Following a few days of high levels of air pollution in Delhi, light showers of rain on Tuesday night (November 9) and on Wednesday led to clearer skies and better air quality.
In early November, the Air Quality Index (AQI) in the region rose to as high as 471, coming under the category of ‘Severe’. The AQI measures the quantity of eight pollutants in the atmosphere and converts it into a number for ease of understanding. On Friday, the 24-hour average of AQI in Delhi, improved to 376 at 9 am, nevertheless falling under the ‘very poor’ category. Why did that happen?
Rain washes away dust, but not for long.
Some constituent pollutants measured by the AQI – such as Ozone, Sulphur dioxide and other pollutants – are not as easily washed away. PM 2.5 and PM 10 can, however, be washed out to a significant extent if it rains for a long period of time.
An earlier article from the Massachusetts Institute of Technology (MIT) explained the process thus: “As a raindrop falls through the atmosphere, it can attract tens to hundreds of tiny aerosol particles to its surface before hitting the ground. The process by which droplets and aerosols attract is coagulation, a natural phenomenon that can act to clear the air of pollutants like soot, sulfates, and organic particles.”
Therefore, prolonged rain can help reduce air pollution in a limited sense, with its impact focused on particulate matter – just some of the pollutants that are part of our atmosphere.
What exactly are PM 2.5 and PM 10?
After the rains in Delhi, the levels of PM 2.5 and PM 10 came down significantly. These are extremely fine particulate matter (PM) particles, with the digits accompanying them referring to their diameter. So, PM 10 and PM 2.5 are smaller than 10 and 2.5 microns in their diameter, respectively. One micron is about a thousandth of a millimetre.
The source of these pollutants is vehicular pollution, emissions from factories, construction activities and road dust. Such particles are not dispersed and stay suspended in the air that we breathe.
PM 2.5 levels sharply rose from around 155 at the end of October to 310 on November 5, according to CPCB data. Following rains, it declined to 174 on Friday afternoon. PM 10 levels also similarly declined – ranging between 400-480 in early November and then dropping to 291 on Friday.
Gufran Beig, founder project director SAFAR, and chair professor, National Institute of Advanced Studies, earlier spoke on the role of rain with The Indian Express when discussing the idea of artificial rain or cloud seeding to ease pollution. “There should be a significant amount of rain so it washes away pollutants. It will only be temporary, but if at all it is successful, it will break the flow of pollutants,” he said.