India, Oct. 26 -- Any serious attempt to cut Delhi's winter pollution starts with knowing where the particulate matter in its air comes from, and a decade of studies has produced only partial answers. The emissions estimate behind IITM's source forecasts still rests on surveys conducted in 2016, and no study has yet traced the largest share of the particles residents breathe, those that form in the atmosphere from gases, to the emitters responsible.

The pollutant at issue is PM2.5, particles no wider than 2.5 thousandths of a millimetre, or roughly a thirtieth of the width of a human hair. Some are released directly as soot from exhausts, smoke from burning crop residue and wood, and dust from roads and building sites (dust in itself is ...