Evolution of Air Pollution in the Monterrey Metropolitan Area, Mexico

The Monterrey Metropolitan Area (MMA) is considered the second most populated metropolis in Mexico, with around 5,341,171 inhabitants, making it necessary to maintain a constant rate of growth of urban services which has caused it to be one of the cities with the greatest increase in air pollution rates in recent years as a result of human activities. Likewise, the MMA is located in a place with diverse natural formations that extend throughout the city, receiving the nickname of the "City of the mountains". These formations together with the polluting emissions product of various sources such as industries, vehicles, natural phenomena; and weather conditions, considerably increase the concentrations of pollutants that impact the quality of life of living beings. Objective: The objective of this research is to show the evolution of air pollution in the first two decades of this century, evaluate its trend and highlight the risks to which urban inhabitants and ecosystems are subjected. Materials and Methods: Data from the Environmental Monitoring Integral System (SIMA) of the Ministry of Environment of the Government of the State of Nuevo León, which has 14 monitoring stations of criteria pollutants, were used to evaluate the evolution and distribution of air pollution in the MMA. Concentrations of CO, NO2, SO2, PM10, PM2.5 and O3 were evaluated. The trend of the hourly, daily, monthly and annual time series was evaluated to see patterns and the distribution of pollutants in the MMA have changed during the last years. Results: The results show that the pollutants have a great variability and these are based on multiple natural factors (thermal inversion, wind speed and direction, orography, among others) and/or anthropogenic factors (mobility, agricultural, industrial, services, fires, among others). They also depend on the time of year with an increase in particulate matter in the dry season and ozone in the wet season. Conclusions: The criteria pollutants that recorded the highest number of days out of the norm and with the greatest tendency to increase during the last two decades are PM10, O3 and PM2.5, which represents a risk to environmental and human health. The pollutants CO, NO2, SO2, so far do not represent a risk to the population, since these are within the norm both for the number of days and for the daily, annual and monthly averages.