the immense use of this public transit, a study conducted on vehicle pollutant measurements discloses the commuter exposure to PM 2.5 , CO and benzene especially in microbuses, buses and metro in Mexico City, which causes carcinogenic health problems (Gomez-Perales et al., 2004; Shiohara et al., 2005).
Since 1940, the Mexico City Metropolitan Area experienced a major air pollution due to urbanization and industrial activities, hence the Mexican government steps into implementing several strategies during the late 20th century to control and reduce the emissions triggering air pollution by restraining the emissions from industries also for the transport sector such as driving restrictions called "No Driving Day" (Hoy No Circula program in Spanish) in 1989. Implementing the control measures, it brought down signi cant changes in improving the air quality, which also happened due to the change in composition of gasoline and veri cation of vehicle engine (Davis, 2008;INEGI, 1988). However, particulate matter (PM) were a major pollutant emitted from the industrial areas present in the City which has happened due to in-house hold activities and different chemical additives from various sector (Soto-Coloballes, 2017; Sicard et al., 2020).
The immediate lockdown period announced across the globe made a substantial difference in the environment and the atmosphere, which was well documented through many research articles and social media. We made use of this period (upto June 2020) to identify and understand the causes, processes accountable for persisting air pollution in Mexico City Metropolitan Area. The main objective of the present study was to document and understand the changes due to the lock down which happened through industrial and transport sectors and the direct effect on the changes in the atmospheric conditions in Mexico City.

Materials And Methods
Geographical information of the Study area Mexico City is an inland basin located at elevated position of 2240 m above mean sea level (MSL) enclosed with high mountains of Ajusco and Sierra Chicchinautzin in the south, Iztaccíhuatl-Popocatépetl dormant volcanic mountains in the east bordering the State of Mexico and Puebla (Fig. 1). Mexico City Metropolitan Area consists of 16 localities formerly known as "Mexico City", comprising 59 metropolises in State of Mexico and 1 metropolis in State of Hidalgo. The topography of Mexico City indicates that it is surrounded with high mountains. In addition, a board opening in the north and a narrow passage in the south-south east at the border of the basin formerly called "Tenango del Aire" acts as natural ventilators for the City between the Mexico City basin and Cuautla-Cuernavaca Valley in the State of Morelos. Since 2000, the drastic transformation in the Mexico City caused urbanization and it expanded over some municipalities bordering the basin from the states of Mexico, Puebla, Tlaxcala, Morelos and Hidalgo which are under increasing population growth forming a grand urban complex in Mexico "Mexico City Megalopolis" as the most polluted City in the World (Fig. 1).

Meteorological settings
The Mexico City basin falls under sub-tropical highland climate, which is classi ed into three patterns: 1) dry winter (November to March); 2) dry summer (April to May and 3) rainy season (June to October) mainly to understand the changes. The summer is the driest season in Mexico adjoined by the presence of clear skies with low humidity and high pressure system with an average ambient temperature of 12 to 24 ˚C. The driest month have westerly current experiencing the anti-cyclonic ow along with strong thermal inversion at night (Collins and Scott, 1993). This ( ow) that often lasts after the sunrise due to turbulent mixing and the strong heating by the sun enhances the photochemical reaction of the ozone (O 3 ) recording higher values of pollutants in Mexico City. In contrary, the rainy season have easterly winds prevailing over the mountains surrounding the Mexico City basin, which is due to convection and the thermal inversions of high moisture concentration. However, during early morning the turbulent eddies are generated from the heat from the sun causing severe vertical mixing of the pollutants (Giovanni et al., 2017).
The average annual rainfall in Mexico City is about 820 mm, which is intense from July to September. The diverse meteorological condition in Mexico city is responsible for the persisting air pollution, triggering activities in the photochemical ozone production and other secondary pollutants such as aerosol loadings and particulate matter (

Pollutant data collection
This study was carried out by analyzing various set of data which is considered as important for determining and the understanding the discrepancies of air quality in Mexico City Megalopolis during the lockdown period. We used the meteorological data such as 1) temperature (in °C); 2) relative humidity (in %); 3) wind speed (in m/s); 4) precipitation (in mm) and air pollutant data (i.e) NO 2 , SO 2 , CO, O 3 , PM 2.5 (in µg/m 3 expect for CO measured in mg/m 3 ) from a real time monitoring stations.
In this article, we have limited our data by mentioning only the diurnal average concentrations of the pollutants from January -May 2020 compared along with the annual average concentration for 2017, 2018 and 2019. The selection of the data and period was selected primarily to identify the periods which is considered as regular movements in the city limits during the past years. The open data was obtained from government network called "Red Autómatico de Monitoreo Atmosférico" (RAMA, 2020). This network comprises 34 stations in Mexico City and State of Mexico with regular maintenance of the laboratories and the monitoring equipment. Mexican government imposed its air quality standards for major air pollutants especially for nitrogen dioxide (NO 2 ), sulfur dioxide (SO 2 ), carbon monoxide (CO), ozone (O 3 ) and particulate matter (PM 2.5 ). The above parameters were used in this study as a quality guideline for determining air quality pertained to human health and the changes during the lockdown periods.

Spatial & temporal distribution of air pollutants in Mexico City
The average concentrations of air pollutants (NO 2 , SO 2 , CO, O 3 , PM 2.5 ) from ve studied monitoring stations in Mexico City is shown in Table 1. Likewise, satellite images for the months of January to May 2020 (    Based on the surface transport movements in Mexico City from January to May 2020 indicates a wide variations and non-reduction of movements in some sectors. The usage (in millions) from January to May (2020) is as follows: metrosystem 130.709 (January) to 35.900 (May); public transport 12.4 (local and long distance buses) (in January) to 33.22 (May) and short transport system 44.16 (micro bus) (in January) to 11.61 (in May) (INEGI, 2020). Eventhough based on the above data there was a reduction in public transport movement during the lock down, other services were operating as it is during the normal conditions. This clearly contributes more towards emissions of NO 2 , SO 2 , CO, PM 2.5 and VOC as they are mostly perceived from the transport sector. The above inference is very well supported by earlier studies in Mexico City that apart from transport sector, industrial source (for NO 2 , SO 2 ) and soil erosion (for PM 2.5 ) and solvent paints (for VOCs) are responsible for the presence of these pollutants (Molina et al., 2019). Likewise, the adaptations from MOBILE 6.2 -Mexico and Moves -Mexico found signi cant reduction in total emissions (in %) for NO x by -37, CO by -52 and VOCs by 26 percent. However, there was an increase observed (in %) for O 3 by 6.6 due to the operations in urban tra c stations PM 10 by + 8 and PM 2.5 by 6 mainly due to the contributions from gasoline based taxis and passenger cars (Guevara et al., 2017).
Industrial sector in Mexico City which has 26 different types (paper, manufacture, mining, fabrication, plastics, rubber) plays a major role in the emission and in transporting the pollutants. The "Monthly industrial activity indice (MIAI)" was calculated based on the data from 2019 (April-May). The calculated MIAI values for the construction industry indicates that it was 105, 99.5 (April, May 2019) and 64.7, 63.8 (April, May 2020) respectively. Likewise, for manufacturing industries it was 115.4, 114.9 (April, May 2019) and 74.3, 74.1 (April, May 2020). Overall, the reduction of industrial activity was between 35.7 to 40.3% (construction) and 40.8 to 41.1% in manufacturing sector. In the mining industry it is almost maintained the same way 70.3 to 73.1% (for April-May 2019) and 68.2 to 72.3% (for April-May 2020) for Mexico City (Suppl. Fig. F2a-j). Air quality/ emission studies from these industries often indicate that toxic metals (As, V, Fe, Cu) dominate due to the increase in PM 2.5 (Morales-García et al., 2014). The above MIAI values suggest that the main contributors exist due to the emissions which also has a direct effect on the higher values of VOCs that often has a direct impact on O 3 (Koupal and Palacios 2019).

Statistical Information
Statistical analysis was done with the available data as well as the meterological variables of temperature and rainfall for the year 2020. Dendrograms were generated which indicated two different clusters which high linkage distance (Fig. 4)

Comparative Studies
Based on the available data compared to the air pollutants reported from different countries during the present pandemia from January till date (Before the lockdown & After lockdown), we have collected the available data and it has been compared with reference to WHO guidelines (Table 3).

Conclusion
Mexico City is well known as one of the most polluted cities in North America and the present article focuses on the changes in the air quality status amid the present pandemic which has shook the whole world in one way or the other.
Primary and secondary data sets were generated to monitor the pollutants (NO 2 , SO 2 , CO, O 3 and PM 2.5 ) in four different monitoring stations located in the Mexico City area. The results indicate that higher values in GAM for all the pollutants which is two to three fold higher than the WHO permissible limits. The increase in pollutants were mainly due to the meteorological factors like rainfall, temperature, relative humidity, wind speed and its direction which foster the photochemical reaction. This is very well supported by the evaluation of statistical analysis indicating that the enrichment of pollutants (NO 2 , SO 2 , CO) were from vehicular eet and industries.
The higher temperatures often promote the intense photochemical reactions which enhances the formation of O 3 and PM 2.5 which is observed in all the stations despite the lockdown period which were are highly controlled by the wind direction and precipitation. It is clearly observed that Mexico City suffers severe air pollution through both natural as well as anthropogenic ways and the persistency of the pollutants happens via broad channel in the narrow passage (Tenango del Aire) and Cuautla-Cuernavaca Valley in the south. It is clear that the government should form strong strategies towards air quality management tools with updated technology for both transport vehicles and cars with changes in emissions standards, which will help cooperate restore the air quality standards. Availability of data and materials All data generated or analyzed during this study are included in this published article (and its supplementary information les)

Competing interests
All authors certify that they have no a liations with or involvement in any organization or entity with any nancial interest or nonnancial interest in the subject matter or materials discussed in this manuscript.

Funding
No funds, grants or other support was received.  Concentrations of major air pollutants in Mexico from January-May, 2020 (In frame: Mexico City) Note: The designations employed and the presentation of the material on this map do not imply the expression of any opinion whatsoever on the part of Research Square concerning the legal status of any country, territory, city or area or of its authorities, or concerning the delimitation of its frontiers or boundaries. This map has been provided by the authors.