Farm fires at 19.18%, low wind & dense fog: What caused ‘severe+’ AQI in Delhi on Monday

On Tuesday, the city again experienced severe plus AQI at 460. Out of the 32 air monitoring stations in Delhi, 23 saw AQI over 450.

Scientists from the Indian Institute of Tropical Meteorology (IITM) in Pune, which issues early warning forecasts for Delhi and the NCR, said on Monday, the emission load from stubble fires transported towards Delhi-NCR were trapped owing to weather conditions. “Based on the models we run, the expectation was the air quality will be in the ‘very poor’ category. However, aerosol load from stubble fires was high. The meteorological conditions were not completely unfavourable, however, they were not as helpful either to disperse the pollutants,” said a senior scientist.

The wind speed was in the range of 6 to 12 km/hour between Monday and Tuesday, as per IITM.

Punjab on Monday had recorded 1,251 incidents of stubble burning — the peak of this year’s paddy harvest season. The previous high was 730, recorded on November 8. The spike on Monday was five to seven times what was recorded over the past week.

According to the IITM scientist, dense fog that descended on the Capital, along with a drop in temperature, did not help matters. “Foggy conditions help in the formation of secondary pollutants. For instance, tailpipe emissions or farm fire smoke mixes with water vapour to form smog and other secondary pollutants.”

Temperature inversion, a phenomenon in which warm air traps cold air below it closer to the surface, also caused the concentration of pollutants closer to the ground as the temperature dropped in Delhi.

Monday’s day temperature was 4 to 5 degrees Celsius below normal and the minimum was below normal by 1 to 5 degrees Celsius. “Due to a western disturbance that arrived a few days ago, the wind patterns changed to north-westerly. That brought moisture and also influenced fog conditions,” said Naresh Kumar, a scientist at the India Meteorological Department.

On Tuesday, 270 cases of stubble fires were recorded in Punjab, as per Ministry of Agriculture data. Among other states, Madhya Pradesh recorded 664 cases of stubble burning, followed by Uttar Pradesh (368), Rajasthan (57) and Haryana (35).