Kolkata: Even a rain-soaked monsoon could not wash Kolkata’s air clean enough to meet World Health Organization’s benchmark for fine particulate pollution.Kolkata recorded an average PM2.5 concentration of 23 µg/m³ in Aug, making it the second most polluted among five major megacities, behind Delhi at 35 µg/m³, according to the latest monthly air-quality assessment by Centre for Research on Energy and Clean Air (CREA). Chennai recorded 20 µg/m³, Bengaluru 19 µg/m³ and Mumbai 14 µg/m³.The figure assumes significance because the monsoon usually provides cities with their cleanest air of the year. Rain removes suspended particulate matter through “wet deposition” while stronger winds and greater atmospheric mixing help disperse accumulated pollution.“Rain droplets capture suspended particles such as PM2.5 and PM10 and carry them to the ground while stronger monsoon winds can also help disperse accumulated pollutants,” said CREA analyst Manoj Kumar.Yet, Kolkata’s Aug average of 23 µg/m³ remained above WHO’s 24-hour PM2.5 guideline of 15 µg/m³, despite favourable monsoon conditions. The WHO guideline is a health-based recommendation, not a legally enforceable “permissible limit”. India’s corresponding 24-hour National Ambient Air Quality Standard is much more relaxed at 60 µg/m³.Experts said the monsoon’s cleansing effect is neither uniform nor permanent. How much pollution is removed depends on the intensity and duration of rain, wind speed, atmospheric mixing and the continuing addition of pollutants from traffic, industries, construction and other sources. High humidity can also cause some particles to absorb moisture and grow.As a result, pollution can fall sharply during prolonged spells of rain but rise again during rainless or stagnant periods within the same monsoon month. Rain is also generally more effective at removing particulate matter than many gaseous pollutants.The persistence of a 23 µg/m³ monthly average in Kolkata during the monsoon, therefore, suggests that local emission sources continue to add a substantial pollution load even when weather conditions favour cleaner air.Nationally, Aug air was considerably cleaner by Indian standards. Of the 258 cities with sufficient continuous monitoring data, 255 recorded monthly average PM2.5 below 60 µg/m³. But only 43 cities were at or below WHO’s 15 µg/m³ daily guideline benchmark. Among the 89 National Clean Air Programme cities assessed, 75 were above that WHO level.Charkhi Dadri in Haryana was India’s most polluted city in Aug, with an average PM2.5 concentration of 68 µg/m³, followed by Gummidipoondi in Tamil Nadu at 62 µg/m³. At the other end, Dombivli in Maharashtra was the cleanest at just 5 µg/m³, followed by Kunjemura in Chhattisgarh at 7 µg/m³.
