On Earth Day, Mumbai saw its cleanest air day of 2020: SAFAR

The air quality index (AQI) was ‘good’ for the first time this year on Wednesday, which also happened to be the 50th Earth Day. The last time the city recorded

The air quality index (AQI) was ‘good’ for the first time this year on Wednesday, which also happened to be the 50th Earth Day. The last time the city recorded good air quality was in October 2019.

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According to the System of Air Quality Weather Forecasting and Research (SAFAR), the pollutant-measuring indicator AQI for PM 2.5 was 46, making it the cleanest air day so far this year and lowest pollution levels during lockdown. Prior to Wednesday, Mumbai had recorded its lowest air pollution levels with an AQI of 61 on Monday, 63 on Tuesday, and 65 on March 22 (Janta Curfew day), all falling under the ‘satisfactory’ category.

SAFAR categorises AQI for pollutants in the 0-50 range as good; 51-100 as satisfactory; 101-200 as moderate; 201-300 as poor; 301-400 as very poor and above 400 as severe. Pollutant concentration for PM2.5 pollutant – breathable particulate matter (solid and liquid particles) 2.5 microns in size or smaller – was 32 micrograms per cubic metre (μg/m³) on Wednesday against the safe limit of 60 μg/m³ for 24 hours. Similarly, PM10 (larger coarser particles) was 55 μg/m³, almost half its safe limit of 100 μg/m³ for 24 hours. Eight of 10 locations in Mumbai, where SAFAR monitors air quality, recorded ‘good’ AQI levels. While Mazagaon had the cleanest air with an AQI of 18 followed by Colaba 32, Navi Mumbai had comparatively poor air at 75 followed by Malad 58, both ‘satisfactory’ levels.

SAFAR said they observed a 10% increase in residential emissions (biofuel burning from residential areas for cooking purposes) mainly emitting carbon dioxide while on the other hand a 90% reduction was witnessed for other pollutants such as PM2.5, PM10, and oxides of nitrogen due to lesser vehicular and industrial emission sources. “The lockdown should be called off in a phased manner or else it will lead to sharp spike in overall air pollution,” said Gufran Beig, director, SAFAR under the Indian Institute of Tropical Meteorology (IITM), Pune. Researchers said changing weather factors were responsible for improvement in air quality. “Wednesday’s AQI is the optimal air quality Mumbai should be recording during lockdown when external emissions are minimal. However, over the past three weeks, we observed that owing to rising temperatures or late setting in of sea breeze, the wind pattern allowed dust and other pollutant particles to settle close to the surface,” said Beig.

The day temperature over Mumbai witnessed a sharp drop from Monday’s high of 37.8 degrees Celsius to 34.3 degrees Celsius on Tuesday, and even the minimum temperature on Wednesday was close to the normal mark. Humidity levels were high with 92% in south Mumbai and 74% in the suburbs. “The drop in temperature, high moisture content and early setting in of sea breeze allowed pollutant concentration to reduce significantly on Wednesday. The drop in pollution maybe temporary due to fluctuating weather and rising temperatures.” An AQI of 75 (satisfactory) has been predicted for Thursday while PM2.5 and PM10 concentration is likely to rise to 46μg/m³ and 61μg/m³ by Friday from current levels.