Enhancement of dry deposition of PM2.5 nitrate in a cool-temperate forest

Abstract

Fine particulate matter (PM2.5) is one of most concerning air pollutants in East Asia. In order to further understand the dry deposition process of PM2.5, measurements were carried out in a cool-temperate forest located in a remote area of northern Japan. These measurements took into account the improvement of simulations by chemical transport models in this region. We measured the vertical profiles of fine (PM2.5), coarse aerosol components, HNO3, and other relevant gases using two denuder/filter-pack sampling systems together with three 4-stage filter-pack sampling systems from July 22, 2017 to August 07, 2017. The vertical profile measurements clearly showed the difference in dry deposition process between fine NO3− and fine SO42−. Concentration gradients of fine NO3− from over to under the canopy were significantly higher than those of fine SO42−. Since most of fine NO3− existed as NH4NO3, it was considered that these high gradients of NO3− were associated with the process of the conversion between NH4NO3 and HNO3/NH3. The concentration gradients of fine NO3− increased with the increase in daytime air temperature from over to under the canopy. The simulations of the thermodynamic equilibrium of NH4NO3 at canopy surface temperatures using ISORROPIA II showed that the equilibrium shift of NH4NO3 into the gas phase during the daytime hot canopy can decrease fine NO3− by approximately 9% when compared to normal atmospheric conditions. The volatilized HNO3 was most probably immediately removed by the canopy as the HNO3 concentrations near the canopy were extremely low. Therefore, the equilibrium shift enhanced the dry deposition of nitrogen within fine NO3−.