Recommendations for dealing with environmental ozone contingencies

To face health complications caused by the increase in ozone levels in the Metropolitan Zone of the Valley of Mexico (ZMVM), a situation to which is added the fact that meteorological conditions favor the formation and accumulation of this gas, in addition to the presence of high temperatures and solar radiation, weak winds and little cloudiness, specialists from the Institute of Atmospheric Sciences and Climate Change (ICAyCC) of the UNAM, make several recommendations to be applied institutionally and individually.

Among the measures to be adopted, in addition to the restrictive measures for motor vehicles in the Metropolitan Zone, for the case of the Mexico City metropolitan area, it is known to reduce both leaks and emissions from burning LP gas, and given that water heating equipment receives little or no maintenance and are the ones that lead to the highest consumption in homes, they suggest shortening bathing time and preferably showering in the afternoon, since there is less solar activity, which reduces the formation of ozone.

Given the estimate of the Megalopolis Environmental Commission that between 3 and 5 environmental contingencies will be registered between March and June, and due to the high-temperature season. Researchers also suggest measures to prevent and reduce emissions, such as the surveillance and continuity of the Vehicle Verification Program; the distribution of gasoline with lower volatility and the readjustment of the regulations in this regard; preventive actions to avoid forest fires, and the coordination among all the federal entities that contribute to the air quality.

They also suggest being informed daily about the weather forecast and air quality; keeping your vehicle in good condition; reducing the use of solvent-based paints; and filling up with gasoline in the morning (before 10 am) or at night (after 7 pm) to reduce its evaporation, preferably favoring telecommuting and online shopping.

Thus this year

On March 29th, the ozone level exceeded 160 ppb in the Santa Fe area in the west of Mexico City, which led authorities to declare an Environmental Contingency with restrictive measures for motor vehicles in the Metropolitan Zone of the Valley of Mexico, to avoid health complications for its inhabitants.

Every year, mainly from March through June, the Megalopolis Environmental Commission declares environmental contingencies. During this period, ozone concentrations reach high levels and exceed the limit values of the Standard (NOM-020-SSA1-2021). During these months, meteorological conditions favor ozone formation and accumulation, in addition to high temperatures and solar radiation, weak winds, and low cloud cover. These conditions change starting in June with the presence of the first rains.

Meteorological conditions during the first contingency of 2022

The first contingency of the year was mainly due to the meteorological conditions during the week of March 28 to April 1 due to the presence of a high-pressure system over most of the country, which reduced the vertical transport of pollutants generated near the surface, and was associated with weak winds, coupled with clear skies, high radiation and relatively high temperatures for the Valley of Mexico.

In the days before the contingency, at surface level, a very deep low-pressure system developed which generated winds that helped mitigate pollution in the metropolis. These phenomena have already been observed on several occasions and are the subject of analysis. The activation of the "green traffic light" led to an increase in vehicular mobility and therefore in emissions of pollutants into the atmosphere, especially ozone precursors.

Ozone irritates the respiratory system, which produces a decrease in respiratory capacity and predisposes to infections in this system. Studies have shown that the incidence and severity of COVID-19 disease are associated with increased concentrations of respirable particles and nitrogen dioxide, in addition to their relationship with ozone concentrations.

Atmospheric chemistry

These meteorological conditions favor the potential for typical ozone formation in the metropolitan area. Ozone at the surface level is formed from emissions of its precursors, nitrogen oxides (NOx), generated mainly by the vehicle fleet, and volatile organic compounds (VOCs) emitted by the volatilization of gasoline, solvents, and even LP gas leaks, which, under the influence of sunlight and photochemical reactions lead to the formation of ozone and other pollutants.

However, this formation is not immediate to the emission of precursors, but as the wind transports these air masses, ozone is formed. Thus, the morning emissions of NOx and VOCs lead to the highest ozone concentrations of the day being recorded downwind a few hours after the emissions. When meteorological conditions are favorable for the dispersion and ventilation of air masses through the mountains surrounding the Valley of Mexico, ozone levels are maintained at a level that is generally close to the limit concentration recommended by national regulations.

No less important is the influence of the increase in ambient temperature, which, in addition to increasing the emission of VOCs, both anthropogenic and biogenic, favors the speed with which ozone-producing photochemical reactions develop. Other factors include the contributions of NOx and VOCs generated by agricultural burning and forest fires from neighboring watersheds that enter the Valley of Mexico.