Rain has little impact as air quality slides to severe in Delhi

Delhi’s air quality deteriorated to the “severe” category on Thursday, with the slight drizzle recorded in the city having little impact on clearing up the pollution.

While rains generally have a positive impact on air quality, experts said that persisting cloud cover and slight drizzle for a brief period may actually have a detrimental impact on air quality as the rain adds to the existing moisture in the air, trapping pollutants locally. The cloud cover also does not allow pollution to escape.

Weather experts suspect this is what happened in the Capital on Thursday, as a combination of calm winds, low temperature and very light rain added to Delhi’s moisture content, taking the AQI to a reading of 429 as per Central Pollution Control Board’s daily 4 pm bulletin – up nearly 60 points from Wednesday’s reading of 370 in the “very poor” range.

While only 1mm of rainfall was recorded at Safdarjung, Delhi’s base observatory, cloud cover through the day meant Delhi recorded a maximum temperature of just 19.8 degrees Celsius (°C) – five notches below normal and the lowest maximum so far this winter season. The minimum temperature was 13.4°C.

Agencies say immediate relief from “severe” air is unlikely soon as calm wind conditions are expected to prevail over the region for the next three days. According to the System of Air Quality and Weather Forecasting And Research (Safar), a forecasting body under the Union ministry of earth sciences, wind speeds will again increase from December 6 onwards, allowing some dispersal to take place.

“For the next three days, winds are likely to be calm, reducing ventilation and leading to deterioration of air quality. From December 6 onwards, winds are expected to increase dispersing pollutants but AQI is likely to remain in the ‘very poor’ category. Partly cloudy sky and low mixing layer height are currently preventing efficient dispersion of pollutants,” said Safar.

Gufran Beig, founder and project director at Safar, said that if there is sufficient rain over a city, pollutants get washed away, but a drizzle may only add to the relative humidity and make the air heavier, thereby trapping pollutants. “Temperatures are cool and these conditions facilitate secondary aerosol formation, which will add to the PM 2.5 content,” he said.

Met officials said that visibility day ranged from 700 -800 metres during the day and improved to 1,200 metres by 1pm, before dropping to 600-800 metres again due to mist and rain.

Meanwhile, Jafarpur in southwest Delhi recorded the city’s lowest maximum temperature at just 18.1°C.

IMD scientist RK Jenamani said while this was the lowest maximum of the season, it cannot be classified as a ‘cold day’ as the minimum temperature also needs to be below 10°C. “For a cold day, the maximum temperature should be 5 degrees or more below normal, with the minimum settling below 10°C. We can expect shallow to moderate fog to now persist over Delhi in the next two days,” he said.

Anumita Roychowdhury, executive director, research and advocacy at the Centre for Science and Environment (CSE), said with the temperature inversion and a drop in mercury at night, the actual impact of Thursday’s rain will become clearer on Friday. “There is a possibility with light rain that you add to the existing moisture... we will see on Friday if pollution levels dip or not,” she said.

Tanushree Ganguly from the Council on Energy, Environment and Water (CEEW) said Delhi’s PM 2.5 concentration had breached the 300 micrograms per cubic metre mark in the morning – five times the safe standard. “The PM2.5 levels have come down from a reading of over 300 ug/m3 in the day to around 180µg/m3, but the air quality continues to remain in the ‘very poor’ category,” she said.