Atmospheric deposition evaluation, as a tool to preserve urban natural green areas: Mexico City case

María Alejandra Fonseca1, Julio Campo2, Luis Zambrano3, Rodolfo Sosa4, Ana Luisa Alarcón5, María del Carmen Torres6, Pablo Sánchez7, Olivia Rivera8 and Mónica del Carmen Jaimes9

 United Nations data indicate that by the year 2030, approximately 60% of the world’s population will live in urban areas. Within urban ecosystems, the anthropogenic pollutants emissions in the atmosphere are concentrated and intensified, mainly linked to economic activities. Since most of these pollutants are harmful to humans, its reduction in the environment is a topic of international, national and local interest. It is important to note that mitigating these emissions is also vital for the conservation of urban natural green areas, which at the same time provide ecosystem services to cities. 

Mexico City Metropolitan Area (MCMA) has a dynamic population of more than 20 million inhabitants being the twelfth largest city in the world and as all megacities faces great challenges in which it refers to environmental sustainability and quality of life of its inhabitants. Actually, MCMA has 23 urban natural green areas and a community area corresponding to 26,047 ha with ecological conservation status (it corresponds to 17.42% of the city territory of 148,500 ha) which offer various ecosystem services such as aquifer recharge, greenhouse gas fixation, climate regulation, agricultural production, noise control, CO2 fixation, local wildlife reservoir, both flora and fauna, and of course, and not least important, recreational and cultural activities. That is why it is essential to evaluate the quality of the atmospheric deposition that is presented in this megacity, to maintain the conservation and management of these natural protected areas. Fortunately, the city is provided with a network of monitoring atmospheric deposition (REDDA) that uses semi-automatic equipment for the collection of dry deposition and wet deposition samples at 16 sampling sites. Some of these monitoring sites are within natural urban areas. The operation of this network is in collaboration with the Government of Mexico City with the Center for Atmospheric Sciences of the National Autonomous University of Mexico. 

The most important findings in relation of the atmospheric deposition evaluation in MCMA include: 

The pH values of the samples at the stations located in the south were more acidic than the samplesfor the stations in the north. It is important to mention that in the south part of the MCMA is where most of the urban natural green areas are located. 

These results are in line with meteorological conditions, prevailing winds blowing from the north to the south, as well as emission sources located in the north sector. 

In most study sites, annual volume weighted decreased from 2003 to 2014. For example, “Montecillos" station located in the North in a rural area, values decreased from 7.48 in 2003 to 5.03 in 2014. 

In the stations located in a rural area (“Montecillos” in the North and “Ajusco” in the South), a strong correlation was found between NH4+ and SO42-, NO3-, and Cl-; this is likely due to their origin in soil (i.e., fertilizer use). The same concentration trend was observed for SO42- and NO3- in the MCMZ stations; this indicates that the sources of acid rain precursors are upwind (i.e., to the north) of the study area. 

It is important to take advantage of the information generated by the existing network and, if necessary, increase the number of sites to generate information for the integral decision-making on the quality of life of people in MCMA and the conservation of the urban natural green areas, as well as studying the critical loads that occur in megacities with local and global impact. 

The actions described will help to establish an integral monitoring of the natural components of the urban ecosystem that consider the potential impact of atmospheric deposition in soils, water bodies, flora and fauna, etc.
2Universidad Nacional Autónoma de México,
3Universidad Nacional Autónoma de México,
4Universidad Nacional Autónoma de México,
5Universidad Nacional Autónoma de México,
6Universidad Nacional Autónoma de México, maria
7Universidad Nacional Autónoma de México,
8SIMAT Gobierno de la Ciudad de México,
9SIMAT Gobierno de la Ciudad de México,