No good air days in Delhi as rains fail to clean pollutants

Delhi might have just experienced the wettest June in more than a decade, but even the heavy rains failed to wash away some of the city’s deadliest air pollutants.

Air Quality Index data between 2016 to June 2017, available with the Central Pollution Control Board, revealed that pollution levels in Delhi refused to drop even during the rainy season. Not even in the month of June 2017, when the city received one of the heaviest rains in the last decade.

“Delhi hasn’t witnessed a single day in the last 535 days, in which air quality could be termed ‘good’. The air quality in nearly 50% of the days had poor category air quality followed by very poor,” said D Saha head of the air laboratory in the Central Pollution Control Board.

Scientists and environment experts were alarmed and apprehends that this could be the effect of the Green Field Gap — a phenomenon in which even heavy rains fail to wash air pollutants.

“Usually rains wash away the particulate matter. But there are some particulate matters, whose size varies between 0.1 micron and 1 micron. They are not cleaned and linger even after heavy rains. They are hydrophobic and tend to bounce away whenever a rain drop hits them,” said SN Tripathi coordinator of Centre for Environmental Science & Engineering at IIT Kanpur.

These are mostly the secondary particles (nitrates and sulphates) which form in the air from primary particles (SO2 and NO2) because of some complex reactions. Even the Supreme Court mandated EPCA had warned of such secondary particles in its action plan.

“Triggers such as high solar radiation and high relative humidity help primary particles to graduate into secondary ones. These particles may at times play havoc with the local rains by delaying and dispersing the rains,” said Abhijit Chatterjee, atmospheric scientist of Bose Institute in Kolkata.

Scientists have warned that long term exposure to pollution levels even in the satisfactory and moderate category could prove harmful to humans and can trigger a range of pulmonary and cardiac diseases.

The AQI is considered good if the PM10 level is below 50 micrograms per metre cube and PM2.5 level is below 30 micrograms per metre cube on a 24 hour average.

“But these are just permissible standards which we maintain for regulatory purposes. The safe standard is much lower and anything above 25 is considered unsafe for humans. Long term exposure even to such low doses of pollution could trigger several diseases,” said Sagnik Dey, associate professor at Centre for Atmospheric Sciences in IIT Delhi.

In the NCR region, air pollution levels in Noida too was equally bad in June 2017 as the city failed to register a single day in which air quality was in the good category. Gurgaon, however, experienced at least five days during June when air quality was in the good category.

“This means we are adding more pollution that what rains can wash away,” said Dey.