During November 1-7, 2016, Delhi residents were caught in a case of what has been termed as a severe air pollution episode (SAPE) or the ‘Great Delhi Smog’. The air quality index (AQI) exceeded 500. It encapsulated the entire Indo-Gangetic Plain.

What caused this spike and suffering? A study by a consortium of institutes has found that it was ‘air stagnation’ that played a vital role in that episode, which is considered as one of the worst in recent years.

Stagnant atmosphere

The consortium used radiosonde observations that showed that stagnant atmospheric conditions led to SAPE in New Delhi by allowing pollutants to accumulate and persist in the near-surface environment.

Also read: Delhi’s air quality plunged sharply in 2016

Radiosonde is a battery-operated telemetry instrument carried by weather balloon into the atmosphere that measures various atmospheric parameters from surface to about 30 km.

Researchers from the University of Hyderabad (UoH), India Meteorological Department(IMD), Indian Institute of Tropical Meteorology (New Delhi), Banaras Hindu University, Finnish Meteorological Institute, Finland; Pacific Northwest National Laboratory, Richland in the US, undertook the study.

The study found that South Asia has been experiencing severely degraded air quality, with particulate matter less than 2.5 μm (PM2.5) reaching unprecedented high levels in recent years.

Concentration of pollutants

“We found that a stagnant weather condition was the dominant cause of the SAPE. Mean concentration of PM2.5 in New Delhi before, during and after the SAPE were 142 μg/m3, 563 μg/m3 and 240 μg/m3, respectively,” said Vijay Kanawade, the lead author.

Satellite-based aerosol optical depth (AOD), ultraviolet-aerosol index (UV-AI) and surface carbon monoxide (CO) concentrations also showed significant enhancements over a large locale spatially by about 50-70 per cent during the SAPE, he said.

A large and simultaneous increase in UV-AI and CO downwind of a large number of fire hotspots (Punjab and Haryana) is a clear indication of biomass burning aerosols. Analysis of absorption Ångström exponent further substantiates this finding, showing a large fraction of light absorbing carbonaceous-type aerosols.

As a result, the heating rate induced by light absorbing aerosols into an atmospheric layer during SAPE was very high (3.1±0.7 k/day) under prevailing atmospheric condition.

These findings will help in understanding air quality and climate effects, as well as in formulating policies to mitigate these complex pollution episodes in future, Kanawade said.