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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. [ABSTRACT FROM AUTHOR]

Ammonia (NH 3) is a major component of reduced nitrogen. A bi-directional exchange of NH 3 takes place between the atmosphere and surface. Although the study of reduced nitrogen is gaining prominence in recent years, quantification of NH 3 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 NH 3 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 NH 3 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 NH 3 concentrations at four or five heights of an observation tower in the forest by using filter-pack holders, and determined NH 3 fluxes by using the aerodynamic gradient method. During daytime, NH 3 showed emission above the canopy in summer, and near the forest floor in winter. There was no clear emission trend in autumn. During nighttime, NH 3 showed deposition from the top of the canopy to the forest floor for all observed periods. NH 3 exchange in the forest possibly had seasonal and diurnal variations. The flux measurements showed large NH 3 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 NH 3 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. • We measured NH 3 vertical concentration profiles and flux using aerodynamic gradient method in a deciduous forest in Japan. • We inferred NH 3 fluxes using a bi-directional exchange model and verified applicability in the forest by comparing with observations. • NH 3 exchange showed seasonal and diurnal variations and flux had a strong positive correlation with solar radiation. • Inferred fluxes using the model agreed with the observations at daytime but failed to reproduce deposition at nighttime. • We improved the discrepancy between observations and model by using more suitable cuticular resistance. [ABSTRACT FROM AUTHOR]

Asia has a considerable risk of nitrogen deposition effects on ecosystems due to the high deposition amounts. Ammonia (NH 3) emissions, primarily from agricultural production, have increased in East Asia, and the increasing contribution of NH 3 deposition to the total nitrogen deposition is concerning. However, NH 3 deposition in this region is uncertain due to lacking flux measurements and the complex bidirectional nature of NH 3 atmosphere−surface exchange. To better understand the NH 3 deposition in this region, we investigated NH 3 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. NH 3 fluxes were mostly downward in both periods, indicating that the agricultural field was a sink for NH 3 during the periods (mean flux = −0.091 μg 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 NH 3 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 NH 3 emission of 0.054 μg 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 μg m−2 s−1). These results indicate that there were possibly other processes enhancing NH 3 deposition in the agricultural field. • We obtained NH 3 fluxes over agricultural field in soybean-growing and fallow periods. • We used the relaxed eddy accumulation method to measure the NH 3 fluxes. • Measured fluxes were mostly downward in both periods and larger than model values. • Correlation between V d and u* showed turbulence was main factor driving deposition. • Chemical reaction enhancing NH 3 deposition in agricultural fields possibly exist. [ABSTRACT FROM AUTHOR]

Starting from an inventory of SO2, NOx, VOC and NH3 emissions for the years 1990 and 1995 in East Asia (Japan, South and North Korea, China, Mongolia and Taiwan), the temporal development of the emissions of the four air pollutants is projected to the year 2030 based on scenarios of economic development. The projections are prepared at a regional level (prefectures or provinces of individual countries) and distinguish more than 100 source categories for each region. The emission estimates are presented with a spatial resolution of 1×1 degree longitude/latitude. First results suggest that, due to the emission control legislation taken in the region, SO2 emissions would only grow by about 46 percent until 2030. Emissions of NOx and VOC may increase by 95 and 65 percent, respectively, mainly driven by the expected increase in road traffic volume. Ammonia, mainly emitted from agriculture, is projected to double by 2030. [ABSTRACT FROM AUTHOR]