The pollution climate of an area is influenced by meteorology and emissions of air pollutants at local and regional scales. The physical and chemical state of the atmosphere determines pollutant transport, dilution, chemical transformation, and ultimately deposition. In many cases meteorology is more important than atmospheric chemistry in controlling the location and the form in which the pollutants are deposited. Estimating pollutant concentrations and loading to forests in the West requires a detailed analysis of emissions, pollutant transport, dilution, chemical transformations, and deposition processes, together with estimates of the relative contribution by each depositional process to total deposition. The first portion of the chapter examines atmospheric conditions that influence the transport and deposition of pollutants and applies this information to conditions experienced within western forests. A brief discussion of the chemistry of sulfur, nitrogen, and ozone is included. A glossary of technical terms is available in Appendix II. The second portion of the chapter discusses air quality and wet deposition in and around western forests. Emphasis is placed on nitrogen and sulfur oxides and their oxidation products. These pollutants contribute to acid rain as well as to other air pollution problems such as ozone and visibility reduction. All aspects of air quality are presented; from emissions of pollutants to ambient concentrations of important chemical species in gaseous, particulate matter, and precipitation forms.