Deposition Velocity of Sulfate and Nitrate in PM_2.5 above a Forest in Suburban Tokyo Using Relaxed Eddy Accumulation

Abstract

To elucidate the dry deposition mechanisms of PM_2.5, we measured the dry deposition fluxes of sulfate and nitrate in PM_2.5 above a forest in suburban Tokyo using the relaxed eddy accumulation method (REA). We carried out experiments four times in the leafy seasons from September 2014 to August 2015. The REA samplings of PM_2.5 fluxes were continuously done in the daytime and nighttime during about one week for each experiment. Concentration gradients above the forest were also measured. The mean deposition velocities of nitrate were higher than those of sulfate in the four experiments. The median deposition velocities of all the valid data were 0.19 cm/s (sulfate) and 1.4 cm/s (nitrate). The concentration gradients were in accordance with the REA results. The nitrate deposition velocity increased with the ratio of temperature increasing from the air to the forest surface. That is probably caused by the volatilization of NH₄NO₃ just above the high temperature surface. This process possibly enhances the nitrate deposition velocity.