@Giorgio *****
I am talking about my experiences and knowledge of the area.
I acknowledged that quite a lot of the smoke comes from Myanmar and Laos, so what's your point?
The Chinese government is very strict on burning, whereas the Vietnamese is kinda like it's neighbors - carefree and lots of burning goes on there, which drifts east and west depending on the wind direction BUT is generally too far away (and there are too many mountains in between) to affect northern Thailand.
I can guarantee you the majority of the smoke in Chiang Mai is from 1) ignorant mushroom foragers who burn the forest searching for mushrooms just outside of Chiang Mai 2) farmers and corporations burning the remains of their harvested rice paddy/corn/sugar cane fields in Chiang Mai/Lamphun and other nearby provinces 3) smoke drifting over from Myanmar (Shan State) and Laos (Bokeo, Saiyabuli) by farmers doing the same thing as their Thai counterparts.
This is combined with Chiang Mai's position in a valley surrounded by quite high mountains (Doi Suthep is 1700m high), construction dust/vehicular emissions and all of this combined creates the problem during the dry season.