India, Feb. 14 -- Residents of the Indo-Gangetic plains are used to an intra-day pattern in air quality on dry and cool days - pollution levels worsen after sunset and improve after sunrise. However, data reviewed by HT show that on at least a few days this February, this trend was reversed at least in Delhi. Air quality improved as the night progressed but began deteriorating as sunrise approached, with pollution levels continuing to worsen even after daybreak.

This curious intra-day pattern cannot be explained by the pollution load in the capital or its surrounding areas this February. Most prominent sources of pollution remain active during the day throughout the year, and that rarely leads to days becoming more polluted than nights, ...