High concentrations of fine particulate matter (PM2.5) in India have severe impacts on public health.   While high PM2.5 levels are primarily due to intensive local emissions, they can be further worsened by meteorological patterns known as atmospheric stagnation, which trap pollutants close to the Earth’s surface. Understanding how climate change might influence these stagnation events is crucial, as changes in stagnation will influence  the effectiveness of air quality policies. A recent Princeton study published in Nature Communications shows that as the climate warms, stagnation events are projected to become more frequent during winter in northern India’s Indo-Gangetic Plain (IGP). This shift is expected to further worsen the already hazardous winter air pollution in the region unless stringent emission controls are implemented. 

PM2.5 is a combination of small particles and droplets that are 2.5 micrometers or less in diameter.  PM2.5 can be transported and diluted by wind and vertical mixing, and is primarily  removed from the air by precipitation.  Atmospheric stagnation, a meteorological event where a lack of precipitation, weak surface winds, and weak vertical mixing cause a buildup of air pollutants,  exacerbates PM2.5 accumulation and has driven researchers to develop region-specific indices to identify stagnation occurrences.  

“Several region-specific stagnation indices have been developed for the U.S., Europe, and China,” explains lead author Mi Zhou, a postdoctoral researcher at C-PREE.  “However, up to the point of our study, no India-specific index had been  developed.”  

To address this issue, Zhou, Professor Denise Mauzerall, and their team developed an India-specific atmospheric stagnation index (India-ASI) that links PM2.5 pollution with three meteorological variables: precipitation, wind speed, and temperature inversions. Built using historical surface PM2.5 measurements and meteorological datasets, this index can estimate potential pollution levels for a given day based on its meteorological conditions. 

In the study, the authors primarily used the index to identify 'stagnation days'—days when meteorological conditions are statistically expected to result in higher-than-average PM2.5 levels.  They found that stagnation is most prevalent in northern, inland regions characterized by weaker surface winds, stronger temperature inversions, and less frequent precipitation.  

Notably, the most polluted region in the country, the Indo-Gangetic Plain (IGP), had approximately 101± 57 days of stagnation occurrences per year, with each stagnation day averaging around 27 ± 10 μg/m³ (34% ± 8%) higher PM2.5 levels compared to non-stagnation days. The most significant impact of stagnation on surface PM2.5 levels is found during winters in IGP, where PM2.5 concentrations increase by about 45 ± 20 μg/m³ (45% ± 13%) during stagnation days.

Building on the link between surface PM2.5 pollution and stagnation occurrences, the authors also projected future stagnation during the late 21st century (2080-2099) under two global warming scenarios, SSP370 and SSP585.  Both scenarios assume increases in global mean temperature, but they make different assumptions about energy and air quality policies.  Specifically, SSP370 assumes continued coal dependence with high pollution levels while SSP585 assumes an energy transition away from coal to natural gas and the implementation of strong air pollution controls.  

Compared to the historical period 1995-2014, the researchers project a decrease in nationwide stagnation occurrences from 2080-2099 under both SSP585 and SSP370. Under the high-warming, lower-pollution SSP585 scenario, the average number of stagnation days across India are expected to decrease by 21±13 days per year, with the most significant reduction of 11±5 days occurring during the monsoon season. The researchers attributed the decrease in stagnation days in the monsoon season to the projected increases in monsoon precipitation and extension of monsoon periods as climate warms. In contrast, the high-warming, high-pollution SSP370 scenario forecasts a much smaller  decrease of 10±9 stagnation days per year during 2080-2099, compared with the historical period.  

However, winter projections tell a different story.  Under the high-pollution SSP370 scenario, annual winter stagnation occurrences over the IGP are projected to increase by 7±3 days by the year 2100.  These more frequent stagnation events alone could lead to a seasonal mean increase of 7 μg/m³ (6%) in PM2.5 concentrations during winters, worsening already severe pollution levels in the region.  In contrast, the low pollution SSP585 will experience fewer increased winter stagnation occurrences, highlighting the importance of emissions controls to further reduce stagnation and associated elevated pollution levels. 

“Increased winter stagnation occurrences are driven by weaker surface winds and weaker vertical mixing,” explains Zhou.  “Though increased winter stagnation occurs  in both scenarios, the increase is less pronounced under the lower-pollution conditions of SSP585 (compared with higher-pollution conditions of SSP370), which shows the crucial role appropriate  air quality and climate policy will play in reducing future stagnation occurrences as the climate warms.”

Given stagnation is projected to increase during the winter over the IGP, wintertime air pollution controls  will be crucial as the climate warms.  Co-author Denise Mauzerall emphasizes the co-benefits of a clean energy transition in addressing both air pollution and climate change induced increases in pollution levels over the IGP.   

“Reductions of air pollutant emissions resulting from direct controls on emission sources as well as reduced emissions from a clean energy transition will improve air quality while reducing future stagnation occurrences, thus providing additional co-benefits for air quality and health” explains Mauzerall, a faculty member at Princeton’s School of Public and International Affairs and the Department of Civil and Environmental Engineering.  

The paper, “Impacts of current and climate induced changes in atmospheric stagnation on Indian surface PM2.5 pollution,” was co-authored by Mi Zhou (School of Public and International Affairs, Princeton University), Yuanyu Xie (School of Public and International Affairs, Princeton University), Chenggong Wang (Program in Atmospheric and Oceanic Sciences, Princeton University), Lu Shen (Department of Atmospheric and Oceanic Science, Peking University), and Denise L. Mauzerall (School of Public and International Affairs and the Department of Civil and Environmental Engineering, Princeton University). The paper appeared in Nature Communications on August 28th, 2024.  This research was supported by  the M.S. Chadha Center for Global India at Princeton University, the Princeton School of Public and International Affairs and its Center for Policy Research on Energy and the Environment.