Till mid-December, Delhi’s monthly average air quality was the cleanest in the last five years. However, the relief was short-lived as air quality deteriorated to the ‘very poor’ category on Monday with an average air quality index (AQI) of 379.
As a result, the capital saw several anti-pollution controls, including a mandatory transition to ‘hybrid’ mode for the junior classes and a ban on BS III petrol and BS IV diesel four-wheelers, kicked in again on Monday. Quality is projected to deteriorate ‘Severe’ category on Tuesday and in the coming days in the ‘very poor’ category.
But what caused the sudden change in air quality? Here are the top reasons:
Low wind speed
Very still air, along with low temperatures, trap pollutants in the atmosphere. As temperatures drop, the inversion height—the layer above which pollutants cannot disperse—lowers. This increases the concentration of pollutants in the air.
Strong winds are needed to disperse pollutants. According to the Central Pollution Control Board (CPCB), wind speed less than 10 km per hour is unfavorable.
The Indian Meteorological Department (IMD) said that the wind speed was less than 6 km per hour on Monday morning and the wind remained calm till evening.
According to the weather forecast division, the wind speed will be 6 to 8 km per hour this week.
Very calm winds from northeast direction of 2 to 4 kmph are likely on Tuesday morning resulting in ‘severe’ AQI conditions, indicated the air quality forecast system developed by IITM Pune.
So far this month, wind speeds of up to 20 km/h have been reported leading to several ‘moderate’ air quality days which is unusual for December.
Cold wave conditions
The IMD defines cold wave conditions when the actual temperature is less than 4.1 °C or when departures exceed 4.4 °C in the plains, if the normal minimum is less than or equal to 10 °C.
According to IMD officials, Delhi witnessed cold wave conditions earlier this year than in the last 23 years. The first cold wave day of the season was observed on 11 December when Delhi’s minimum temperature dropped to 4.9 °C, making it the earliest cold wave day reported since 1996 (with a temperature below 5 °C).
According to officials, the last time the minimum temperature fell below 5 degrees Celsius was on December 6, 1987 at 4.1 degrees Celsius. On December 11, 1996, the minimum temperature was 2.3 degrees Celsius.
The IMD has forecast cold wave conditions to persist in Delhi till Wednesday as the mercury is expected to touch 5 degrees Celsius.
Contribution of transport sector and other emissions
The transport sector remained the major contributor (16.94 per cent) to the capital’s pollution problem till December 12, the latest data available from Delhi’s Decision Support System for Air Quality Management. Among the other important contributors to particulate pollution, the industrial sector accounts for 8.36 percent. The contribution remained significant as the second phase of the Classified Response Action Plan was implemented in the capital.
No significant rainfall
On December 9, gusty winds and mainly clear skies washed away the pollutants in the city. However, it has not rained much this month. The IMD forecast for the next seven days also does not indicate any cloud cover.
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