Net Zero Summit

There is change in winter smog patterns in Delhi-NCR: CSE

The worst of the winter in Delhi and the National Capital Region (NCR) is over. For the purposes of implementation of the Graded Response Action Plan (GRAP), winter in this region is officially demarcated as the period from October 15 to February 1. A new analysis by Centre for Science and Environment (CSE), of the winter pollution levels in Delhi-NCR, says that while the seasonal average in this period was higher than in the previous winter, the severity and duration of smog episodes was lower.

“There is always a special interest in understanding winter pollution trends — given the extraordinary situation due to the pandemic this year, as well as the fact that winter remains the most difficult season in this region due to atmospheric conditions of inversion, calm wind, and cold weather,” says Anumita Roychowdhury, executive director-research and advocacy, CSE, and the head of its sustainable urbanisation, air pollution and mobility programmes. “The trapped winter air traps local and regional pollution in turn, and gives rise to the deadly winter smog that we, the residents of Delhi-NCR, are so familiar with.”

While Delhi has been battling a serious challenge of prolonged smoggy days every winter, in the time horizon of 2015-2020, several multi-sector actions have been initiated. These have been a part of the Comprehensive Clean Air Action Plan (CCAAP) as well as the GRAP. Avikal Somvanshi, programme manager in CSE’s Urban Lab team of the Sustainable Cities programme, says: “On a year-on-year basis, annual average PM2.5 levels are declining in Delhi. Is this also moderating the impact on the pollution build-up and severe peaks during winter? This analysis of winter pollution trends since 2018 helps understand the patterns of change.”

“Winter season is always a special challenge when inversion, and cool and calm weather traps and spikes daily pollution. But this is also the bar to understand the effectiveness of round-the-year action in reducing long-term pollution in the region. Despite the declining trend (year-on-year basis) due to action taken over the last few years on clean fuels for industry and transport, power plants, trucks, old vehicles etc, the winter PM2.5 concentration has bounced back, unmasking the impacts of local and regional pollution. This demands quicker and more ambitious regional reforms to curb pollution from all sources with scale with speed,” says Roychowdhury. 

Somvanshi adds to this: “This analysis has helped us understand the regional patterns as well as the local variations. Even though there is considerable regional variation, peak pollution episodes increased and synchronised within the region. The uneven rise across monitoring locations — even contiguous locations — brings out the impact of local pollution.” 


The analysis is based on publically available data from various government agencies. Most granular data (15-minute averages) has been sourced from the Central Pollution Control Board’s (CPCB) official online portal Central Control Room for Air Quality Management – All India ( This has analysed the data recorded by 81 air quality monitoring stations (or 100 per cent of the current NCR network) under the Continuous Ambient Air Quality Monitoring System (CAAQMS) of the CPCB.

Farm stubble fire data has been sourced from System of Air Quality and Weather Forecasting and Research (SAFAR). Weather data has been sourced from the Palam weather station of India Meteorological Department (IMD).

There is variation in the number of air quality monitoring stations: Delhi has 40, while Ghaziabad, Noida, Gurugram and Faridabad have four stations each; Meerut has three and Greater Noida, two. Therefore, city-wide average has been used for comparative analysis: it is defined as average of all city stations that meet the minimum 75 per cent data availability criteria.   

Key highlights

Severe intensity of winter smog comparatively lower, but seasonal average marginally higher

There is an interesting trend this winter. The number of days with ‘severe’ and ‘very poor’ levels of PM2.5 concentration (as per the concentration classification of the National Air Quality Index – AQI) has been comparatively lower this winter. But the number of days with the concentration level in the ‘poor’ category has increased. At the same time, the overall the city-wide average for the season has been comparatively higher this year. For Delhi, this average stood at 186 μg/m— 7 per cent higher than the seasonal average of 2019-20 winter.

But, the seasonal peak has been about 8 per cent lower compared to last winter. Meteorologically – mainly from the perspective of rainfall and temperature — this winter has been somewhat similar to the winter of 2018-19; the seasonal average this winter has been 4 per cent lower than the 2018-19 winter. But the 2019-20 winter saw more rains.

Says Somvanshi: “Overall, higher seasonal average but lower peak can be attributed to meteorology and changes in farm stubble burning patterns. But considerable variation has been noted among individual stations in the city. This also indicates dominance of local pollution sources.”

 Number of days with severe concentration of PM2.5 declined, smog episodes of shorter duration

This winter, 23 days had a city-wide average of PM2.5 concentration in ‘severe’ or ‘worse’ AQI category — down from 25 such days in the previous winter and 33 in 2018-19. Technically, a smog episode is defined for the purpose of implementing emergency action under GRAP, when levels of PM2.5 remain in the ‘severe’ category for three consecutive days.

From this perspective, there were two continuous smog episodes during this winter. The first episode was of longer duration as it started on November 3 and lasted seven days. The second started on December 22 and lasted three days. Thus, the continuous smog episodes were fewer and shorter compared to previous winters. The 2019-20 winter had three smog episodes of eight, six and five-days durations; the 2018-19 winter had four smog episodes of 10 days, two episodes of six days, and one of a three-days duration.

Relatively faster dissipation of smog episodes without any major rainfall or pollution control-emergency action this year points towards a downward trend in the annual average concentration. This period also coincided with the gradual unlocking of economy and travel in the airshed (perhaps a residual impact of lockdown). But the continuing persistent high levels suggest impact of local pollution.   

Difference between various monitoring locations in the city more varied, indicating dominance of local pollution sources

Twelve stations witnessed an improvement in their seasonal averages over last year. Most improvement was noted at NSIT Dwarka, Wazirpur and Shadipur. Maximum increase was noted at Patparganj, Vivek Vihar and RK Puram (all residential areas). Stations in north and east Delhi continued to exhibit highest PM2.5levels, while west Delhi was relatively cleaner.

A bewildering aspect of the trend is that there is no clear clustering of stations that shows improvement or deterioration. In fact, many neighboring stations have indicated a diverging trend. For instance, the station at Shadipur shows 34 per cent improvement, while its next door PUSA IMD station registered a 13 per cent worsening from the previous winter. This heterogeneous spatial distribution further indicates impact of local sources and micro-climate.        

16 of the 18 recognised hotspots registered worsening of air quality

Except Wazirpur and Sahibabad, all the locations in the Delhi-NCR pollution hotspots list saw a spike in the seasonal PM2.5 levels (compared to last winter). Jahangirpuri, with a seasonal average of 256 μg/m3 was the dirtiest among the recognised hotspots. Bahadurgarh that recorded an almost 50 per cent jump in its PM2.5 level was cleaner than other hotspots. But heavier deterioration was noted in many other locations that are not yet designated as hotspots. Originally, hotspots were defined as those with annual average levels higher than the mean value of the city – that in any case is much worse than the national ambient air quality standards.

However, during the winter months, at least 14 more locations registered higher seasonal averages than the mean of recognised hotspots (197 μg/m3). These were Alipur, DTU, ITO, Nehru Nagar, Patparganj, Sonia Vihar and Vivek Vihar in Delhi; Sectors 1 and 116 in Noida; Loni, Sanjay Vihar and Indirapuram in Ghaziabad; Knowledge Park V in Greater Noida; and Bulandshahr. This again points towards impact of local pollution.

Ghaziabad most polluted among the four major neighbouring towns of Delhi — all registered an increase in seasonal averages compared to last winter

Haryana cities were relatively less polluted than the cities of Uttar Pradesh. North Delhi was the dirtiest place in the region, with Jahangirpuri recording the worst seasonal average and Mundkaa with the worst seasonal peak. In the larger NCR, there were wide variations in trends among neighbouring towns.

Twenty-five out of the 27 NCR towns have adequate data for previous winter that makes a comparison possible. Eight towns show an improvement in sesonal averages from last year, with Bhiwani and Palwal in Haryana indicating the most improvement (exceeding 60 per cent). But Sonipat (69 per cent) and Bahadurgarh (49 per cent) lead the pack of towns with maximum detoriation in seasonal averages.

It is noteworthy that Hapur and Bulandshahr are neighbours, but Hapur saw a 24 per cent improvement while Bulandshahr deteriorated by 32 per cent. In fact, the seasonal average of Bulandshahr is three times that of Hapur. There are many such pairs of cities and towns in the NCR and the larger north Indian region. This heterogeneous spatial distribution further strengthens the observation that local pollution and micro-climate has been the driving factor this winter. It must be noted that these pairs are mostly of smaller towns that have some higher seasonal fluctuations compared to bigger cities of the region.

End note

The winter of 2020-21 coincided with the gradual unlocking of the economy after the hard lockdown phases during summer. There were still some residual effects, as all establishments had not fully opened. While the entire region was affected by the winter inversion and trapping of the pollution at a regional scale, the data indicates an interesting trend towards very wide variation across monitorong locations and towns and cities of Delhi-NCR. There is no clear clustering of cities. This indicates impact of local pollution while micro-climates may have also played a role. This winter, other than the initial implementation of the basic GRAP measures in October in terms of reinforcing enforcement of pollution control measures, the additional measures for severely polluted days were not implemented.

At the same, lesser number of smog episodes and lesser number of days in ‘severe’ categories is also a reflection of the overall decline in year-on-year average levels due to the implementation of some of the critical sectoral measures over the last five years. This includes closure of coal based power plants in Delhi, expansion of CNG programm for public transport and commercial vehicles and use of natural gas in industry, banning of dirty fuels like petcoke and furnace oil (and coal in Delhi), controls on truck entry into Delhi, banning of old vehicles among others.

“The arresting of the trend is the cummulative impact of all these measures. This, however, has not been uniformly implemented across the entire NCR,” says Roychowdhury.  

She adds: “This is clearly an inflexion point for Delhi-NCR. Urgent nexts steps are needed for scale and speed of action across all sectors and across the entire NCR.  The region needs massive clean fuel and technology transition in industry; time-bound implementation of power plant standards; massive transportation and mobility transition across Delhi and NCR; and a paradigm shift in waste management with strong compliance and accountability.”