A modeling study of the impact of urban trees on ozone

In this study, a doubling of tree canopy cover, to 40% from 20%, for the northeastern coastal United States was modeled to determine the effects on ozone levels. Six scenarios were considered: (1) the base condition; (2) an increase to 40% canopy cover but trees not planted specifically to shade buildings and thus no corresponding decrease in utility air-pollutant emissions; (3) an increase to 40% canopy cover with a 5% reduction in utility emissions owing to strategic tree planting to conserve energy; (4) similar to #3 but with all species low BVOC emitters; (5) land use change with conditions similar to #3 but with anthropogenic emissions removed in 40% of urban cells; and (6) a deposition velocity and BVOC change in which trees were removed from urban areas to isolate how deposition velocity and BVOC emissions affect ozone formation when no meteorological changes are included. The study concludes that urban trees reduce ozone concentrations in urban areas but may increase ozone levels at the regional level, particularly at night. Changing the species composition to include only low-BVOC-emitters did not affect the ozone concentrations, most likely because this region tends to be NOx limited. The most significant effects of trees on ozone levels seemed to be due to changes in the atmospheric physical environment, including how trees reduce wind speed, affect boundary layer heights, and reduce air temperature.