‘Pollution down by at least 15% as a result of air panel’s curbs’

Scientists at IITM, Pune said that when CAQM announced a slew of directives for Delhi and NCR states to control pollution sources on November 16, the early warning system predicted the air quality in the national capital will slip into the ‘severe’ category on November 17 and November 18.

Pollution levels in Delhi went down by at least 15% as a result of the curbs implemented by the Commission for Air Quality Management in Delhi-NCR and adjoining areas (CAQM), observations by the Indian Institute of Tropical Meteorology’s (IITM) Early Warning System has shown, even as experts asked why the forecasts were not used to announce pre-emptive measures.

IITM scientists restrictions on polluting activities, such as construction, thermal power plants and truck movement in the NCR, helped bring down pollution. (Sanchit Khanna/HT)

Scientists at IITM, Pune said that when CAQM announced a slew of directives for Delhi and NCR states to control pollution sources on November 16, the EWS predicted the air quality in the national capital to slip into the ‘severe’ category on November 17 and November 18. But, the 4pm average air quality index of Delhi remained in the very poor category.

IITM scientists attributed this to the restrictions on polluting activities such as construction, thermal power plants and truck movement in the NCR.

Air levels in Delhi were consistently in the poor zone since October 27, and plunged further to very poor between November 2 and 4 (the day Diwali was celebrated). Between November 5 and 13, Delhi recorded seven ‘severe’ air quality days.

Taking cognisance of the persistent poor run of air quality, the Supreme Court on November 13 ordered the Delhi and central governments to take emergent measures to curb hazardous levels of air pollution in the national capital. The same day, the Delhi government announced shutting of schools, banned construction and announced work from home for the government staff.

The CAQM held a meeting, on the orders of the Supreme Court, on November 16, and announced ban on construction activities, ordered schools to shut down and prohibited entry of trucks into Delhi till November 21. The federal panel is expected to decide on the continuation of these measures in a meeting on Wednesday.

“When the commission announced these measures, our air quality forecasts for the week were already in place. We did not recalibrate it to see the difference and found that instead of falling into the severe zone, the AQI in Delhi showed an improvement because the polluting sources were not contributing as much as they did on a regular day,” said Sachin Ghude, a scientist at IITM Pune.

The EWS observations showed that on November 16, the day the directions were announced by the CAQM, the AQI forecast for Delhi was 391 and the final recorded AQI that day was 393, a difference of only two points.

On November 17, the EWS forecast said that the AQI may touched 487, the highest AQI value that Delhi would have recorded this season. However, with the pollution restrictions, the AQI levels settled in the very poor category, with a reading of 355 on the EWS scale. Similarly, on November 18, the forecast said that the AQI should have been 414, also in the severe zone, but the actual recording was 357.

The AQI recordings of EWS are slightly different from the Central Pollution Control Board (CPCB) data because of the different technology used by the two agencies. According to CPCB, Delhi recorded an average AQI of 403 on November 16, 375 on November 17 and 347 on November 18.

“We have now recalibrated our forecasts keeping the restrictions. But our observations do say that Delhi saw around a 15-22% reduction in pollution levels because of these measures by the CAQM,” Ghude added.

Scientists at the India Meteorological Department (IMD) said better wind speed also played an important role in the improvement of air quality during the period for which the IITM analysed its forecast data.

A senior Met official, who asked not to be named, said that from November 16 to November 19, Delhi was battling low wind speeds and considerably unfavourable ventilation conditions, which was not allowing pollutants to disperse easily. But, wind speed improved from November 20, played a huge role in improved air quality.

“The winds were in the low to moderate range till Friday, after which it improved. The stubble burning instances have also been on the lower end throughout last week. All these factors cannot be ignored while assessing the reasons behind an improvement in air quality,” a senior Met official explained.

Environment experts said the post-Diwali toxic air spell in the Capital could have been prevented if the commission had implemented these measures earlier.

Anumita Roychowdhury, executive director (research and advocacy), Centre for Science and Environment (CSE) said forecast-based forecast-based monitoring of pollution needs to be leveraged in order to ensure pre-emptive action

“What stopped the agencies to take this action when Delhi had already recorded so many severe air days right after Diwali? Agencies need to focus on pre-emptive action based on air quality forecasts to prevent such spikes,” Roychowdhury said.