ABSTRACT
It has been reported that ambient ozone concentrations in Japan have not been effectively suppressed by precursor emission controls. In this study, we developed an unprecedented long-term emission inventory of ozone precursors, including nitrogen oxide (NOX) and volatile organic compounds (VOCs), in Japan. The developed emission inventory, which explicitly represents changes in emissions caused by emission controls and variation in activities from 2000 to 2019, revealed that emission controls implemented for vehicles, large point sources, and fugitive VOC sources effectively reduced precursor emissions. The impact of emission changes on ozone concentrations at six different timescales was evaluated using air quality simulations. Three of them corresponded to annual mean values of daily ozone concentrations, and the others corresponded to annual high ozone concentrations. The simulations performed better at higher ozone concentrations. The simulation results suggested that emission controls have helped suppress annual high ozone concentrations, whereas the annual mean values of daily ozone concentrations may have increased in populated urban areas because reduced NOX emissions weakened the titration of ozone. Overall changes in the simulated ozone concentrations over different timescales were generally consistent with those in the observed ozone concentrations. These findings highlight the importance of evaluating the impact of emission controls on ozone concentrations over multiple timescales. The differences in the sensitivities of ozone concentrations to emission controls were clearly explained by the ozone sensitivity regimes implied by the ratios of formaldehyde to nitrogen dioxide. The outcomes of this study could assist countries beyond Japan in developing effective strategies to manage ozone pollution.
ABSTRACT
The decreasing trend of the atmospheric 137Cs concentration in two cities in Fukushima prefecture was analyzed by a regression model to clarify the relation between the parameter of the decrease in the model and the trend and to compare the trend with that after the Chernobyl accident. The 137Cs particle concentration measurements were conducted in urban Fukushima and rural Date sites from September 2012 to June 2015. The 137Cs particle concentrations were separated in two groups: particles of more than 1.1 µm aerodynamic diameters (coarse particles) and particles with aerodynamic diameter lower than 1.1 µm (fine particles). The averages of the measured concentrations were 0.1 mBq m-3 in Fukushima and Date sites. The measured concentrations were applied in the regression model which decomposed them into two components: trend and seasonal variation. The trend concentration included the parameters for the constant and the exponential decrease. The parameter for the constant was slightly different between the Fukushima and Date sites. The parameter for the exponential decrease was similar for all the cases, and much higher than the value of the physical radioactive decay except for the concentration in the fine particles at the Date site. The annual decreasing rates of the 137Cs concentration evaluated by the trend concentration ranged from 44 to 53% y-1 with average and standard deviation of 49 ± 8% y-1 for all the cases in 2013. In the other years, the decreasing rates also varied slightly for all cases. These indicated that the decreasing trend of the 137Cs concentration was nearly unchanged for the location and ground contamination level in the three years after the accident. The 137Cs activity per aerosol particle mass also decreased with the same trend as the 137Cs concentration in the atmosphere. The results indicated that the decreasing trend of the atmospheric 137Cs concentration was related with the reduction of the 137Cs concentration in resuspended particles.
