Abstract
The formation mechanism for high concentrations of NO2 over a nocturnal urban area was analyzed based on the field observation data collected at Sapporo in November 1984. In the nocturnal urban area, the NO2/NOx ratio increased with height and was proportional to O3 concentration. The vertical NO2 concentration profile was formed through a combination of turbulent diffusion and chemical reaction (NO with O3), and the vertical gradient of NO2 was very small throughout the nocturnal period. The data analysis showed that 8.3% of NO2 was directly emitted from sources and about 85% of observed ambient NO2 may be caused from the reaction of NO+O3→ NO2+02. Furthermore, the vertical profile of NO2+O3 was nearly uniform. These vertical profiles of air pollutants (NO, NO2 and O3) are important for the understanding nocturnal NO2 problem. In addition, background levels of O3 and the role of turbulent diffusion in urban areas are significant factors in establishing appropriate environmental standards for NO2.
