Urban Air Pollution and Forests. Resources at Risk in the. Mexico City Air Basin. Foreword by Mario J. Molina. With 97 Illustrations, 10 in Full Color. Springer ...
Mark E. Fenn L.I. de Bauer Tomas Hernandez-Tejeda Editors
Urban Air Pollution and Forests
Resources at Risk in the Mexico City Air Basin
Foreword by Mario J. Molina With 97 Illustrations, 10 in Full Color
Springer
13. Nitrogen and Sulfur Deposition in the Mexico City Air Basin: Impacts on Forest Nutrient Status and Nitrate Levels in Drainage Waters Mark E. Fenn, L.I. de Bauer, Karl Zeller, Abel Quevedo, Claudio Rodriguez, and Tomas Hernandez-Tejeda
Based on published emission inventories for metropolitan Mexico City (Bravo 1987; INEGI 1999; Ruiz and Gasca 2002), elevated deposition of nitrogen (N) and sulfur (S) pollutants is expected to occur in forests downwind of Mexico City. Air pollution concentrations are elevated in montane areas to the south-southwest (SSW) of Mexico City because of their proximity to the large urban pollution sources and because of the prevailing northeasterly winds (de Bauer 199 l ). Fossil fuel consumption, mainly for transportation, is the primary source of atmospheric N pollutants in Mexico City. Sulfurous air pollutants come primarily from indus trial activities and burning of fossil fuels (Ruiz and Gasca 2002). There were 128,646 tons of nitrogen oxides (NOx) emissions in 1994 compared with 45,468 tons of S02 (INEGI 1999). Data from the acid precipitation wet deposition network in Mexico City (INEGI 1999) and from a recent throughfall study in an adjacent forested area southwest (SW) of the metropolitan area (Fenn et al. 1999) suggest that sulfate deposition is still greater than nitrate deposition, but similar to the combined deposition of inorganic N forms (nitrate and ammonium). Forest production in northern temperate forests is normally N limited. However, a large body of evidence now indicates that in forested areas exposed to chronic atmospheric deposition, the N status of the forest shifts from N limi tation to a condition of N excess. The syndrome of effects or forest responses that occur in forests with elevated atmospheric N deposition is referred to as N saturation (Aber et al. 1989). Nitrogen saturation can be concisely defined as the long-term removal of N limitations on biotic activity accompanied by a decrease 298
