On the 13th, calm conditions suddenly appeared from 5:00 pm
and the first NRTI in December was recorded in the 7:00 pm
upper-air sounding. The temperature inversion and calm conditions
coincided with the evening traffic rush hour and cooking making
particulate and gaseous pollutants from traffic and coal burning
trapped in a shallow layer just above the ground. Pollutant concentrations
drastically soared from 5:00 pm, reached maxima at about
9:00 pm then declined gradually overnight and throughout the
daytime reaching the lowest levels at around 4:00 pm ahead of
the next day NRTI (Fig. 6). The pollutant loadings occurred predominantly
from 6:00 pm to midnight and were much reduced in
daytime. The evening peaks occur at about 3.5 h later than the
rush hours as a result of pollutant buildup in poor nighttime dispersion
conditions (Venkatram and Cimorelli, 2007). In the meantime,
the morning pollutant peaks were weakly pronounced suggesting
the inversion layer had already dissolved before the morning traffic
rush hour.