Pollution and its hazy mystery

Air quality deterioration is a daunting problem in a developing country like India often linked with reduced visibility in winter. Chloride-rich particulate matter (PM) found to be largely responsible, and if controlled, may yield positive results.

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On one of his routine business trips to the national capital of India, New Delhi, Sachin’s flight from Chennai got delayed, as operations at Delhi airport were suspended due to the dense fog. He missed his important meeting! Upon his next regular summer visit to Prof. Scot Martin’s laboratory at Harvard University, while attending a discussion in Pierce Hall, he told Pengfei, the then postdoc at Harvard University, “We need to do something to understand the smog issue in Delhi. I do not think only measuring the specific parameters to report “increased and decreased”, setting a few more pollution monitoring stations, and concluding that PM2.5 is high during winters is going to solve the issue”. He was refereeing to Pengfei about understanding the precise role of aerosol particles (commonly referred to as PM2.5, particulate matter 2.5 µm in size by diameter) in the chemically complex atmosphere of Delhi, and predominantly in dense haze and fog formation leading to reduced visibility.

Fig. 1 | India Gate on a typical hazy day in the winter morning (Photo credit: Aishwarya Singh).

The formation of the dense smog and subsequent visibility deterioration during winters in Delhi is a longstanding problem, disrupting flight operations causing unprecedented financial losses and increased road accidents risking human lives (Fig. 1). As a part of an international field measurement campaign, advanced aerosol characterization instruments were deployed in Delhi during Jan-Feb 2018, which itself was a very important but laborious exercise. Within 24 hrs of the instruments started recording the ambient aerosol composition, Sachin asked one of his students to send the data to him with routine corrections (recorded every 15 minutes interval in online and real-time mode). Immediately he noticed something unusual; the inorganic fraction of PM2.5 was dominated by chloride, something unheard of, as for all previously reported polluted locations across the globe it was dominated by sulfate. He immediately sent these results to Pengfei, and subsequently during his 2018 visit to Harvard, upon a week of intense scientific discussions, they found something intriguing! During the same time, other studies also started reporting unusual high chloride in Delhi aerosols. Pengfei was eagerly interested to answer two important questions a). Where this chloride comes from? and b). If this has any direct relation to the smog/visibility?

Fig. 2 | Schematic showing the HCl emission from trash burning and industrial sources, co-condensation of chloride, and the possible influences on haze and fog formation. Other potential environmental impacts, such as the influence on ozone chemistry and co-emission of persistent organic pollutants are also depicted.

Making this study a classic example and demonstrating why atmospheric chemistry research requires a multi-institutional approach and how international collaborations with leading experts can significantly enhance the scientific understanding, Pengfei and Sachin brought in together 23 researchers from 15 institutions across the globe. Using state-of-the-art instruments and complicated chemistry models they proved that Delhi's atmosphere is chemically complex and distinctive from other global pollution hot spots. Briefly about what they found (Fig. 2): burning of plastic-contained waste along with the use of acids for various industrial processes releases hydrochloric acid (HCl) in the atmosphere of Delhi. Under the regime of excess NH4, the HCl partitions in particle phase leading to the formation of NH4Cl making chloride the dominant factor of PM2.5 thus explaining the source of chloride. Further, gas-phase HCl partitions into particle-phase NH4Cl and highly water-absorbing and soluble chloride in the aqueous phase strongly augments the water absorption capacity of the aerosol particles. Under the favorable meteorological during chilly winter nights in Delhi the relative humidity reaches the point of making enough water available in the atmosphere, which is then efficiently and readily absorbed by chloride-rich PM2.5, eventually leading to haze and fog formation. Contrary, in the absence of atmospheric HCl, the water-absorbing capacity of the same PM2.5 would be substantially low, which would improve the visibility by 50%.   

 Contrary to the previous finding that highly polluted megacities across the globe exhibit sulfate as the dominant inorganic fraction of PM2.5 mass burden, Delhi turned out to be totally different. The burning of plastic-contained waste is not only imperiling climate but also releases the dioxins in the atmosphere, which may accumulate in the food chain.

You can read more about this study in Nature Geoscience.

Pengfei Liu

Assistant Professor, Georgia Institute of Technology

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