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CSE warns of growing air pollution in cities of India’s northeast; Guwahati and Agartala top the list of most polluted cities in the region

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New analysis of urban air pollution in the northeast points to early signs that bust the myth of pristine blue skies in this ecologically sensitive region.

A new analysis by Centre for Science and Environment (CSE), the New Delhi-based think tank, says that the problem of air pollution is growing steadily in the states of India’s northeast, putting paid to the impression of pristine blue skies and clean air that people usually have about this region.

“The current obsession with high pollution concentration in the Indo-Gangetic Plain and in overall northern India overshadows and sidelines the early signs of the crisis in our northeastern states in the national discourse on air pollution and public health. Weak and inadequate air quality monitoring and paucity of data do not allow proper assessment of the risk. But even the limited evidence shows several cities – especially the state capitals — are already vulnerable to poor air quality and winter smog,” says Anumita Roychowdhury, executive director, research and advocacy, CSE.

“Gaps in air quality data and lack of quality control of data make it difficult to construct reliable air quality trends for these cities,” says Avikal Somvanshi, programme manager, Urban Lab, CSE. “The air quality in the region is worsening. But this has not drawn adequate public attention. In winter, air quality of cities like Guwahati can be almost as bad as what we see in the National Capital Region (NCR) and cities of Uttar Pradesh. Pollution is also high in smaller cities like Agartala and Kohima.”

The study methodology

CSE has analysed the urban air quality status in the states of Assam, Meghalaya, Tripura, Nagaland, Mizoram, and Arunachal Pradesh. This is part of the air quality tracker initiative of the Urban Data Analytics Lab of CSE, which was initiated last winter. The objective of this new analysis is to understand the magnitude and trend in winter pollution in major cities of the region which have recently started real-time air quality monitoring.

This is an assessment of annual and seasonal trends in PM2.5 concentration for the period January 1, 2019 to December 7, 2021. This analysis is based on the real-time data available from currently functioning air quality monitoring stations in the northeast. A huge volume of data points have been cleaned and data gaps have been addressed based on the USEPA (United States Environmental Protection Agency) method.

This analysis covers seven continuous ambient air quality monitoring stations (CAAQMS) spread across six cities in an equal number of states: two stations in Guwahati (Assam) and one station each in Shillong (Meghalaya), Agartala (Tripura), Kohima (Nagaland), Aizwal (Mizoram) and Naharlagun (Arunachal Pradesh).

There is no real-time monitoring in Manipur and Sikkim, which is why the analysis is unable to cover these states.

Even from the states covered, the data is limited. Guwahati and Shillong have data available for over two years. Real-time monitors in Agartala, Kohima and Aizwal became operational only near the end of 2020, which limits the possibility of doing long-term trend analysis for these cities. Naharlagun got its real-time monitor in March 2021. Due to excessive amount of missing data from this station, any meaningful analysis is not possible.

Seasonal and annual pollution patterns in key cities of northeast India

Data quality remains poor despite setting up automatic real-time air quality monitoring stations: The region has historically been low on monitoring due to hilly terrain and limited infrastructure. With the introduction of automated monitoring in the region under the CAAQMS (continuous ambient air quality monitoring system) programme of the Central Pollution Control Board (CPCB), it was expected that air quality data generation would improve — but even this online system is plagued with data gaps. Data availability calculated as number of days with adequate data for commutation of a valid 24-hour average has been low in three of the six cities in the region. In the last six months (May to November 2021), data availability at Naharlagun station of Anurachal Pradesh has been just 26 per cent. Stations at Shillong in Meghalaya and Aizwal in Mizoram fare just marginally better with 33 per cent and 34 per cent data availability, respectively.

Stations at Kohima in Nagaland, Agartala in Tripura and Guwahati in Assam meet the minimum 75 per cent data availability requirement. In contrast, the CAAQMS stations in Delhi-NCR have over 95 per cent data availability. Says Somvanshi: “It is not clear why these stations have such poor data availability but it probably has to do with poor electricity and internet connectivity in the region.”

Guwahati has the most polluted air in northeastern India: Average PM2.5 levels in 2021 (up till November 30) has already surpassed the 2019 annual average in Guwahati. The city’s 2020 annual average was also higher than its 2019 average which indicates continuous worsening of air in the city. Shillong is the only other city in the region that has a station generating data for over two years, but due to poor data availability its annual averages cannot be considered credible. Nevertheless, the city’s average is considerably below the annual standard.

Among other cities meeting the minimum data availability requirements, Agartala with a 2020 average of 45 microgramme per cubic metre (ug/m3) is the second most polluted city in the region. Kohima with a 2020 average of 35 ug/mis the third most polluted city in the region. Aizwal and Naharlagun do not meet the minimum data availability requirement, but the limited data available indicates that these two would most probably be meeting the annual standard.

Guwahati has almost two months of very poor air quality days: This year, so far, the number of days with air quality in ‘very poor’ or ‘severe’ categories stands at 54 days in Guwahati city. This is comparable to cities of north India which are more polluted compared to those in the northeastern states.

In other cities, ‘good’ and ‘satisfactory’ days dominate but ‘poor’ and ‘very poor’ days have also begun to emerge. Agartala registered 10 ‘very poor’ days, while Kohima had two. There is a considerable number of days in these cities for which the AQI (air quality index) could not be calculated due to poor data availability. All three cities meet the 24-hour standard for about half of the days in 2021 so far. The AQI chart for Shillong, Aizwal and Naharlagun could not be made as these cities do not have enough data.

High pollution episode common during winters despite low annual levels: Except Guwahati, rest of the cities in the states in northeast have low annual PM2.5 levels; but during winters, episodes of high pollution are common. Weekly PM2.5 levels can go as high as 189 ug/m3 in Guwahati (as recorded in the week ending on January 17, 2021). This winter, so far the highest weekly level has been reported from Agartala where it hit 91 ug/m3 (as recorded in the week ending on November 28, 2021). Last winter, it had gone up to 112 ug/m(as recorded in the week ending on January 10, 2021). Similarly, high pollution has been recorded in Aizwal and Kohima.

Guwahati, Kohima and Naharlagun show elevated NO2 (nitrogen dioxide) levels in winter: There is a significant increase in amount of NO2 in the air of Kohima and Naharlagun during November compared to October and September, 2021. Kohima registered a 124 per cent jump in monthly NO2 levels, while Naharlagun registered a 67 per cent increase. In other cities including Shillong, Agartala and Guwahati, there seems to be a problem with the monitors as the data points are showing an almost flat line. This implies their NO2 monitors might not be working properly. Guwahati’s data for this year is reporting an almost flat line since May 2021. But in 2020, NO2 level in Guwahati had increased as the winter progressed — there was an 85 per cent jump in monthly NO2 levels between September and November. Aizwal has not reported NO2 data since May 2021.

Daily NO2 peaks with traffic peaks: Only three cities have reliable NO2 data and all of them show a peaking of the hourly NO2 concentration at 6 PM which coincides with the evening rush hour in cities. Hourly NO2 in Guwahati and Agartala increases five-fold between 1 PM and 6 PM. The NO2 cycle is not as sharp in Naharlagun, but despite that, a 40 per cent increase is noted between 1 and 6 PM. All three cities have a morning NO2 peak which happens around 7-8 AM, but it is relatively smaller compared to the evening peak. In Guwahati, night-time NO2 is high indicating impact of night-time truck movement in the city.

Diwali is a mega pollution event for Guwahati and Agartala: Pollution levels on Diwali night (8 PM to 8 AM) shot up by 1 to 3.6 times the average level recorded seven nights preceding Diwali. Guwahati had the greatest pollution build-up on Diwali night, with a 3.6-fold increase in night-time PM2.5 levels, while Kohima had no impact of Diwali on its air quality. Naharlagun had no data for Diwali night while Shillong and Aizwal registered very low PM2.5 levels with negligible impact of Diwali. In absolute concentration terms, Guwahati dominates the list of most polluted Diwali nights with a 225 ug/m3 PM2.5 level. Agartala with a Diwali night PM2.5 level of 192 ug/m3 enters the AQI categorisation of ‘very poor’ level.

Spotlight on Guwahati

Guwahati has comparatively better data sets that allow more insights into the pollution behaviour in the city. It has had a stable annual average of PM2.5 since the last three years, but it does not meet the annual standard for PM2.5. It needs to cut its pollution level by 33 per cent to meet the standard.

Further, November pollution this year has been lesser compared to the last two Novembers. But the most polluted months for the city are December and January. Analysis of days as per the categorisation of the national AQI shows that the city is experiencing increasingly higher number of days with poor or worse air quality, and most of these days are concentrated during winter months. Number of days with good air quality has remained the same in the last two years. 

The next steps

Says Roychowdhury: “Cities of northeastern states need urgent attention and support under the National Clean Air Programme to implement locally appropriate clean air action and robust air quality monitoring network for proper risk assessment.”

She adds: “This is urgently needed to cut pollution from growing motorisation and congestion, use of solid fuels and open burning, and dispersed industrial sources at the early stages to prevent worsening of the public health crisis in this ecologically vulnerable region.”

(Inputs by Pratyusha Mukherjee)

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