Global scale transport of acidifying pollutants

During the past few years several attempts have been made to use three-dimensional tracer transport models to simulate the global distribution of sulfur and nitrogen compounds from both natural and anthropogenic sources. We review these studies and show examples of estimated distributions of the total deposition of sulfur, oxidized nitrogen and ammonium as well as the pH of precipitation. The simulated patterns are compared with observations. Weaknesses in these estimates resulting from lack of knowledge of emissions, chemical transformations and removal processes are emphasized and discussed. We also show examples of how the models can be used to estimate past and future deposition patterns. In particular, we use the IPCC scenario IS92a to estimate the possible sulfur deposition around the world in the year 2050. A comparison with critical load values for sulfur deposition indicates that substantial parts of South and East Asia are at risk for acidification problems in the future.