Why does air pollution peak around midnight?

[RamenRaven]

We all know that Chiang Mai air pollution is getting worse now that the rice harvest season is kicking in. Rice straw and stubble are being burnt as agricultural waste accumulates. Cool dry air is blowing in from the north, as opposed to warm humid air from the south during the rainy season.

 

The question now is:

Why does air pollution in the outskirts of Chiang Mai peak during the late evenings and early mornings, when all the farmers are supposed to be asleep? Hang Dong, Saraphi, Chang Pheuak, and San Sai AQI's go way up to around 150 around midnight, while downtown does OK with AQI's of around 50. By noontime, AQI's in the outskirts of Chiang Mai dip into the 50-100 range.
 

Check out AirVisual.com and you'll see that AQI values jump up at night.

 

https://www.airvisual.com/thailand/chiang-mai

[canthai55]

At night the air cools. Cool air is more dense than warm air, and will settle.

All the monitors are at ground level, hence the spike.

[sfokevin]

My guess is two fold... One as stated before as the sun goes down the whole atmosphere cools and settles... Secondly as soon as it’s dark 7-8pm the locals feel secure in lighting the fires as the smoke can not be seen... a few hours later the high pollution levels results.. 

[sfokevin]

Curious?... Can my fellow OCD members who have PM 2.5 meter post thier readings and location...

 

My PM Meyer reads 28 indoors and am located just south of the Big C/HomePro  near Mae Hai... I suspect my high reading is due to a high pollen count from the flowering tree that smells strong - My house is in a forrest of them... And we have a stand of palm trees that are also in bloom...

Edited November 7, 2018 by sfokevin

[natway09]

And after 11pm often gets very still. ie ; no wind to blow away the pollution as in Bangkok

[themerg]

Cooler air sinks along and compresses the pollution towards the ground.

This is 4th grade science.

[cmsally]

Temperature inversion traps the polluted air, probably peaks about midnight.

[simoh1490]

4 minutes ago, cmsally said:

Temperature inversion traps the polluted air, probably peaks about midnight.

I understand that the inversion breaks as the sun warms the air cooler air around mid morning, I think the AQI site confirms that pollution peaks mid morning and early evening, that's consistent with the sun warming the air in the AM and farmers burning in the evenings after dark.

[cardinalblue]

So if the cool air compresses the air pollution, then why is March and April the hottest months of the year the worse months for air pollution? 

 

The main rice harvest occurs in nov/dec time frame...the 2nd crop usually is not rice due to water shortages over the last 5 years at least in the north....

 

yes, the forest burning is a big player in the northern air pollution but never seen estimates as to percentages...

[Rockhopper]

Also check http://www.chiangmaiair.org/

[RamenRaven]

Check the AQI history for these two locations.

 

https://www.airvisual.com/thailand/chiang-mai/lanna-international-primary-school

 

https://www.airvisual.com/thailand/chiang-mai/payap

 

Air pollution suddenly spikes around 8-9 pm, and then starts to subside at around 3 am, give or take. Around 10-11 pm, I have to wear an N95 mask while I'm in my office, even though I've tried to insulate all the windows and seal off everything.

 

Could this mostly be due to locals burning trash? Farmers are usually done with the fields by 6 pm, and I see stubble burning around 2-4 pm, not after dark. Pollen from trees also is a small factor, but that can't possibly send the AQI shooting way up to 150.
 

[simoh1490]

10 hours ago, cardinalblue said:

So if the cool air compresses the air pollution, then why is March and April the hottest months of the year the worse months for air pollution? 

 

The main rice harvest occurs in nov/dec time frame...the 2nd crop usually is not rice due to water shortages over the last 5 years at least in the north....

 

yes, the forest burning is a big player in the northern air pollution but never seen estimates as to percentages...

Air is hottest closest to the ground, it cools the further away it gets from the ground. The inversion air reverses that, a layer of hot air traps a layer of cold air close to the ground, by mid morning, the ground has warmed sufficently thus the  mass of cooler air has also warmed and it dissipates. Why March/April: hot air only rises if there is cooler air that can be drawn in beneath it, during the hottest months of the year the intense heat from the sun allows this to happen and cooler air from the ocean is drawn in.

[RamenRaven]

'm wondering when the Chiang Mai air pollution is going to wind down this winter. The air quality index in my area is now peaking to about the same level in March and April. Nasty stuff.