Nitric acid dry deposition associated with equilibrium shift of ammonium nitrate above a forest by long-term measurement using relaxed eddy accumulation

Abstract: The dry deposition process of nitrogen compounds still has considerable uncertainty due to the complex compound behavior in the atmosphere. To enhance understanding of the nitric acid (HNO3) dry deposition process that largely contributes to the total nitrogen deposition, we associate this process with the equilibrium shift of ammonium nitrate (NH4NO3) to HNO3 and ammonia (NH3) near the surface. We obtained the flux of HNO3 and PM2.5 components above a forest in suburban Tokyo using a relaxed eddy accumulation (REA) system incorporating the denuder/filter-pack sampling technique. We conducted continuous samplings on a weekly basis over a long-term period, from October 14, 2016 to October 3, 2018, including leafy and leafless conditions. The measured deposition velocities of fine NO3− particles were significantly (p