What could be causing such high and sustained NOx?

[Bangkokdi]

Hi, I’ve been keeping an eye on the NOx readings and I sometimes get peaks randomly during the day, but last night it was very high from 1am all through to 6.30am.
I live in a residential street. There is a building site opposite which has trucks, which I think cause some of the daytime peaks, or possibly gas cooking in the outdoor kitchen. But what would cause NOx to be above 400 for 6 hours overnight?
PM2.5 levels are currently high but it doesn’t seem correlated, the NOx went down when the PM2.5 went up.
I’m quite concerned as this sensor is outside my children’s bedroom window, so as a gas, they would probably be exposed to it to some extent.

Could it possibly be an aircon compressor that is emitting this, as the air-conditioner was turned off around 6.30am (it was turned on at 7pm). TVOC wasn’t high when the NOx was high (see charts)

[Altair_AirGradient]

Thanks for sharing the details. The fact that NOx dropped when PM2.5 went up suggests an airflow shift to me.

It’s worth keeping an eye on whether this repeats. If they do, trying to correlate them with wind direction or specific nearby activities is probably the most practical next step. If this pattern repeats on multiple nights at similar times, that’s a strong hint of a local, recurring source, for example a nearby generator, boiler. If it’s a one-off, it could be a temporary local activity combined with stagnant air.