The decline in air quality during Mumbai's winter months is quickly turning into a yearly subject of discussion, mirroring the situation that Delhi has faced over the last ten years owing to stubble burning and other factors.

Historically, Mumbai was thought to be resistant to air pollution issues, including haze and smog, due to its seaside position.

The robust ocean winds were known to disperse dust and other airborne particles, thus maintaining cleaner air in the city.

However, the last two years have revealed that this geographic benefit is no longer as effective.

Last year marked the city's most prolonged period of degraded air quality, spanning from November to January. At times, the pollution in Mumbai's air exceeded that of Delhi.

This issue recurred just last week when the air quality index (AQI) in certain city regions rose above 300. An AQI over 200 is deemed 'poor,' and readings over 300 are categorized as 'very poor.'

These notably severe periods of pollution have been caused by a combination of unfavourable weather conditions.

Scientists have noted that, aside from these incidents, there has been a consistent downturn in Mumbai's air quality.

Wind patterns, specifically their direction and force, play a crucial role in shaping the air quality of Mumbai.

The city's production of pollutants is not less than that of Delhi. Emissions from transportation, industrial activities, and various other contributors are comparable to those in Delhi and other significant urban areas in India.

However, the strong winds typical of coastal regions are advantageous in this context.

Winds typically oscillate, shifting from sea to land and then back again, with this pattern recurring every three to four days during this season.

When winds blow seaward, they carry away dust particles, effectively purifying the air. However, any temporary interruption in this cycle can adversely affect the city's air quality.

The particularly poor air quality last year was linked to regular and extended disturbances in the usual wind patterns. Rather than the typical 3-4 day alternation, wind directions were shifting after intervals of eight to ten days.

It was not immediately evident at the time, but the prevailing La Nina conditions in the eastern Pacific Ocean could have had a role to play in that, according to Gufran Beig, a scientist who has closely monitored air pollution in Mumbai and other parts of the country, Indian Express reported.

La Nina refers to a climatic event characterized by cooler-than-average sea surface temperatures in the eastern Pacific Ocean.

This large-scale phenomenon influences weather events across the world, and has a wide variety of impacts. Last year was part of the longest and strongest La Nina events on record.

Beig, who was the project director of SAFAR (System of Air Quality and Weather Forecasting and Research) — a government-backed initiative for air quality monitoring and prediction in key cities — is preparing to release a study on how La Nina affected Mumbai's air quality last winter.

“There were two aberrations last year. The frequency of the cleansing mechanism was disrupted, and the relatively calm wind conditions that Mumbai sees for about a week after the withdrawal of the monsoon prevailed for almost two months," Beig was quoted as saying in the IE report.

"That meant that even when the wind was moving towards the sea, it was not strong enough to blow away the pollutants effectively. Both of these could have been influenced by La Nina,” he added.

However, La Nina has concluded, and its opposite phenomenon, El Nino, has taken over.

It's not as intense as La Nina, and its impact remains uncertain.

Beig noted that the recent decline in Mumbai's air quality stemmed from an entirely different cause.

“Such meteorological conditions are not unusual, and keep happening once in a while. These would have gone unnoticed had there been no dust sources in the way. The problem is the increasing number of sources of pollutants, and not the meteorological conditions that make the situation worse," Beig said.

He, however, believes that the "worst is already over for Mumbai, for the time being".

"The conditions have changed, and the immediate impact could be an improvement in air quality. This does not ensure that the rest of the season would be clean, but what we saw last week, might be already over,” he added.

It's not the local weather conditions that are primarily to blame for Mumbai's poor air quality. The city is generating pollutants at a rate that is increasingly surpassing its capacity to carry them.

"There is no ambiguity. Long-term trends show a clear decline in Mumbai’s air quality, particularly in the last one decade," S N Tripathi, a professor at IIT Kanpur and one of India’s leading experts on air pollution, was quoted as saying in the IE report.

He points out that the deterioration is due to a significant increase in economic activity, the number of vehicles, construction projects, consumption, and emissions.

Tripathi emphasises that efforts to mitigate these impacts are not keeping pace with the rate of pollution.

He also notes that Mumbai has begun to experience haze-like conditions on certain days, a new development for the city that was not observed in the past.

Beig also pointed out that even last week, some areas of Mumbai suffered from extremely low visibility, a condition that closely resembles the frequent winter visibility issues in Delhi.ChatGPT