**5. Conclusions**

This paper investigated the behavior of reactive pollutants inside a realistic urban street canyon by coupling a CFD model with a chemical reaction model (CBM-IV). The complexity of the urban street canyon geometry was represented by the CFD model and the dynamical mechanism involved in the dispersion of the pollutants were incorporated using mesoscale and radiation models. The

spatial distribution of the reactive pollutants, in particular, NO, NO2, and O3 were researched over a 24-h period on 23 August 2010. The dispersion of the contaminants was highly dependent on the reaction processes, boundary conditions, and emission rates all integrated at the same time within the urban street canyon. The production of NOx or fading of O3 were especially found in regions with low wind speed and high turbulence, and NOx titration was noted to be of grea<sup>t</sup> importance. The O3 behavior was directly a ffected by the chemical reactions near the roadside, where fresh NO was being emitted, and was consequently controlled by the NOx distribution. The grid resolution of WRF-CMAQ appears to have a strong influence when representing the boundary conditions, and still, because of their limitations (1 km × 1 km minimum grid size) the particularities that accompany urban areas such as street, highways, unequal height of buildings, sidewalks cannot be well represented.

Finally, we can conclude that the prevailing wind flow mainly carried the air pollutants in the windward direction with small vortices recirculating pollutants inside the street canyon. This work is an intent to find better representations of boundary conditions and to step forward in the incorporation of radiation models and reactive models with the purpose of simulating urban-like environments, satisfactorily. Further e fforts in this kind of research are necessary to reproduce the realities of the urban areas and the implications that it may have on the people who are exposed to these concentrations during the day.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2073-4433/10/9/479/s1, Figure S1: Boundary conditions for air temperature and wind speed at 08:00, 12:00, 16:00, and 20:00 JST. Figure S2: Boundary conditions for NO, NO2, and O3 concentrations at 08:00, 12:00, 16:00, and 20:00 JST. Figure S3: Spatial distributions of mean NOx emission intensity in CMAQ modeling domains from D1 covering East Asia to D7 covering Osaka Prefecture, and locations of observation sites in Osaka City used for model validations. Figure S4: Diurnal variations of the CMAQ-simulated and observed NO2/NOx concentration ratio at the Kokusetsu-Osaka station for monitoring ambient air pollution on 23 August 2010. Figure S5: Diurnal variations of the CMAQ-simulated, observed and CFD-simulated NO2/NOx concentration ratio at the Umeda-Shinmichi station for monitoring roadside air pollution on 23 August 2010. Figure S6: Vertical air temperature profiles at 08:00, 12:00, 16:00, and 20:00 JST on National Route 25 for the points (**a**) P1 located at y = 230 m; (**b**) P2 located at y = 350 m; (**c**) P3 located at y = 390 m. Table S1: WRF and CMAQ configurations.

**Author Contributions:** F.G.G.O. analyzed the data and also wrote the paper; Q.Z. performed the numerical simulations; T.M., H.S., and A.K. provided significant suggestions on data analyses. All the authors have read and approved the final manuscript.

**Funding:** This research received no external funding.

**Conflicts of Interest:** The authors declare no conflict of interest.
