OR/16/028 Conclusions

Beriro, D J, Field, L P and Cave, M R. 2016. Validation of a gravimetric PM2.5 impactor using particle sizing techniques. British Geological Survey Internal Report, OR/16/028.

The results of this investigation show that the DustScan DS500X system fitted with the PM_2.5 size selective impactor is capable of separating particles with a d_PA of ≤2.5 µm from ambient air and retaining them on filter media for subsequent quantification. This statement relates to test sampling conducted at Ballidon Quarry for the limestone particulate matter generated in the primary crusher shed. The overall median particle size measured on the filters was 0.46 µm, the mean was 0.74 µm and the 95th percentile was 2.15 µm (n=6343). The results of the particle size analysis of the oversize fraction collected from the impactor plate by Escubed Limited report the median as 4.01 µm.

The median values are lower than might be expected for the filters if the following PM_2.5 definition is applied:

“Particulate matter suspended in air which is small enough to pass through a size-selective inlet with a 50%
efficiency cut-off at 2.5 μm aerodynamic diameter”            British Standard EN 12341:2014

The particle size analysis conducted by BGS and Escubed report the d_PA on a 2D plane and equivalent sphere diameter in solution as oppose to dae referred to in the British Standard definition. Although the definition is applicable validating the collection efficiency of the DS500X system, the results are essentially surrogates for dae. Further analysis of dae would only be possible if significantly greater sample mass was made available, this is not an option with the current configuration of the DS500X. Notwithstanding, the measurements presented in this study are extremely encouraging and show that the majority of the particles collected on the filter are less than 2.5 µm and that those collected on the impactor plate are greater than 2.5 µm. This indicates that the DS500X fitted with the PM_2.5 size selective impactor is an effective method for collecting the desired particle sizes.

Exploratory data analysis of the particulate matter collected on the filters showed that the particle size distribution between and within sampling visits was very consistent. These indicators of the precision of the DS500X system suggest a good level of collection repeatability between stations and reproducibility between visits.

The following observations were made during the monitoring visits to Ballidon Quarry and the subsequent gravimetric and particle sizing analyses:

1. The primary crusher shed at Ballidon Quarry was a very dusty environment (Figure 3);
2. The particulate matter collected and analysed for this study is limited to a single principle source comprising limestone rock with similar density of morphology characteristics;
3. When the impactor was dismantled for oversize particulate matter collection and cleaning a significant mass of particulate matter remained on the funnel section of impactor (Figure 6);
4. When the filter cassette was removed there was a significant mass of particulate matter (>1 µg) in the tube connecting the box inlet to the cassette (Figure 7);
5. Physical differences observed between filters from the first two weeks of sample collection and the last week of sample collection (quantity of material, delamination in the centre of the filters, and colour of material collected);
6. When the petroleum jelly was re-applied to the impactor it was difficult to ascertain whether the thickness applied was the same each time. It is not known how significantly this factor contributes to collection efficiency;
7. Some of the larger particles observed using secondary STEM conditions were a determined to be aggregate particles. It was not possible to differentiate between whether this is due to flocculation during the dilution process or agglomeration in ambient air prior to sample collection.

The experiments demonstrated that the DS500X samplers fitted with nominally PM_2.5 size-selective inlets could collect samples in a dusty environment with a high degree of precision. BGS recommend that the causes of the observations reported above are investigated. Further investigations are also recommended to evaluate the device in other environments and further consideration should also be given to how the d_PA results reported relate to the d_ae to help fulfil the British Standards definition of PM_2.5.