The chemical regimes of various photochemical products were investigated using the CBMIV photochemical model. According to the analyses by isopleths diagrams, clear regime boundary lines were detected for the main products, NO₂, HNO₃, O₃ and PAN, and minor products, HNO₂, N₂O₅, HNO₄ and NO₃, along with further radicals generated in the HO_x-cycle. A similarity law of regime boundary, the same initial concentration ratio of precursors (VOC₀/NO_x0) producing the same regime boundary, is confirmed for the various photochemical products. Based on the analyses, the NO_x consumptions during the photochemical process are described by a pseudo-first-order reaction model solely depending on the VOC₀/NO_x0 condition. This result is thought to be one of the reasons for the similarity law. As a useful result of our analyses, a clear inhibition role of NO_x emissions in the VOC-limited regime for HNO₃, O₃ and PAN formation is found. NO_x reduction may cause an increase in these species concentrations. Further research is needed for the NO_x reduction effects on these important species.

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In order to verify the NO_x reduction effect due to traffic control, nitrogen oxides (NO_x) were determined on the mountain road of Mt. Fuji one day a week from the end of July to the beginning of September, 2012, using a NO_x analyzer and an electricity supply system which were installed in a monitoring car. The average concentrations per day of NO_x in and out of the traffic control period were measured as 6.2 ppbv and 15.9 ppbv, respectively. The traffic control reduced the NO_x concentration to 39%. The NO_x reduction effect by the traffic control was confirmed by the observation. The average number of automobiles that used the mountain road was 105±31 when traffic congestions occurred during the observation period. The concentrations of NO per vehicle were estimated as 0.05±0.03 ppbv and 0.22±0.04 ppbv for the number of automobiles less than 110 and exceeded 110, respectively. The concentrations of NO per vehicle increased when the number of automobiles was around 110, which might be caused by the traffic status changing from an ordinary condition to congestion.

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Ultrafine particles, that is, particles with a diameter of less than 100 nm, have drawn increasing attention due to their toxicity and associated health risks. To determine the health risk of inhaled ultrafine particles based on the deposited dose of the particle surface area in a roadside environment, we measured the fraction of agglomerates and their morphology, such as particle surface area, using a transmission electron microscope for roadside environmental ultrafine particles and diesel exhausts particles (DEP) with six electrical mobility diameters ranging from 30 to 300 nm. We determined the deposited particle surface area in 3 case scenarios for different contents of agglomerates. The daily deposition dose for an adult man was estimated to be 1.2×10^-4 cm² for the highest dose case that is all aerosols in the roadside environment are agglomerates, which was 2.2 times higher than the case scenario that is all aerosols are spherical-shaped. We compared the daily deposited dose of the highest dose case to the threshold value of acute inflammation and the ratio was 1.2×10^-4, which indicates that the risk was quite low. However, it is noted that this result was based on the assumption that the DEP was low-toxicity, low-solubility particles and the end point was restricted to acute inflammation.

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