Yesterday we had a few points where our nearby sewage works was upwind of us and the whole area stank.

The MQ135 sensor picked up the atmospheric pollutants reporting values of 24-25 as compared to the usual 19-20. I don't know about the units these things report yet, but I am delighted that it responds to these sorts of concentrations. Also, given the range of gases the MQ135 responds to it is difficult to know exactly which pollutant was present, however it was there in detectable quantities.

There didn't appear to be much response on the readings from the SDS011, but strangely the PPD42NS showed a noticeable uptick in readings that corresponded directly with the periods of bad smell. I have  been given to understand that the SDS011 is superior to the PPD42NS so this bears further consideration.