Why Saturday rains worsened air quality in the capital?

On Saturday, we were blessed ( at least we thought then ) with tempered rains. Amidst the billowing PM2.5 level, the residents of National Capital Region cheered, expecting a respite from the "hazardous" air pollution. However, waking up on Sunday was a different affair altogether. Thick smog permeated through the streets and what initially was considered as a blessing, made atmospheric conditions worse, raising the question: Why didn't we see a dip in the AQI level?

As a raindrop falls through the atmosphere, it has the potential to attract hundreds of tiny aerosols before it falls on the ground. Such attraction is termed as coagulation, a happening that is known to filter out the pollutants from the air. Although in the past we have experienced such weather phenomenons easing pollution, especially in the NCR region, this time dichotomous to expectations Delhiites experienced a spike in PM level. Why did this happen? Atmospheric experts at Massachusetts Institute of Technology ( MIT) might have the answer. They suggest that the effectiveness of rain vis-a-vis cleaning the atmosphere is attributed to "the altitude of a cloud, the size of its droplets, and the diameter and concentration of aerosols". This means that the likelihood of a raindrop, sweeping away pollutants will depend upon many factors, and not just the fact that it rained.

Further, the researchers in Lanzhou, China, have systematically tracked what happens to particulate levels when it rains. The size of the particulate plays a pivotal role in deciding rain's cleansing effect. According to them, even during the most torrential rains, small particles were reduced by just 8.7%. Surprisingly, in their research, the impact on PM2.5, for light to medium rains, was close to zero. Hence, it's simpler to wash away larger particles than the tiny ones.

Delhi pollution has a higher concentration of PM2.5 and PM10, making it more harmful and dangerous, especially for small children and elderlies. According to the Lanzhou research, PM2.5 won't be washed away as quickly and efficiently as the other particulates ( larger in size ) will. In this scenario, the best we can hope is for winds to pick-up speed that can effectively disperse away the pollutants or pray for a heavy downpour. Saturday drizzle, instead of bringing relief has rather caused pollutant particles to consolidate owing to an increase in the humidity levels.

Air pollution is proven to aggravate the cardiovascular and respiratory illness. The World Health Organisation (WHO) reports that air pollution accounts for 29% of all deaths and disease from lung cancer, 43% of all deaths and disease from chronic obstructive pulmonary disease, 24% of all deaths from stroke, 25% of all deaths and disease from ischaemic heart disease, 17% of all deaths and disease from acute lower respiratory infection. Hence, till the time the sky doesn't clear up, stay indoor, use air filters or masks and put air-filtering indoor plants in your rooms. Cover Picture: PTI

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