Recently, it has been reported that the O₃ concentrations on the weekend are higher than those during the weekday despite the lower concentrations of the O₃ precursors such as NO_x and NMHCs in urban areas. This phenomenon is called the “ozone weekend effect” (OWE). We investigated the weekly patterns of O₃ and its precursors and the cause of the OWE, using the data observed in Osaka from 2006 to 2008. The weekly patterns for both the ozone and its precursors on the weekdays were similar among the air monitoring stations but varied year to year. On the other hand, the weekday-Saturday-Sunday variations of the precursors were similar for all the examined periods. The main cause of the OWE is the titration of O₃ by NO in terms of the comparison between the weekdays and weekends. However, the comparison between the weekdays and Saturday in 2007 suggests that the photochemical ozone production on Saturday would be greater than that on the weekdays. The analyses of NO_x and Ox (the sum of O₃ and NO₂) between the weekdays and Saturday indicated that the ozone production regime in 2006, on the weekdays in 2007 and on Saturdays in 2008 would be the boundary region, NMHCs- and NO_x-limited, respectively.

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In recent years, the increasing photochemical oxidant concentration in Japan has been discussed as one of the social issues. In the Kanto area in the summer, the effect of transboundary air pollution is not very high, therefore, a domestic pollutant source could effect this issue. Based on previous studies, a high concentration of inland ozone results from the transportation of ozone from the south, the Tokyo Bay area and the Pacific Ocean driven by sea breezes. However some studies have shown that a high concentration of ozone was observed in the northern Kanto area, before the ozone from south arrives. To understand this ozone elevation in the northern Kanto area, we consider the contribution of the upper-layer ozone to the ground-level ozone by a box model with no chemical reactions. The difference between the simulated results and observed results at 12:00 is 30-60ppb. This difference would be caused by chemical reactions. From the doppler lidar observations and observations of the ozone concentration at Mt. Akagi, the ozone concentration was almost constant and about a 4 to 5 m/s horizontal wind blew on the night of July 27^th and August 4^th at a 1300m height. These results would mean that the northern Kanto area was covered by a constant concentration ozone and transported by the wind.

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The present study aimed to measure the air concentration and exchange flux of ozone (O₃) in a paddy field for the single cropping of rice in central Japan from June 2010 to September 2011. A passive sampler method and a gradient method were used to determine the weekly means of the air concentration at two heights (6m and 2m above ground surface) and the exchange flux of O₃, respectively. The present study also tested a solution to eliminate the overestimation by the filters to measure the O₃ concentrations, which originated from the collection of nitric acid gas in the atmosphere. The solution was the combined use of two types of filters made of the same material ; one is the nitrite and alkaline impregnated filter and the other is the alkaline impregnated filter. The collection effect of nitric acid gas, i.e., the ratio of the nitrate that originated from nitric acid gas to the total nitrate, was on average 9.2% in the present study. The weekly mean O₃ concentrations at the study site during the rice cultivation period were frequently higher than 40 ppb, which tended to be higher than those at the neighboring observatories for air quality. The estimated deposition velocities of O₃ in the daytime and the nighttime in the cropping season were 0.94 and 0.40 cm s^-1, respectively.

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The dispersion of motor vehicle exhaust gas was computed using the k-ε model in a built-up residential area of two story buildings. The near neutral atmospheric stability and wind direction perpendicular to the street were adopted as the model conditions. The computed concentrations were compared to the observed SF6 concentrations from a dispersion experiment. The dispersion domain was within 150 m from the street. We investigated the size of the upstream domain (driver region) required for generating the turbulence intensity that agreed with the observations. The driver region of 250 m produced a good agreement in the turbulence intensity between the computed value and the observation at a z=15 m height, although we used an estimated exponent value for the vertical wind profile as the upstream boundary condition. A sensitivity analysis was done for agreement between the computed and observed concentrations when we varied the number of grid points within the street canyons. This analysis showed that more than 10 grids in the vertical and span-wise directions of the street canyons produced a good agreement. An accurate simulation of the turbulence intensity and a high-resolution simulation of the flow and vortices within the street canyons yielded a good agreement between the observed and computed concentrations. The motor vehicle exhaust gas is dispersed by the mechanical turbulence generated by buildings near the grid points in urban areas. Therefore, it is possible to approximate the turbulence generated upstream, far from the dispersion area, by the upstream boundary condition. However, the turbulence generated near the grid points should be computed at a high resolution.

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