Unlikely. Usually mostly due to agricultural burning and motor vehicles.

Road vehicles and agricultural vehicles have both been using ULSD for the past 10 or so years in America, and it's my impression that this is common elsewhere in the developed world too. ULSD has two orders of magnitude less sulfur than common jet fuel.

Kathmandu is far from this, by the way. IIRC a major component of the smog was from smugglers adulterating the fuel, e.g. adding kerosene to gasoline.

Is there any reason jet aircraft couldn't burn something like ULSD? ULSD has a sulfur content of 15ppm in North America, whereas jet fuel can apparently be 2000ppm or more.

Because it's cheaper, I believe. aviation SE has a larger answer https://aviation.stackexchange.com/questions/13042/why-do-je...