It has become our unconscious habit during the winter months to keep looking at the trees for signs of leaves swaying in the wind. This is our way of knowing if there is wind in the city to blow away and lighten the heavy blanket of smog that enwraps Delhi and the entire Indo Gangetic Plain (IGP) during winter. Stillness of air is bad news.

Every winter, the deadly smog in Delhi and IGP takes the centre stage of public attention when changes in atmospheric conditions trap massive swathes of pollutants close to our nose level. The complex interplay of wind and temperature that contributes to this serious health crisis is often not well understood.

The killer smog experienced every winter is the result of what is commonly known as ‘winter inversion’. Cooler earth surfaces during winter reduce temperatures near the ground, and prevent the air from rising up to disperse. The upper layer of warmer air caps and traps the cold air beneath. This leads to a massive trapping of pollution under the inversion layer.

In such situation, the wind is the only saviour. Pollution can disperse only if the wind blows in the city and the region. Wind speed and direction, combined with the changes in air temperature, influence the pollution concentration across the land surface. The temperature changes in the air cause air pressure differences. The warm air rises and moves and leaves behind low pressure areas. The gases move from high to low pressure areas and disperse pollution. While gases are less dense in low pressure areas, they are highly concentrated in high pressure ones.

The insidious link between wind patterns and pollution concentration came out starkly from a study carried out by Delhi-based Centre for Science and Environment during the winter of 2023-24 in Delhi. Even though the crop fire incidents in Punjab and Haryana during that winter had not changed much compared to the previous winter, there was no other unusual increase in episodic pollution activities in the region. At the same time, there were considerable rains during that season. But the concentration of PM2.5 still increased in Delhi, impacting the overall annual levels.

The contributory factor was change in wind speed. During November, the average surface wind speed in Delhi was about 9.8 m/s that was the slowest average speed recorded compared to the previous six years. The wind speed was about 21 per cent slower during that period. While the vertical movement of air and pollutants that was already restricted due to the inversion, the horizontal movement of pollutants also got restricted due to slower wind speed.

This contributed towards increase in annual average level of PM2.5, undoing the longer term of downward trend in Delhi pollution. This was a deadly combination of very high pollution level in the region and slowing down of wind speed.

Such an overwhelming influence of atmospheric conditions also came out very sharply in the Mumbai region during the 2023 winter when the pollution episode in the city hit the headlines. Mumbai otherwise has a natural advantage of a coastal geography, where comparatively higher surface wind speeds enable faster dispersion of pollutants. Also, wind reversal helps blow away the pollutants.

Gufran Beig, a noted atmospheric scientist formerly with the Indian Institute of Tropical Meteorology, has observed on several occasions that a large-scale disturbance in weather systems has been noted in the region since 2022. This, combined with growing local pollution, has impacted the urban air quality of the city and the region. During the October-November of 2023, the surface wind speed had slowed down in most parts of western India that led to the accumulation of pollutants. Also, delayed retreat of the monsoon in 2023 and anticyclone circulation impacted the trend.

Insidious link with global warming

Scientists have now cautioned that the growing global warming that is expected to impact the weather systems worldwide will also have a profound impact on pollution concentration across different local geographies. Local evidence of such an impact has begun to emerge even in India.

Scientific studies carried out in the IGP show that increase in carbon dioxide (CO2) emissions and the attendant warming tend to reduce the surface wind speed in the IGP. This, in turn, is expected to result in higher wintertime fine particulate. Thus, meteorological changes associated due to global warming can aggravate the pollution challenge in the IGP, requiring more accelerated reduction in pollution.

Another study published in the journal Advancing Earth Space Sciences in September 2022, shows similar correlation between PM2.5 concentration and the trend in heat trapping CO2emissions in the IGP. The study has estimated the reduction of surface wind speed with increasing CO2. This is expected to result in higher average wintertime PM2.5 concentrations (1 per cent per degree K of global warming) and more frequent high-pollution events. A reduction in the frequency and intensity of western disturbances with increasing CO2 may contribute to the reduction in the surface wind in the IGP. This is a double whammy.

Towards airshed approach to controlling pollution

The way wind disperses pollution is a determining factor in the overall air quality management in regions. This not only needs to look at the local sources of pollution but also the pollution brought to the area from outside sources by wind. The downwind areas will be more impacted by the major pollution sources in upwind areas. Pollutants can travel long distances making it tougher for the downwind states to meet the clean air standards.

Already, evidence suggests that a city like Delhi receives nearly 60 per cent of its pollution from outside the city, while Delhi itself contributes about 40 per cent of the winter particulate pollution in downwind Noida in Uttar Pradesh.

This phenomenon has become a major concern in the IGP area due to its unique meteorology, landlocked ecosystem and high pollution and population. The National Green Tribunal in its directive dated September 9, 2021, has taken cognisance of the high particulate pollution in IGP compared to other regions and sought advanced air pollution abatement measures.

This has led to an integrated assessment of the IGP and a regional emissions inventory by the expert committee under the Central Pollution Control Board. It has highlighted that Uttar Pradesh is the highest PM2.5 emitter in the region followed by West Bengal, Bihar, Punjab and Haryana. The industry sector alone accounts for 48.5 per cent of the total emissions in the region. Solid fuels for household cooking contributes as much as 19 per cent with highest contribution from UP, Bihar and West Bengal. Affordable cooking energy sources remain a challenge. The main contributors to transport pollution are the metropolitan cities like Delhi, Kolkata, Lucknow and the industrial areas.

It is challenging for any city or town in IGP to meet the clean air targets if the regional clean-up is not achieved. The framework for a formal adoption of integrated management of airshed is not yet in place.

This requires an aligned and coordinated action across the airshed that may have several administrative and political overlaps. This will require an operative framework and state council.

Such a precedent has now been set in Delhi and the National Capital Region (NCR). Public movement, judicial intervention and the subsequent setting up of the Air Commission for the Delhi-NCR and beyond has established the principle of regional integrated planning for four states in the NCR region. This needs to be leveraged to create a framework for all other regions.

Globally, national governments have begun to develop such a framework for management of transboundary pollution within the country and between countries.

Operationalise regional action

Given the complex connection between meteorology, atmospheric chemistry and pollution concentration, it is necessary to take an airshed-based approach to air quality management. The most recent effort under the National Clean Air programme (NCAP) to prepare state-level action plans has created an opportunity for more harmonised action across districts of the state and also inter-state cooperation. NCAP has also taken on board the need for regional airshed management, with a special focus on the IGP.

More steps are needed to develop regional action plans, monitoring strategy, legal framework, operative mechanism for integrated action and alignment of responsibilities of different authorities and compliance system within the region and the federal system. This strategy is needed to meet the clean air standards across all geographies.

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