Characteristics of a highly polluted air site
In highly polluted regions, a sampling period of 24 hours may not be achievable. This can be easily checked by monitoring the drop in rotameter flow rate. With the WHILEY valve fully open, the flow rate will be 16-17 lpm at the start of the sampling. Sampling should proceed until the flow rate is reduced to about 14 lpm. This may result in sampling period of much less than 24 hours. If the sampling period can only be up to 8-10 hours (refer to 2.7), then it is recommended to sample two filters per day, i.e. one filter during day-time and another filter during night time or to use the procedure under 2.4b,c.