Among Indian metros, Delhi gets the most attention for its notoriously high pollution levels. But it has been evident for some time now that bad air problems affect large parts of the country. As a series in this newspaper has pointed out, Mumbai has experienced an air quality crisis during the post-monsoon period for the second consecutive year. For most of October so far, the Air Quality Index (AQI) in Maharashtra’s capital has registered “moderate” or “poor”. The city has not had a single “good” air day this month.
In the first few days of this month, the pollution was much more palpable than what the AQI indicated because a layer of haze blanketed major parts of the Mumbai sky, leading to reduced visibility. However, there is a thin silver lining: The basket of pollutants is dominated by coarser particles which can cause irritation but are less harmful than finer particles like PM2.5. Meteorological conditions play a role in pollution. But it should be clear that the weather or climate cannot generate air pollution. Air quality deteriorates mainly due to anthropogenic sources of emissions, whether local or distant. Meteorological conditions only help to manoeuvre the pollutants in the air.
Mumbai’s geography gives it a distinct advantage: The coastal city has a natural cleaning mechanism. Strong surface winds lead to faster dispersal of pollutants, and the strong sea breeze sweeps away these particles from the land. This wind reversal cycle is a feature of the city for most parts of the year. However, the post-monsoon season has been ringing alarm bells for Mumbai, mainly due to the unfavourable meteorological conditions. Things get complicated in the city if the withdrawal of the monsoon is delayed. In recent years, the city has witnessed some strong and longer spells of bad air quality, that defies our basic scientific understanding of a “usual” environment.
The question is why has the air quality been bad in Mumbai of late? The answer lies in the fact that the “usual” is becoming “unusual” and getting triggered by additional anthropogenic factors. Last winter, the city experienced record-breaking particulate pollution, which was largely related to the unusual triple dip La Nina conditions — understood as linked to climate change. An abnormal drop in surface temperature in the Pacific Ocean had a telling effect on coastal wind speeds around Mumbai. There was scarcely any wind reversal from across the Arabian Sea, which would otherwise occur every two to three days and disperse pollutants in the air. In other words, the emission from various anthropogenic sources was not cleaned up and the city experienced its worst air of the decade.
La Nina has gone this year. However, the withdrawal of the monsoon was delayed till October. The withdrawal time of monsoon always plays a critical role in Mumbai’s air quality. The withdrawal is always followed by an anticyclonic circulation that was right above Mumbai and the surrounding regions during the initial days of October. This led to weak ventilation in the city.
At the same time, the transport level winds, much above the surface in the atmosphere, were moderately fast and were blowing towards Mumbai from Lonavala and Khandala in the Sahyadri ranges, around 2,040 ft above sea level. These chilly moderate winds are capable of transporting pollutants towards Mumbai. When these winds started losing energy, they descended near Navi Mumbai and the surrounding coastal region. There they encountered warmer winds which were laden with local dust. When this dust-filled cloud pushed towards Mumbai, it was arrested because of the calm winds, deteriorating the city’s air quality. The fact that PM10 continues to be the major pollutant in the city and the visibility of Mumbai’s residents has been impacted suggests that the major blame for the current worsening of air quality should be ascribed to coarser particles, largely a result of dust emissions.
Many development and construction activities are being undertaken across the city — these include the coastal corridor, work on the Metro and other digging activities. Unfavourable weather conditions exploit the emissions from such sources to create pollution. Under normal conditions, windblown dust contributes 26 per cent of PM10 and 19 per cent in PM2.5. However, the spike in pollution indicates that PM10 levels have gone up significantly. Under normal conditions, the transport sector contributes to 14 per cent of PM10 and 31 per cent of PM2.5. However, the stagnating conditions are likely to have caused a change in this mix.
On some days this month, Mumbai’s AQI was worse than that of Delhi. This is a coincidence. Delhi experienced rains that washed away the polluting particles while the intensity of these particles in Mumbai’s air increased due to the lack of ventilation. In the long-term, however, Mumbai’s air is still better than Delhi — at least, so far.
The festival season this year will reach its peak around mid-November, when winter will set in. It would be a concern if the festivities lead to an increase in emissions. However, that would still be a short-term issue, lasting for two or three days. We need to prioritise long-term mitigation planning based on mapping airsheds — areas where pollutants get trapped — because climate change is leading to extreme and unusual weather events and precipitating changes in the ecosystem. Coastal regions are highly vulnerable and hence, Mumbai needs to be extra-attentive. We need to address the root cause of the problem — anthropogenic emissions from the source.
The battle against air pollution could prove long and difficult, but we must be steadfast in following the science. Understanding and accepting the problem is half the solution. We need to own the data coming from reliable scientific sources rather than living in denial. Cordoning off construction sites with green curtains, regular spraying of water to dampen dust particles and prevent them from becoming airborne, faster implementation of fossil fuel-less transportation and ensuring smooth traffic flow to overcome snarls check pollution in the short-term. Transitioning to electric vehicles, addressing solid waste management, cleaning up dumping grounds and adept industrial toxin management can improve air quality in the medium term. Given the seriousness of climate change, no time should be lost in bringing about these changes.
The writer is Chair Professor, National Institute of Advanced Studies, Bengaluru and Founder Project Director, SAFAR
