Anatomy of the winter 2017 air quality emergency in Delhi.

The Indian capital Delhi experienced an environmental emergency in early November 2017 when levels of toxic PM2.5 particles surpassed WHO guidelines by 25 times (11 times by Indian Standards) for a prolonged period of a week (peak 24 h average ~650 μg/m3). We hereby demonstrate the role that monsoon dynamics played in linking and mixing dust emitted from a large natural dust storm, 3000 km away in the Middle East, with smoke from agriculture fires in northwest India. This dust and smoke rich air was then transported to Delhi where, under stagnant conditions, it mixed with local emissions resulting in very high pollution levels. The heavy aerosol-laden air altered the land-skin surface air temperature difference resulting in increased surface wind speeds, favouring faster dispersion and an unusual sharp decline in PM2.5 (PM2.5-110 μg/m3). Understanding the multi-scale nature of such events is important in improving our abilities to forecast these events and in developing effective air quality management strategies for the mega cities.