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AirGradient Forum

High NOx during night hours

#1

Hi,

I’ve had an Airgradient outdoor unit running through the last week or so. Initially I purchased this to help investigate a burning smell observed in the area during the night. Please note that we are a kilometre or so away from a light industrial area.

I was expecting to see high PM emissions during the night hours however that was not the case. I am constantly measuring between 15-25 PM2.5 depending on the hour of day.

One thing that I noticed is that the NOx measurement spikes during the night and matches the pattern we observed with the burning odours. At the same time the NOx measurements are not precise and they just give you an arbitrary level above the baseline (being 1).

I am attaching a screenshot of the NOx measurements from the last week. Is there more information to know based on these? I.e. are the levels dangerous, what could be causing them, etc.

Any help is greatly appreciated.

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#2

Disclaimer: talking outta my bum for the most part.

NOx are a known pollutant from poorly tuned engines (diesel more-so than petrol), so perhaps you’ve a neighbour who burns diesel in the evenings / at night?

NOx are a known pollutant from burning any sorts of fuel (e.g. coal, NH₄.) These in particular are likely in peaker plants and/or district heating. In developed nation both see peak demand throughout the evening and night. Are there installations of this sort in your proximity?

Neither NO nor NO₂ are described as having a burnt smell in literature, but in general authors are known to be pretty bad at correctly describing odours. However it is VOCs that are a general category of smelly particles. Do you have a VOC sensor, and does it show any correlation here? This could help to corroborate the theory that the increased NOx is a product of fuel burn (and not e.g. fertilizer production processes)

Regardless, it is more likely that NOx readings you see are a correlation, rather than root cause of the smell.

(Sorry, no idea what the dangerous levels are, however. WHO has some guidelines for NO₂, but it isn’t clear how comparable are their numbers and those from an AG sensor.)