Accuracy of PM2.5 measurements and what is the ground truth of the local concentration?

In recent months I have been contemplating both the accuracy of the low cost sensors and indeed that of the so-called certified equipment. As part of this consideration, I have also been trying to establish what are the exact measurements the WHO used to decide on its guidelines for health. What technology did they use?

In the UK, technologies such as TEOMs and FIDAS are used by local authorities and DEFRA. Yet it is has been noted that the output from these can vary by as much as 20%. FIDAS has been found to read lower values than a collocated TEOM. One possible reason for this is that the TEOM is thought to have a lower threshold for counting. That is, it will count more of the very small particles than the FIDAS.

When I look at the Air Quality England data, I find that local authorities who use TEOMs tend to perform relatively poorly in terms of meeting regulatory standards than similar locations that use FIDAS to monitor the PM2.5 concentrations. So, which one is correct?

When looking at my local SDS011 Sensor Community data, I would say that the output from these is more in line with that of the FIDAS (not surprising as they are both based on the same technique). Could the sensor community data therefore be an undercount?

Does anyone have any thoughts on this?