Sample collection
The samples had been collected for five years (January
2005eDecember 2009) by using the four-stage filter pack (Nilu,
Norway). Each sampling was conducted over a period of 10 days. The
four-stage filter pack is composed of four filters in line with the air
stream (Fig. 2). The first two layers are polytetrafluoro ethylene;
PTFE filter (F0) and polyamide filter (F1). The other two layers
(F2, F3) are cellulose acetate filter. The F2 was impregnated with
potassium carbonate, while the F3 was coated with phosphoric
acid and both layers were coated with glycerin solution. Air
was continuously pumped through the filter pack. Aerosols were
collected on the first filter while the gases substances were trapped
on the next filter layers. HNO3, partial SO2, HCl and NH3 in the air
sample were collected on the second filter. The remaining SO2 and
HCl reacted with an alkali-impregnated substance on the third filter.
While NH3 was trapped on the fourth filter using an acid-
impregnated substance. The inlet of the air sampling holder was
set up at a height of about 3 m above the ground (Fig. 2). During the
sampling period, the flow rate of a diaphragm pump was set at
0.01 L min1
. Details of sampling, such as time and ambient air
temperature were recorded before the start the sampling. The filter
pack holder set was closed with a plastic cap or para-film and stored
in a clean plastic zip bag to avoid absorption after the sampling
period (Acid Deposition Monitoring Network in East Asia, 2000).