The inter-comparison of regional air quality models is an effective way to understand uncertainty in ambient pollutant concentrations simulated using various model configurations, as well as to find ways to improve model performance. Based on the outcomes and experiences of Japanese projects thus far, a new model inter-comparison project called Japan’s study for reference air quality modeling (J-STREAM) has begun. The objective of J-STREAM is to establish reference air quality modeling for source apportionment and effective strategy making to suppress secondary air pollutants including PM_2.5 and photochemical ozone in Japan through model inter-comparison. The first phase focuses on understanding the ranges and limitations in ambient PM_2.5 and ozone concentrations simulated by participants using common input datasets. The second phase focuses on issues revealed in previous studies in simulating secondary inorganic aerosols, as well as on the three-dimensional characteristics of photochemical ozone as a new target. The third phase focuses on comparing source apportionments and sensitivities under heavy air pollution episodes simulated by participating models. Detailed understanding of model performance, uncertainty, and possible improvements to urban-scale air pollution involving secondary pollutants, as well as detailed sector-wise source apportionments over megacities in Japan are expected.

A model inter-comparison of secondary pollutant simulations over urban areas in Japan, the first phase of Japan’s study for reference air quality modeling (J-STREAM Phase I), was conducted using 32 model settings. Simulated hourly concentrations of nitric oxide (NO) and nitrogen dioxide (NO2), which are primary pollutant precursors of particulate matter with a diameter of 2.5 µm or less (PM2.5), showed good agreement with the observed concentrations, but most of the simulated hourly sulfur oxide (SO2) concentrations were much higher than the observations. Simulated concentrations of PM2.5 and its components were compared to daily observed concentrations by using the filter pack method at selected ambient air pollution monitoring stations (AAPMSs) for each season. In general, most models showed good agreement with the observed total PM2.5 mass concentration levels in each season and provided goal or criteria levels of model ensemble statistics in warmer seasons. The good performances of these models were associated with the simulated reproducibility of some dominant components, sulfates (SO42−) and ammonium (NH4+). The other simulated PM2.5 components, i.e., nitrates (NO3−), elemental carbon (EC), and organic carbon (OC), often show clear deviations from the observations. The considerable underestimations (approximately 30 µg/m3 for total PM2.5) of all participant models found on heavily polluted days with approximately 40–50 µg/m3 for total PM2.5 indicated some problems in the simulated local meteorology such as the atmospheric stability. This model inter-comparison suggests that these deviations may be owing to a need for further improvements both in the emission inventories and additional formation pathways in chemical transport models, and meteorological conditions also require improvement to simulate elevated atmospheric pollutants. Additional accumulated observations are likely needed to further evaluate the simulated concentrations and improve the model performance.

East Asian oceans are possibly affected by a high nitrogen (N) burden because of the intense anthropogenic emissions in this region. Based on high-resolution regional chemical transport modeling with horizontal grid scales of 36 and 12 km, we investigated the N burden into East Asian oceans via atmospheric deposition in 2010. We found a high N burden of 2-9 kg N ha^-1 yr^-1 over the Yellow Sea, East China Sea (ECS), and Sea of Japan. Emissions over East Asia were dominated by ammonia (NH₃) over land and nitrogen oxides (NO_x) over oceans, and N deposition was dominated by reduced N over most land and open ocean, whereas it was dominated by oxidized N over marginal seas and desert areas. The verified numerical modeling identified that the following processes were quantitatively important over East Asian oceans: the dry deposition of nitric acid (HNO₃), NH₃, and coarse-mode (aerodynamic diameter greater than 2.5 μm) NO₃^-, and wet deposition of fine-mode (aerodynamic diameter less than 2.5 μm) NO₃^- and NH₄⁺. The relative importance of the dry deposition of coarse-mode NO₃^- was higher over open ocean. The estimated N deposition to the whole ECS was 390 Gg N yr^-1; this is comparable to the discharge from the Yangtze River to the ECS, indicating the significant contribution of atmospheric deposition. Based on the high-resolution modeling over the ECS, a tendency of high deposition in the western ECS and low deposition in the eastern ECS was found, and a variety of deposition processes were estimated. The dry deposition of coarse-mode NO₃^- and wet deposition of fine-mode NH₄⁺ were the main factors, and the wet deposition of fine-mode NO₃^- over the northeastern ECS and wet deposition of coarse-mode NO₃^- over the southeastern ECS were also found to be significant processes determining N deposition over the ECS.

This study developed a database of emission inventories from ships around Japan using the activity data in the 2015 Japanese fiscal year. The emission of air pollutants from ships was estimated for fuel combustion in main engine, auxiliary engines, and boilers onboard. The ships’ operations for the emission estimation in the exhaust gas consist of ships in navigation, ships at anchor, and fishing boats. For the emission estimation regarding navigation ships, data from Automatic Identification System (AIS) received at the stationary stations on land in Japan were used in this study to calculate the activity. The emission amounts were compared between the sea (Seto Inland Sea (SIS) and Tokyo Bay) and prefectures surrounding those ocean areas. The ship emission ratios in total anthropogenic emissions including the land part reached SO₂: 29%, PM: 40%, and NO_x: 22% for the SIS area. In Tokyo Bay, the ship emission ratios in total reached SO₂: 36%, PM: 28%, and NO_x: 13%, and the emission intensities per unit area for SO₂ and PM_2.5 are approximately three times higher than those in the other regions, respectively. Therefore, the shipping traffic density is relatively higher compared to transportation on land, and the emission is condensed locally. Assuming that the 2020 global sulphur limit switched to a sulphur content of 0.50% m/m, SO₂ emissions could be reduced to 24% in SIS and 22% in Tokyo Bay, sulphate emissions could be reduced to 23% in SIS and 21% in Tokyo Bay, and PM emissions could be reduced to 39% in SIS and 36% in Tokyo Bay. Therefore, the 2020 global sulphur limit should reduce the emission from ships around the Japan coastal area and improve the air quality in congested water areas such as SIS and Tokyo Bay.