Distinctive diurnal patterns of pollutant concentrations were
found for NRTI and STI periods as NRTI occurs only at night while
STI may also exist in both nighttime and daytime. During each NRTI
period, pollutant loadings dramatically rise after sunset reaching
maxima sometime before midnight then gradually decline overnight
and throughout the daytime before reaching the lowest levels at
around 4:00 pm ahead of the next-day NRTI. The evening traffic
peak is much more pronounced than the morning one and pollution
is about two times higher at night than in daytime despite a considerable reduction of traffic emissions during nighttime. On the contrary,
in STI periods, broad morning and evening traffic peaks are observed
and pollution is as high at night as in daytime