Change in weather factors may keep air pollution lower than expected in Mumbai post-Diwali: SAFAR

With Mumbai recording the hottest day this season so far on Saturday, the city is expected to witness marginally lower air pollution after Diwali owing to change in weather factors such as hotter conditions and rise in wind speed, said an official at the System of Air Quality Weather Forecasting and Research (SAFAR).

Santacruz weather observatory, representative of the city, recorded day temperature at 36 degrees Celsius, 2.4 degrees Celsius above normal. Colaba, representative of south Mumbai, recorded 35 degrees Celsius, 2 degrees Celsius above normal.

Meanwhile, air pollution levels fell to ‘moderate’ category on Saturday, the day of Diwali, as opposed to a prediction of ‘poor’ air quality. Air quality index (AQI) for all pollutants on Saturday was 111 (moderate) with PM2.5 levels in the satisfactory category at 54 microgrammes per cubic metre (µg/m3) against the safe limit of 60 µg/m3, and AQI for PM10 at 98 (satisfactory).

SAFAR published a revised air quality forecast for Sunday stating, “A positive development is that the prevailing temperature is not as cooler as expected. Also increase in wind speed may bring relief for Mumbai, and AQI is going to be towards the lower end of the ‘very poor’ category in a 50% firecrackers (half of the average emissions from firecrackers in 2016-2018) scenario. In case of no firecracker emissions, AQI is likely to remain steady and within ‘poor’ category.”

The most polluted areas are expected to be Chembur, Mazagaon, and Malad on Sunday. On Saturday, not a single location in Mumbai recorded ‘poor’ or ‘very poor’ air. Bhandup and Navi Mumbai had the cleanest air.

“The highest levels of PM10 and PM2.5 are expected between 1am to 4am between Saturday night and Sunday morning. Air quality will start to improve from Monday,” SAFAR said.

In contrast to the previous AQI prediction at 338 (very poor), which would have been the worst post-Diwali air quality in six years, SAFAR’s revised forecast said AQI was expected to be 309 (very poor) on Sunday and 267 (poor) on Monday with 50% cracker emissions, and 227 (poor) on Sunday and 203 (poor) on Monday with no crackers.

SAFAR categorises AQI levels from 0-50 as good, 51-100 satisfactory, 101-200 as moderate, 201-300 is poor, 301-400 is very poor, and above 401 is severe.

Gufran Beig, project director, SAFAR said, “When temperature and wind speed is low, pollutant particles get suspended closer to the surface, but in this case, current weather factors can allow slight increase in pollutant particle dispersion.”

According to the India Meteorological Department (IMD), the wind direction changed from cooler northeasterly winds to warmer easterly to southeasterly winds on Saturday, allowing the rise in temperatures. Wind speed ranged between 11 and 14 km/hour against the average wind speed of 5-6 km/hour over the past three days. The minimum temperature on Saturday increased to 22 degrees Celsius (a degree Celsius above normal) from 19.6 degrees Celsius on Friday in the suburbs while south Mumbai recorded a 1 degree Celsius rise from Friday to 24 degrees Celsius on Saturday.

SAFAR also predicted that the concentration of PM2.5 on Sunday would be over twice the safe limit if firecrackers were used. Against the safe limit of 60µg/m3, Mumbai could record 132 µg/m3 on Sunday with 50% firecracker emissions and 98 µg/m3 with no emission from firecrackers.

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