The pollution situation in Bangkok has improved dramatically since the year 2000.
The main source of air pollution used to be buses which belched out diesel smoke. That changed around 2005 when buses were converted into Natural Gas Vehicles (NGV) and Liquified Petroleum Gas (LPG) vehicles.
There was also a campaign to do better maintenance of the vehicle engines.
The government has also invested in a lot of infrastructure and tax incentives to encourage production and use of natural gas to fuel commercial vehicles. Natural gas is more economical than diesel. Also, Thailand has abundant reserves of natural gas but not much oil, so use of natural gas instead of imported oil helps the trade balance and local economy, in addition to creating a whole new economic sector and jobs.
Back in the year 2000, stand in one place, and there would periodically be vehicles like this, mainly green buses, other buses (like the white one below), private commercial vehicles, private cars and even motorcycles.