Dry deposition of nitric acid gas by long-term measurement above and below a forest canopy

Reactive nitrogen negatively affects terrestrial ecosystems by excessive deposition. Nitric acid gas (HNO₃), a component of reactive nitrogen, is readily deposited on ground surfaces due to its high reactivity. However, there have been recent cases in which suppressed deposition fluxes, including upward fluxes, were observed above forests. As the mechanisms of HNO₃ dry deposition on forest surfaces are not fully understood, the accuracy of dry deposition estimates remains uncertain. To reduce uncertainties in the estimation, we investigated dry deposition of HNO₃ by 1-year measurement in a forest. We measured the vertical profiles of HNO₃, nitrate, and sulfate in PM_2.5 in a deciduous forest in suburban Tokyo (FM Tama). We observed their concentrations above the forest canopy (30 m) and near the forest floor (2 and 0.2 m) using the denuder/filter pack from October 2020 to September 2021. The HNO₃ concentration decreased significantly from 30 to 2 m. However, the decrease in HNO₃ was not as significant, and occasionally, emission profiles were produced between 2 and 0.2 m. This was likely caused by HNO₃ generated by the volatilization of NH₄NO₃ near the forest floor, which was warmed by sunlight during daytime in both leafy and leafless periods. Conversely, HNO₃ concentrations at 30 m were much higher than those at 2 m and 0.2 m, indicating that the forest acted as a sink for HNO₃ from a long-term perspective. It is presumed that HNO₃, generated just above the forest canopy, could cause an upward flux if a temperature difference of several degrees occurs between 25 and 20 m.