A big new study, with researchers from Harvard and CREA, just uncovered something troubling about Delhi's air: almost a third of the city's PM2.5 -- the really fine, dangerous air pollution -- is actually ammonium sulfate. The twist? This stuff forms in the air itself, not straight from tailpipes or smokestacks. That changes how we look at Delhi's smog. Turns out, there's a hidden chemical driver making things worse for millions of people.
The details are even more striking. When Delhi's pollution is at its worst -- right after the monsoon and through winter -- ammonium sulfate makes up as much as 49% of PM2.5. In the winter, it's around 41%. So the haze isn't just dust or smoke you can see. It's also invisible chemistry, where gases like sulfur dioxide (from burning coal and diesel) and ammonia (mostly from farms and fertilizer) mix in the air and turn into these tiny, harmful particles.