For a better appreciation of the diurnal variations of pollutant concentrations
in NRTI and STI periods, Fig. 8 shows the average diurnal
trends in 2003. Both the morning and evening traffic peaks clearly
show up. The rush hours for motorcycles and gasoline cars, which
are the major sources of CO and NO2 in Hanoi, are marked with triangles
on the horizontal axis. The rush hours for diesel trucks and buses,
which contribute largely to SO2 and PM10 loadings, occur about
1 hour later. According to CENMA (2009), the vehicle fleet of Hanoi
is dominated by motorcycles followed by gasoline cars, diesel trucks
and buses in the proportions of 1:0.06:0.04:0.008.