Current Issue

Photochemical oxidant (or simply ozone) air pollution first occurred in Japan in the 1970s, and was then considered a social issue. Annual mean ozone concentration in Japan has increased since 1980s. Possible causes of high ozone concentration include transboundary air pollution from Asian industrial countries, increased total ozone in the troposphere, settling of stratospheric ozone, and the effect of domestic pollutant sources. Focusing on the Kanto region in Japan (the area in and around metropolitan Tokyo on the Pacific coast of Japan) during summer, seasonal wind from the Pacific Ocean dominates wind patterns and blocks air masses approaching from inland. Pochanart et al. (2002) showed reduced inland air mass to the Pacific coast of Japan in summer relative to other seasons by monitoring pollutants at remote islands and performing backward trajectory analyses. Therefore, the effect of transboundary air pollution is relatively smaller than in other seasons. It is well known that amounts of ozone settling from the stratosphere peak for mid-latitudes in springtime (Muramatsu, 1980) and that there is a reduced effect in summertime. Precursor sources in and around the Kanto region will therefore have the greatest effect on ozone concentration in this area during summertime. One factor behind high ozone concentration inland from large coastal cities is the transportation of precursors by sea breezes (e.g. Schade et al., 2011). There have been many studies of transportation by sea breeze in the inland Kanto region. For example, Wakamatsu et al. (1983) showed typical sea-land breeze transport patterns and vertical distribution of precursors. There are also cases of high ozone concentration in the inland Kanto region before sea breezes approach in the morning (Kiriyama et al., 2012). As a result, high ozone concentration is sustained from mid-morning to early evening in the region. Long exposure to high ozone concentration is a health and environmental problem. Local emissions are considered a major cause of high ozone levels, but no detailed evaluation of morning high ozone concentration has been conducted in the inland Kanto region. It therefore remains unclear whether precursors emitted from the inland Kanto region, which are relatively less than those generated in the metropolitan area, can result in ozone concentrations nearly equal to those of the metropolitan area alone (Hosoi et al., 2012). The contribution of precursors emitted from the metropolitan area and that of inland areas remains unknown. If ozone exists above the nocturnal layer containing the low concentration NOx, ozone is in a better position to stay over night on the layer because NO and ozone is difficult to react. This ozone residual layer forming above the nocturnal boundary layer was reported by Niwano et al. (2007) and Lin et al. (2010), the latter of which showed that downward transport of residual ozone can affect morning ground-level ozone concentration. In the Kanto region, however, there have been no evaluations of the extent to which residual ozone is involved in morning surface ozone concentrations, or of the contribution by precursors from metropolitan and other areas.

The present study uses a meteorological model and a chemical transport model to evaluate the effect of regional precursors and residual ozone on ozone concentrations, especially in the morning inland of the Kanto region during the summer season.

For air quality simulations, we used the Weather Research and Forecasting (WRF) model version 3.2.1 (Skamarock et al., 2008) and the Community Multiscale Air Quality (CMAQ) model version 4.7.1 (Byun and Schere, 2006). The calculation period was 16 July to 8 August 2010, with the first 4 days used as a spin-up period. The results and discussions below therefore use the results from 20 July to 8 August. During this period, high ozone concentrations were continuously observed in the inland Kanto region. The model consists of three domains from domain 1 (D1), covering a wide area of East Asia, to domain 3 (D3), covering most of the Kanto region (Fig. 1). In the WRF simulation, D1 consist of 109×93 grids (64 km×64 km per 1 grid), domain 2 (D2) consist of 73×73 grids (16 km×16 km per 1 grid) and D3 consist of 97×97 grids (4 km×4 km per 1 grid). As input data to the WRF model, for meteorology we used final analysis data provided by the National Center for Environmental Prediction (NCEP), and meso-analysis data provided by the Japan Meteorological Agency (JMA) (used for D2 and D3), and for sea surface temperature we used real-time global sea surface temperature analysis data provided by NOAA/NCEP. Three-dimensional analysis nudging was applied to the west-east and north-south wind components in D1 and D2, with the nudging coefficient set to 1.0×10^-4 s^-1 for the entire simulation period. For initial and boundary conditions of the CMAQ calculation, we used the results calculated by MOZART4/GEOS5 (Emmons et al., 2010) for D1, and simulated results for the outer domains D2 and D3. WRF/CMAQ was configured with the same physics and chemistry options as those used by Shimadera et al. (2013). The emission data for the base case simulation except for volcano emission were produced from the dataset described by Shimadera et al. (2014).

Fig. 1. 
Horizontal model domain used for CMAQ. The black circle divides the Kanto region into two areas, a metropolitan area (inside area of the circle) and a non-metropolitan area (outside area of the circle). Black circle was defined by reference to Route16. Gray points indicate locations of diagnostic points in the Kanto region: Isesaki, Kumagaya, Oomiya, and Shinjuku.

In addition to the base simulation, in D3 we conducted emission-controlled simulations and simulations with no chemical reaction. As Fig. 1 shows, we divided the Kanto region into two areas. One is the interior of the circle, defined by reference to Route 16, with radius about 40 km centered on central Tokyo (labeled “metropolitan” in Fig. 1). The other area is the exterior of this area (labeled “non-metropolitan” in Fig. 1). In this study, metropolitan cases indicate no emissions in the metropolitan area, and non-metropolitan cases indicate no emissions in the non-metropolitan area. Table 1 summarizes the amounts of emission from metropolitan and non-metropolitan area and the total amount of emissions in D3. That table shows that precursors emitted from metropolitan area include 6.90×10⁹ g of NOx, and 1.61×10¹⁰ g of VOCs over 25 days, accounting for approximately 40% of total NOx and VOC emissions. Non-metropolitan areas have more precursors than the metropolitan area, but metropolitan areas have far more precursors per unit area. Contributions from each area were calculated as the difference between the base and each emission-controlled case for ozone concentration. We investigated causes of high ozone concentration by evaluating the contribution of each emission area from these differences. If the ozone concentration increases under no chemical reaction condition, the increase will be influenced by horizontally or vertically transported ozone as distinct from in situ generation by photochemical reactions. Moreover, the increase of ozone will be mostly influenced by vertical ozone transport if horizontal ozone distribution is almost uniform and wind speed is slow. Therefore, to evaluate the effect of downward transport of residual ozone for ground-level ozone concentration, we conducted the no chemical reaction (No_chem) simulation. In the no chemical reaction (No_chem) case, the simulation starts at 05:00 Japan Standard Time (JST, UTC+9 hours; times are given as JST below) and uses base simulation results of D3 for initial conditions and no chemical reaction, and the simulation stops at 05:00 the next day, a 24-h cycle. In the No_chem case, O₃ concentrations for this case will be shown. In this study, boundary condition which is used in base case applied to all cases. Initial condition of base simulation applied to metropolitan and non-metropolitan cases. In this study, emissions in each emission-controlled case from corresponding ocean area is also zero.

Observed ozone concentration data used in this study were obtained from the Atmospheric Environmental Regional Observation System (AEROS, http://www.nies.go.jp/igreen/, in Japanese). In this study, the high ozone concentration criterion was defined as 120 ppb, in accordance with the standard criterion for photochemical oxidant advisories in Japan.

In this study, the words “Inland areas” are used in the sense of the area in and around Kumagaya and the north.

To address the problem of high ozone concentrations, especially during morning, in the inland Kanto region of Japan, we investigated the ozone concentration response to emissions from emissions from metropolitan and non-metropolitan using the WRF model and the CMAQ model. Focusing on 22 July, the metropolitan contribution accounted for less than 10% of the base simulation result. On other high ozone days in the period from 20 to 24 July, the metropolitan contribution was nearly zero in the mornings, and the non-metropolitan contribution accounted for 45-70%. In contrast, during the period from 5 to 8 August, during which relatively high ozone occurred with regularity in Isesaki and Kumagaya, both metropolitan and non-metropolitan contributions appeared throughout the day. But also during this period the non-metropolitan area significantly affected inland ozone concentrations, with contributions of 20-60% at Isesaki and 20-50% at Kumagaya. The spatial distribution also showed that non-metropolitan areas contributed to inland ozone concentrations. The example of 22 July shows that precursors from the metropolitan area had a small, narrow effect inland. The example of 6 August shows that sea breeze transport precursors from the metropolitan area transport to remote parts of the Kanto region and affect ozone concentrations along the penetration path. Moreover, results of the No_chem case simulations show increasing ozone concentrations without secondary formation. This increase results from entrainment of upper residual ozone through the development of the planetary boundary layer. An upper air residual ozone was contributed to by the non-metropolitan area being larger than the metropolitan area.

Two episodes shown in this paper indicate that contributions to high ozone concentration in the inland Kanto region, especially in the morning, and the contribution of non-metropolitan areas constitute the majority of high ozone generation, while the contribution of the metropolitan area was small in the calculation period. But as shown in Figs. 6 and 8, contributions of precursors emitted from the metropolitan area to ozone concentration were sometimes comparable to contributions of non-metropolitan area along the path of the sea breeze. Therefore, the results of the present study will not deny the importance of transportation of precursors emitted from metropolitan area for air quality of Kanto region.