Differences of Dry Deposition between Sulfate and Nitrate in PM_2.5 to a Forest in Suburban Tokyo by Vertical Profile Observations

Abstract

To elucidate the differences in the dry deposition process between sulfate and nitrate in PM_2.5, vertical profiles of the inorganic components were measured in a forest in suburban Tokyo (FM Tama site) for one year from December, 2012 to November, 2013. We carried out weekly samplings of PM_2.5 at 4 heights (2 heights above canopy, 2 heights below canopy) on a tower built in the forest. The major inorganic aerosols consisted of NH₄NO₃ and (NH₄)₂SO₄ at the 4 heights through out the year. Significant differences between the SO₄^2- and NO₃^- profiles were observed. NO₃^- relatively decreased compared to SO₄^2- toward the forest floor through out the year. In the winter and spring, the air temperature of the forest floor was higher than that of the other heights. On the other hand, in the summer and autumn, the air temperature of the forest floor was lower than that of the other heights. The profiles of the concentrations and the air temperatures indicated that volatilization of NH₄NO₃ by the higher temperature on the floor caused the rapid decrease of NO₃^- in the winter and spring, and the volatilization of NH₄NO₃ by the low concentration of HNO₃ due to its significant removal by the leaves caused the rapid decrease of NO₃^- in the summer and autumn. These volatilization processes indicated that the dry deposition of ammonium nitrate to forests was more efficient than that of ammonium sulfate.