Nitric acid dry deposition associated with equilibrium shift of ammonium nitrate above a forest by long-term measurement using relaxed eddy accumulation. (1st July 2021)

• Record Type: Journal Article
• Authors: Xu, Mao; Kasahara, Kenta; Sorimachi, Atsuyuki; Matsuda, Kazuhide
• Is Part Of: Atmospheric environment. Volume 256(2021)
• Publication Date: 2021-07-01
• Subjects: Deposition velocity -- REA -- HNO3 -- Semi-volatile aerosols -- Asia -- Deciduous forest; Air -- Pollution -- Periodicals; Air -- Pollution -- Meteorological aspects -- Periodicals; 551.51
• DOI: 10.1016/j.atmosenv.2021.118454
• Languages: English
• ISSNs: 1352-2310

Abstract

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 (NH4 NO3 ) 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 < 0.05) larger than those of fine SO4 2− particles and similar to those of HNO3, regardless of the leafy and leafless periods. The larger deposition velocities of fine NO3 − particles caused by the equilibrium shift of NH4 NO3 were found to be similar to previous measurements at the same site. The measured deposition velocities of HNO3 were low and included many emission cases compared to those of fine NO3 − particles. The lower HNO3 deposition velocities were associated with the equilibrium shift of NH4 NO3 to the gas phase, causing the suppressed deposition and apparent emission of HNO3, particularly during the leafless period. The emission of HNO3 is possibly due to high HNO3 concentration near the surface, which is attributed to a small removal of the leafless canopy. The turbulence enhanced both the upward and downward fluxes of HNO3 . These results will contribute to the better understanding for the dynamics of the nitrogen compounds in the atmosphere and the accurate assessment for the nitrogen deposition. Highlights: We conducted long-term flux measurements of HNO3 and NH4 NO3 above a forest. We applied the relaxed eddy accumulation method to measure the fluxes. Fine nitrate Vd was significantly larger than fine sulfate Vd . Suppressed deposition of HNO3 were observed particularly in leafless period. Apparent emission of HNO3 is due to the volatilization of NH4 NO3 near the surfaces.