Air slips into severe zone, city witnesses first smog episode

Delhi Pollution Control Committee (DPCC) recordings show that the concentration of PM 2.5 (particulate matter with diameter less than 2.5 micrometres) started rising from Thursday midnight and reached above the 300ug/m3 mark.

Low temperature, dense fog and cracker bursting on New Year’s eve, pushed Delhi’s air quality into the severe zone on Friday.

A man crosses the road at Tilak Marg, near ITO, on Friday when the capital witnessed dense fog. (Arvind Yadav/HT PHOTO)

Central Pollution Control Board (CPCB) data shows that the overall air quality index (AQI) in the city on Friday was 441 -- a major drop from Thursday’s AQI recording of 347, which is categorised as very poor.

Delhi Pollution Control Committee (DPCC) recordings show that the concentration of PM 2.5 (particulate matter with diameter less than 2.5 micrometres) started rising from Thursday midnight and reached above the 300ug/m3 mark. The ambient air quality standard for PM 2.5 levels in India is 60ug/m3.

At 8pm on Friday, the average PM2.5 levels in the city were 313.3ug/m3.

“There were instances of cracker bursting around some residential neighbourhoods. The emissions from these combined with the dense fog cause the smog that we saw in the early morning hours. The radiative fog mixed with high PM (particulate matter) levels also reduced visibility,” said VK Soni, head of IMD’s environment monitoring and research centre.

IMD recordings show that at 6am, the visibility at Safdarjung and the Palam observatory was zero metres, and it stayed below 200 metres till around 9am.

The mixing height is the height of vertical mixing of air and suspended particles above the ground. This height is primarily determined by temperature. When the temperature increases, the air expands and pushes the height higher, giving pollutants closer to the ground more space to mix with the atmosphere.

He also said that the change in winds direction will also improve wind speeds on Saturday and help disperse pollutants.

“Our forecast had said that there was a possibility of the AQI deteriorating on Friday and that was primarily because the wind speeds were calm for a large part of Friday morning. However, wind speed picked up from afternoon after the change in direction,” Soni said.

He added that on Saturday, the average wind speeds are likely to be around 15kmph and improvement in air quality will be visible from Saturday morning itself. The IMD has also forecasted a western disturbance to impact the city from Saturday, which will also lead to rainfall activities in some parts of Delhi. “This will also bring relief.”

Despite the assurance of a better weekend, on Friday evening at least 28 out of the total 37 pollution monitoring stations were in the ‘severe’ zone. These areas included Mundka, Alipur, RK Puram, Vivek Vihar, Anand Vihar, Rohini, Dwarka sector-8, Bawana, and Narela among others.

Union ministry of earth science’s air quality monitoring centre, System of Air Quality and Weather Forecasting and Research (Safar) also said that high moisture content in the city’s air and calm winds led to the deterioration in pollution.

“Surface winds became extremely calm as forecasted and the low dispersion conditions led to accumulation and trapping of pollutants near surface. The radiative fog mixed with high PM levels made it smog and reduced the visibility further,” Safar analysis read.