With the winter approaching, air quality in the city touched 188 (moderate) on Monday, up from 155 the previous day, according to the Central Pollution Control Board (CPCB). PM2.5 (tiny respirable particles less than 2.5 micrometres in diameter) was noted as the day’s primary pollutant by the CPCB.
An AQI between 100 and 199 is considered ‘moderate’, while between 50 and 99 ‘satisfactory’ and below 50 is considered ‘good’. AQI in excess of 200 is considered ‘poor’, above 300 ‘very poor’, above 400 ‘severe’ and above 500 ‘severe +’. As per the CPCB’s national ambient air quality standards (NAAQS), ‘moderate’ air “may cause breathing discomfort to people with lung disease such as asthma, and discomfort to people with heart disease, children and older adults”.
A fortnight ago, on October 15, Mumbai’s air quality stood at 141, and has been consistently rising. Experts said the spike in airborne pollutants is typical at this time of year, due to falling temperatures and slower wind speeds. “Till the first week of October, air quality was ‘good’ to ‘satisfactory’ because of rain, which washes pollutants out of the air, and high wind speeds which disperse them,” said Gufran Beig, meteorologist with IIT-M’s System of Air Quality and Weather Forecasting And Research (SAFAR).
In the last week, Beig added, the city has seen a slowing down of “local winds”, with speeds not exceeding above 10kmph in most parts of the city. During the monsoon, the winds can travel with speeds well over 20kmph. “To add to this, the temperature is also dropping as winter approaches. Warm air rises, and with it pollutants also settle higher in the atmosphere. But when air is cooled, it reduces the height of the ‘inversion layer’, which is the part of the atmosphere where temperature stops decreasing with height. This in turn reduces the ‘mixing height’ of airborne pollutants and settles them much closer to the earth’s surface, so much so that the air around us can start turning hazy,” Beig said.
{{/usCountry}}In the last week, Beig added, the city has seen a slowing down of “local winds”, with speeds not exceeding above 10kmph in most parts of the city. During the monsoon, the winds can travel with speeds well over 20kmph. “To add to this, the temperature is also dropping as winter approaches. Warm air rises, and with it pollutants also settle higher in the atmosphere. But when air is cooled, it reduces the height of the ‘inversion layer’, which is the part of the atmosphere where temperature stops decreasing with height. This in turn reduces the ‘mixing height’ of airborne pollutants and settles them much closer to the earth’s surface, so much so that the air around us can start turning hazy,” Beig said.
{{/usCountry}}An unseasonal bout of rain is expected over Mumbai towards the end of this week, which may bring some respite from rising pollution levels. An official with the India Meteorological Department’s regional forecasting centre in Mumbai said that a low-pressure area may form over the Arabian Sea by Friday. “It’s hard to predict now but most parts of west Maharashtra coast could get rain for a day or two. Heavier showers may be felt in the south Konkan, but for Mumbai also there is a chance of rain,” they said.
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