Why Mumbai’s AQI readings may be hazy: Monitors could be to blame

A study of data from all the 30 AQI monitoring stations revealed that Mumbai’s AQI readings may not be entirely accurate

MUMBAI: A thick blanket of haze continued to cling to Mumbai, a day after the city experienced some of its worst air quality in recent times. Oddly, though, the city’s Air Quality Index (AQI) dropped on Friday, from 160 to 154 (both figures in the ‘moderate’ range) a day earlier, suggesting cleaner air and improved visibility. Clearly, it doesn’t add up.

Despite the murky haze, AQI readings in only four parts of Mumbai were ‘poor’ (201 – 300), that is, Borivali East, Malad West, Mazgaon and Navy Nagar. (Bhushan Koyande/HT Photo)

A study of data from all the 30 AQI monitoring stations revealed that Mumbai’s AQI readings – an index Mumbaiites keenly track, especially during winter – may not be entirely accurate. As a result, what you read and what you see could be markedly different.

On Friday’s hazy conditions, Sushma Nair, scientist with the Indian Meteorological Department, Mumbai, said, “The weather conditions continued to be the same as they were on Thursday.” She said a weather phenomenon called a Western Disturbance is bringing increased moisture to the city, allowing suspended particles, mostly pollutants, to cling to it. Add to this slow wind speeds that don’t allow the particles to disperse. The result is a thick haze.

“There is also temperature inversion at play. Due to a layer of warmer temperature higher in the atmosphere, pollutants stay trapped as they can’t rise. The build-up accumulates. Visibility has taken a hit due to the haze,” Nair added.

Despite the murky haze, AQI readings in only four parts of Mumbai were ‘poor’ (201 – 300), that is, Borivali East, Malad West, Mazgaon and Navy Nagar; they were ‘moderate’ (101 – 200) in most other parts of the city. However, during different periods during the day, for several hours at a time, the density of pollutant particles in the air, also judged on the AQI scale, shot up to ‘very poor’ levels (301 – 400). This could partly explain the low visibility. These areas included Borivali East, Navy Nagar, Deonar, Malad West, Mazgaon and Worli. In many other areas, readings were at ‘poor’ levels.

However, Hindustan Times discovered an alarming truth about the 30 monitors that measure AQI in Mumbai, some of them installed by the Indian Institute of Tropical Meteorology (IITM) and others by the Maharashtra Pollution Control Board (MPCB). We discovered that the data they yield was missing crucial inputs.

HT studied the readings from December 20 and found that several pollution hotspots had no data inputs for significant periods of time. Some monitoring stations, like the ones in BKC and Bandra, had no inputs at all. A second monitoring station in BKC had no signal for several hours on most days, as was the case with Kandivali East, Deonar, Siddharth Nagar in Worli, and Vile Parle East.

Gufran Beig, Founder-Director of SAFAR, a research agency with the Indian Institute of Tropical Meteorology (IITM), pointed to a limitation in the air quality monitors. “According to CPCB guidelines, there is an inlet in the air quality monitoring system which is 5 to 10 meters above the ground that takes in the air and calculates the particulate matter. It is possible that dust particles sit above their vertical monitoring range and do not reflect in the AQI.”

Hindustan Times reached out to Sachin Ghude, Project Director at SAFAR, a part of IITM, who said he would get back with more details but did not. A senior official from the MPCB, did not answer our phone calls.