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I live in Maryland. I bought a HEPA air purifier with automatic, smart sensor-based operation to help with things like particulates during the central Canada wildfires.

The air purifier's integrated air quality sensors constantly monitor fine dust particles (PM2.5), CO2, temperature and humidity. Its air quality sensors monitor room air quality throughout the day, adjusting fan speed and purification to the necessary level, saving energy when air cleaning is not needed. The air quality indicator projects a soft glow behind the air purifier to indicate current air quality, based on measurements from the internal air quality sensors.

My purifier's air quality sensors circumstantiate that air quality is better at night; the purifier automatically slows down at night.

Why?

Do the wildfires burn less at night? Or does less smoke wind down to the USA at night? Or something else?

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  • $\begingroup$ I suspect that britannica.com/science/land-breeze breezes play a role. During the day smokey air is trapped from Canada above Maryland by the sea breeze, in the night the land breeze pushes smoke away from Canada and Maryland $\endgroup$
    – EarlGrey
    Jun 9 at 13:23

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Your air filter is not a good way to qualify the smoke in your area, since it will run based on the air quality inside your home which can vary quite differently than the outdoor air. Select a nearby site at EPA's fire.airnow.gov site to see how air quality changed during the June 2023 wildfire smoke event in your region.

I selected a few monitors in your state and pasted a representative one below. It looks like the smoke got bad after midnight on June 7 and then again it got even worse after midnight on June 8. This phenomena could be due to a shallower boundary layer at night which kept smoke closer to the surface. When you woke up in the morning it was probably quite smoky, but as the day progressed there was convective activity that kept more of the smoke aloft.

Yes, wildfires usually burn less at night because temperatures are cooler. However, wind, terrain, available fuels, and humidity play major roles in the dynamics of combustive activity in wildfires.

The timing of smoke impacts is circumstantial to the meteorology and geographic locations of the source and receptor. I wouldn't look for many patterns in the timing of the smoke effects that you experienced, since you are far away from the source with many dynamic meteorological phenomena in play.

There are detailed satellite imagery with PM2.5 monitor overlay at Aerosol Watch, if you would like to see how the event progressed through time.

fire.airnow.gov pop-up

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  • $\begingroup$ "Yes, wildfires usually burn less at night because temperatures are cooler..." and likewise note... the fires are far away from Maryland. So there'd be a many many hour delay in seeing the consequences of any nighttime fire reduction... and so it wouldn't be a particularly nighttime effect in MD anyways. $\endgroup$
    – JeopardyTempest
    Jun 10 at 18:39

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