Limited effect of urban tree vegetation on NO2 and O3 concentrations near a traffic route.

Concentrations of NO2 and O3 were measured inside and outside a dense broad-leaved forest canopy adjacent to a busy traffic route in the City of Gothenburg, Sweden, with duplicate passive diffusion samplers during six one-week periods starting well before leaf senescence and ending when leaves were largely senescent. Concentrations of NO2 were lower inside the forest canopy during all periods (representing a significant effect, p = 0.016), on average by 7% or 2.7 μg m(-3). O3 showed a more variable response with an average non-significant effect of 2% lower in the forest stand. There was no systematic trend of the difference in concentrations inside and outside the forest stand of the pollutants with the progression of autumn leaf senescence. Our study indicates that the effect of urban vegetation on air pollution concentrations is small, although it seems to exist for NO2 in a traffic polluted environment.