Computer modeling estimates that urban trees in the contiguous U.S. remove 711,000 metric tons of carbon, monoxide, nitrogen dioxide, ozone, particulate matter, and sulfur dioxide each year, a service with an annual value of $3.8 billion. The analysis used meteorological, pollution concentration, and urban tree cover data and was performed for 55 U.S. cities and for the entire nation. Typical air pollutant removal per city was hundreds to thousands of metric tons per year.
Nowak, David J., Daniel E. Crane, and Jack C. Stevens. (2006). Air pollution removal by urban trees and shrubs in the United States. Urban Forestry & Urban Greening, 4(3), 115-123.