conditions, and the particulate mass was measured by the
gravimetric method using an electric ultramicrobalance
(Mettler UM3) with a sensitivity of 0.1 µg. PM10 concentration
is the sum of the PM2.5 concentration calculated
from the increase in weight of the backup filter and the
PM2.5-10 concentration calculated from the gain of the 2-µm
impactor filter.
The precision test of the personal air sampler was performed
by placing two personal air samplers close together
and sampling at the same flow rate at the low-pollution
sites—Prachasumran and Talad-Ku—nine times (see
Table 1). All the air samplers were calibrated by the same
flow meter. The filter holders were randomly selected from
a large stock, and the precision was evaluated by coefficient
of variation (CV).
Sampling Procedure. The particulate samplings were conducted
at road intersections both inside and outside the
police booth for 12 hr each during the day (approximately
6:00 a.m.–6:00 p.m.) and at night on weekdays. The personal
exposure and intersection measurements were performed
simultaneously from approximately 6:00 a.m. to
6:00 p.m., and when an officer completed his shift in the
daytime, the same personal sampler was taken to the officer
on the next shift. Normally, police stand in the center
of the intersection when traffic is busy and stay in the
police booth while resting. The time period for sampling
and the number of samples collected at each site are shown
in Table 1.