Co

<g 0.01 0.10 1.00 10.00 100.00 Active sampler based air concentration (ng m 3)

Fig. 6.3. Plot of various PAH air concentrations measured using active and passive samplers at sites in the UK and Australia.

<g 0.01 0.10 1.00 10.00 100.00 Active sampler based air concentration (ng m 3)

Fig. 6.3. Plot of various PAH air concentrations measured using active and passive samplers at sites in the UK and Australia.

results are useful for SPMDs; however, because the nature of the sampler surface will affect how particles are sampled, further work is required to characterize how particle-bound PAHs are accumulated by other samplers. For example, the SPMD membrane is non-porous but it usually develops a sticky external surface after being deployed in the field, whereas the surface of PUF disks is clearly porous. Furthermore, the design of the sampler chamber also plays a role in how much particle-bound material reaches the sampler surface. Preliminary reports from recent studies suggest that the bowl deployment chambers (Fig. 6.4) used in several studies actually stop a significant proportion of coarse particles from reaching the sampler surface [29].

Unless sampling sites experience very high wind velocities, the mechanism of particulate accumulation by samplers deployed in the bowl chambers (Fig. 6.4) is likely to be Brownian diffusion. Diffusion becomes the dominant mechanism of contact with surfaces only when particle diameters are <0.3 mm. In the case of active samplers, the particulate fraction sampled by glass fibre filters is generally X0.3 mm in diameter. The probability of fine particles (<0.3 mm) sticking on a solid surface (e.g. the PUF sorbent) is generally controlled by local adhesive forces. In light of this information, it is surprising to find that the active sampler total concentrations (gas and particulate phases), of individual high molecular weight analytes, correspond well to the total

Fig. 6.4. Schematic representation of the bowl chamber (transparent side view). Adapted with permission from Ref. [15]. Copyright (2004) American Chemical Society.

Mounting bracket

Mounting bracket

Fig. 6.4. Schematic representation of the bowl chamber (transparent side view). Adapted with permission from Ref. [15]. Copyright (2004) American Chemical Society.

Position of PUF disk, SPMDs or PSD

concentrations (gas and particulate phases), of the same individual analytes, accumulated by passive samplers enclosed in protective deployment chambers.

Discrepancies between air concentrations measured using passive and active samplers at the same site can occur for various reasons. Firstly, if passive and active samplers are not deployed for the same periods, changes in air concentrations during these periods will obviously influence results. Particles can also affect results, particularly for SOCs such as PAHs and PBDEs, many of which are associated with particles in ambient air. However, certain environmental factors can also affect the performance of passive air samplers to a greater extent than active air samplers.

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