Deposition velocity of PM_2.5 nitrate and gaseous nitric acid above a forest in suburban Tokyo using relaxed eddy accumulation with denuder sampling technique

Abstract

In order to contribute to the improvement of the assessment of nitrogen deposition in Asia, direct measurements of the deposition velocity of nitrate in PM_2.5 and gaseous nitric acid above a forest were carried out. We improved the measurement accuracy of the deposition velocity of sulfate and nitrate in PM_2.5 and gaseous HNO₃ by using relaxed eddy accumulation together with a denuder sampling technique. Observations were carried out in a forest in suburban Tokyo from 14th October to 14th December 2016. Representative values of the deposition velocity during the period were estimated to be 0.80 cm/s for SO₄^2-, 1.4 cm/s for NO₃^- and 1.9 cm/s for HNO₃. The deposition velocities and the decreases in the concentrations from 30 m to 8 m indicated that PM_2.5 nitrate and HNO₃ were more efficiently deposited and removed in the forest than PM_2.5 sulfate. At the upper canopy, there was a process in which NH₄NO₃ converts to HNO₃ by a higher temperature due to solar radiation and is quickly removed. On the other hand, below the forest, there were probably both processes of a low supply of HNO₃ due to the lower temperature and inhibition of NH₄NO₃ volatilization due to a shift in the gas-particle equilibrium.