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Air pollution is a major environmental concern in most urban areas. Atmospheric emissions of gaseous and particulate pollutants have caused profound environmental and health implications in last years. Sulphur dioxide (SO2), nitrogen oxides (NOx) and ammonia (NH3) are gaseous pollutants commonly present in both, man- made and natural emissions in the lower atmosphere. Industrial processes, vehicular traffic, open biomass burning (including forest fires), dairy farming, intensive animal husbandry, and other anthropogenic activities are the main sources for these gaseous pollutants in the troposphere.

These pollutants may be removed from the atmosphere by two ways: wet deposition, which includes all pollutant material reaching the earth's surface by precipitation; and dry deposi‐ tion, comprising the processes of adsorption of particulate and gaseous material by land or water surfaces [1]. In a general way, both processes govern the transfer of beneficial and toxic chemicals from the atmosphere on to surfaces.

Sulfur and nitrogen deposition (N and S) occurs as a result of these removal processes and is associated with acidification of soils and surface waters. Some environmental indicators have demonstrated that acidification related to air pollution may have already occurred in devel‐ oping countries [2- 3]. The deposition of sulphur compounds cause changes in the chemistry and biology of the soils. These changes include decreasing pH and alkalinity, elevated concentrations of soluble aluminium and an imbalance in nutrient cycling leading to a change of ecosystem diversity [4]. There are evidences that nitrogen compounds deposition causes changes in ecosystems through eutrophication and acidification of soils and waters, decline in

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trees growth and losses of vitality in forest [5- 6]. In addition, N and S deposition can cause deterioration to historical monuments and diverse materials and can cause damages to human health.

In Mexico, N and S deposition monitoring, its spatial and temporal distribution, and its effects on historical and cultural heritage and sensitive vegetation species have not been sufficiently studied. Comparing current conditions with ten years ago, S emissions' scenarios have decreased as a result of reformulation of fuels and the application of rigorous regulations focused to reduce them; however, it has been reported that the relative importance of N deposition has increased [7].

The coast of the Gulf of Mexico is characterized to be a complex region where the co- existence of oil refineries, offshore platforms and other facilities for oil and gas exploration and produc‐ tion, historical monuments, archaeological zones, and many valuable aquatic and terrestrial ecosystems (this region has the widest mangrove forests cover in the country) occurs. Cam‐ peche State is located at the southeast of the Gulf of Mexico and it constitutes the most important oil and gas producer in the country. A total annual emission of 205.64 Gg of NOx and 336.79 Gg of SO2 has been reported for this state, mainly produced by urban and industrial sources. Consequently, it is expected that N and S deposition, in ecosystems located downwind from these sources, would be high. This region has important natural reserves, national parks, protected natural areas, historical monuments and archaeological sites that constitute the main support of the eco- tourism industry which generates significant economical resources for Campeche State. Only in Campeche State, mangrove cover accounts for 29.98% of the total country cover, which is, approximately 196,552 ha. Therefore, the potential ecological effects derived from atmospheric deposition on these sites constitute a key concern for this region.

Critical loads estimation method allows to quantify the grade of damage derived from atmospheric pollutants deposition on ecosystems. A critical load can be defined as the input of one pollutant at levels below of which harmful ecological effects do not occur in the long term. To establish these critical load values and estimate their exceedances in a given site, atmospheric deposition measurements in field are required. N and S levels, their spatial and temporal distribution and their deposition fluxes are used in order to determine the actual inputs of these pollutants to ecosystems. This information is commonly represented in deposition maps, most of them are based on three- dimensional chemical transport model results which need to be validated by comparison with field measurements [8- 10]. Such studies represent an opportunity for policy- makers to identify the potential impacts associated with different emission sources and their spatial and geographical distribution. Sensitivity mapping and the critical load approach are methods that may allow analysis of these risks.

Particularly, in Mexico, measurement- based maps are required to assess the current deposi‐ tion fluxes and the vulnerability of the ecosystems. Considering that studies about critical loads and their exceedances are scarce in tropical humid forests, this research work had the following aims: 1) To establish a solid base line about throughfall deposition of N and S in mangrove ecosystems, and 2) To assess the temporal and geographical distribution of N and S deposition along Carmen Island in order to identify critical zones and seasons in which N and S deposition can be high as a result of the prevailing meteorological conditions.
