Atmospheric contingencies by ozone air quality at Mexico City

摘要

The Atmospheric Environmental Contingency Program at Mexico City is an emergency program which is activated in response to an episode of atmospheric pollution in which the concentration of ozone or particles reaches levels that compromise the population health. It is intended to alert the population when exists the presence of high concentrations of atmospheric pollutants and apply then immediate, temporary and single actions for reducing atmospheric emissions of the main sources and avoid in this way, the worsening of the episode. In Mexico City and its conurbated area this program was implemented since 1988 and since then has had 12 modifications, mainly in the reduction of activation values, implementation of a new phase (precontingency) and the clear definition of its evaluation for deactivation. Comparing the limit values to declare Phase I of 1988, which was 232 ppb with the modified value in 2016 of 155 ppb, we performed a count of days considering the period from 1988 to 2016 and it was found that with the value of the base year 1988, the year with the highest frequency of events was 1991 with 182 cases that correspond to more than 50% of days of the year, and from 2004 to 2016 there was no event that exceeded the value of 232 ppb. In contrast, taking 155 ppb as the limit value, it was found that from 1988 to 2001 it presented more than 50% of days above this value, being year 1991 with the highest number of events with 314 (86% of days of the year) and the year with the lowest number of events was 2012 with only 4 days. Recently, it should be mentioned that on April 30, 2016, in which an environmental contingency for ozone was presented in Mexico City, meteorological factors played an important role. Analyzing the radiosonde results of the international airport of Mexico City for this day, it shows in the first levels of height a thermal inversion, the atmosphere was stable and it is resistant to the vertical circulation, resulting in a non-favorable dispersion of pollutants, and at 3 pm the contingency alert was issued. The use of radiosonde allows us to explain the dynamic process in the atmosphere with the height, and this measurement is performed at 7 am in the morning, under this scheme and using weather forecast models, we can anticipate in advance with 24 Hours, weather conditions and not only in surface but also in height, becoming a useful tool to predict the contingencies and thus take proactive actions rather than reactive to improve ozone air quality in Mexico City.