Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：Springer Science and Business Media LLC  

Abstract

Ammonia (NH₃) has received considerable attention as a major reduced nitrogen. However, accurate estimates of the deposition amount are difficult due to its complex behavior characterized by bidirectional exchange between the atmosphere and the surface. We observed the vertical profile of NH₃ concentration in a deciduous forest in Japan for 1 year to further advance the studies on NH₃ bidirectional exchange in Asia, especially focusing on the process near the forest floor. The observation period lasted from September 29, 2020, to September 28, 2021, including leafy and leafless periods. Using the denuder sampling technique, we measured NH₃ concentration in the forest at three heights (above the forest canopy, 30 m, and near the forest floor, 2 m and 0.2 m). NH₃ concentrations tended to be highest at the top of the canopy (30 m). Focusing on the concentration near the forest floor, the concentrations at 0.2 m were frequently higher than those at 2 m regardless of the leafy and leafless period, thus suggesting NH₃ emissions from the forest floor. NH₃ concentration near the forest floor showed strong positive correlations with air temperature during the leafy period. The NH₃ emissions from the forest floor during the leafy period were possibly due to the decomposition of leaf litter with increased air temperature. The decrease in leaf area index might induced the increase in NH₃ concentration and emission. NH₃ emission during the leafless period was also possibly dependent on the state of the deposition surface, apart from air temperature, relative humidity, and leaf area index.

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DOI： 10.1007/s44273-024-00042-z

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Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：Elsevier BV  

Ammonia (NH3) is a major component of reduced nitrogen. A bi-directional exchange of NH3 takes place between the atmosphere and surface. Although the study of reduced nitrogen is gaining prominence in recent years, quantification of NH3 deposition is technically challenging due to its complex properties; consequently, this generates significant uncertainty regarding the estimation of total nitrogen deposition. Based on past studies, it has been reported that Asia has numerous high-risk nitrogen deposition sites; however, only a few studies have investigated the NH3 exchange in this region. To clarify the exchange process and achieve improved accuracy regarding the estimation of nitrogen deposition in Asia, we conducted measurements of vertical profile and flux of NH3 in a forest in Japan during summer (July 21–August 1, 2015), winter (February 23–February 29, 2016), and autumn (September 27–October 11, 2016). We measured daytime and nighttime NH3 concentrations at four or five heights of an observation tower in the forest by using filter-pack holders, and determined NH3 fluxes by using the aerodynamic gradient method. During daytime, NH3 showed emission above the canopy in summer, and near the forest floor in winter. There was no clear emission trend in autumn. During nighttime, NH3 showed deposition from the top of the canopy to the forest floor for all observed periods. NH3 exchange in the forest possibly had seasonal and diurnal variations. The flux measurements showed large NH3 emissions at daytime in summer and small depositions at nighttime in summer and autumn. Mean flux during the observation periods was 0.026 μg m−2 s−1. The daytime flux during leafy periods was found to have a strong correlation with solar radiation. At daytime in summer, flux also had strong correlations with relative humidity and horizontal wind speed. We also estimated NH3 fluxes in summer by employing a bi-directional exchange model to verify the applicability of the model at the study site. Inferred fluxes obtained via the model showed large emissions during daytime and agreed with the observation results; however, the model could not reproduce the nighttime deposition. This discrepancy was improved by applying more suitable cuticular resistances.

DOI： 10.1016/j.atmosenv.2023.120144

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Abstract

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.

DOI： 10.1007/s44273-024-00034-z

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Authorship：Lead author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：PERGAMON-ELSEVIER SCIENCE LTD  

Asia has a considerable risk of nitrogen deposition effects on ecosystems due to the high deposition amounts. Ammonia (NH3) emissions, primarily from agricultural production, have increased in East Asia, and the increasing contribution of NH3 deposition to the total nitrogen deposition is concerning. However, NH3 deposition in this region is uncertain due to lacking flux measurements and the complex bidirectional nature of NH3 atmosphere-surface exchange. To better understand the NH3 deposition in this region, we investigated NH3 fluxes over an agricultural field in Tokyo, Japan. Fluxes were determined using the relaxed eddy accumulation method incorporating the denuder/filter-pack sampling technique and continuously conducting daytime and nighttime measurements in soybean-growing (July 28-August 5, 2020) and fallow (March 4-March 10, 2021) periods. NH3 fluxes were mostly downward in both periods, indicating that the agricultural field was a sink for NH3 during the periods (mean flux =-0.091 mu g m(-2) s(-1)). The deposition velocities in the growing period were lower than in the fallow period possibly due to the smaller turbulence. The deposition velocities correlated positively with friction velocity, indicating that the turbulence is the dominant factor driving NH3 deposition. We compared inferred fluxes using a compensation point model with the measured fluxes. The model did not reproduce the observation results when using default inputs, but showed a mean NH3 emission of 0.054 mu g m(- 2) s(-1). Revised inputs considering on-site soil and foliage information improved the model's results; however, the inferred fluxes were still smaller than the measured fluxes (mean flux =-0.017 mu g m(-2) s(-1)). These results indicate that there were possibly other processes enhancing NH3 deposition in the agricultural field.

DOI： 10.1016/j.atmosenv.2022.119195

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DOI： https://doi.org/10.1016/j.envpol.2021.117842

Language：Japanese   Publishing type：Article, review, commentary, editorial, etc. (scientific journal)  

DOI： https://doi.org/10.11203/jar.36.32