Behavior of nocturnal urban boundary layer and air pollutants

The turbulence structure of the nocturnal urban boundary layer (UBL) and accompanying high concentration of air pollutants at Sapporo, Japan, were analyzed experimentally. A strong, elevated inversion formed over the Sapporo urban area. Its base height was approximately 40~60 m, which corresponded to twice the average building height. Velocity fluctuations σ(u), σ(w) and Reynolds stress u'w' had nearly uniform profiles within the nocturnal UBL and decreased with height above the UBL. On the other hand, heat flux w'θ' had a large negative peak at the inversion base. Thus, the mechanism of UBL formation was found to be controlled by the downward transport of sensible heat from elevated inversion which was caused by mechanically-generated turbulence. Analysis based on the entire data of November 1984 showed that the nocturnal UBL was created when the averaged surface wind speed over the city was less than approximately 1.8 m/s. Under such conditions, air pollutants showed very high concentrations within the UBL and NO/NO(X) ratio showed the inverse correlation with O₃ concentration. The existence of nocturnal UBL prevented the development of thermal connective layer in the next morning and worked to increase the level of ground base concentration. These facts are very important to understand the formation mechanism of the high air pollution occurring in the large cities.