A modeling study of the impact of urban trees on ozone

Modeling the effects of increased urban tree cover on ozone concentrations (July 13-15, 1995) from Washington, DC, to central Massachusetts reveals th at urban trees generally reduce ozone concentrations in cities, but tend to increase average ozone concentrations in the overall modeling domain. Duri ng the daytime, average ozone reductions in urban areas (1 ppb) were greate r than the average ozone increase (0.26 ppb) for the model domain. Interact ions of the effects of trees on meteorology, dry deposition, volatile organ ic compound (VOC) emissions, and anthropogenic emissions demonstrate that t rees can cause changes in dry deposition and meteorology, particularly air temperatures, wind fields, and boundary layer heights, which, in turn, affe ct ozone concentrations. Changes in urban tree species composition had no d etectable effect on ozone concentrations. Increasing urban tree cover from 20 to 40% led to an average decrease in hourly ozone concentrations in urba n areas during daylight hours of 1 ppb (2.4%) with a peak decrease of 2.4 p pb (4.1 %). However, nighttime (20:00-1:00 EST) ozone concentrations increa sed due to reduced wind speeds and loss of NOx scavenging of ozone from inc reased deposition of NOx. Overall, 8-hour average ozone concentration in ur ban areas dropped by 0.5 ppb (1%) throughout the day. (C) 2000 Elsevier Sci ence Ltd. All rights reserved.