Aerosol hygroscopicity and photodegradation in the polluted Indo-Gangetic Plain compared with the clean Himalayas examined using a quartz crystal microbalance

The air we breathe is never truly empty. It is filled with tiny, floating particles of dust, soot, and salt, known as aerosols. These microscopic specks play a massive role in our planet's climate. They can cool the Earth by reflecting sunlight back into space, or warm it by absorbing heat. Perhaps more importantly, they act as the seeds upon which clouds form. Without these particles, water vapor in the atmosphere would struggle to condense into raindrops or ice crystals. However, the behavior of these particles is not fixed; it changes depending on what they are made of and how much moisture is in the air around them. When the air is humid, some particles soak up water and swell, while others remain dry. This ability to attract and hold water, called hygroscopicity, determines how big the particles get, how long they stay in the sky, and how effectively they can turn into clouds. Understanding these subtle shifts is crucial for predicting weather patterns and modeling future climate change, yet the exact rules governing how different types of pollution behave in humid air remain a puzzle.

To solve part of this puzzle, a team of researchers traveled to two vastly different corners of India to compare the air they found there. One location was Leh, a high-altitude desert in the Himalayas, where the air is thin, cold, and remarkably clean. The other was Delhi, a sprawling, heavily populated megacity in the Indo-Gangetic Plain, known for its thick haze and intense pollution. The scientists wanted to see how the tiny particles from these two worlds reacted when exposed to moisture and sunlight. They collected samples of fine dust, specifically particles smaller than 2.5 micrometers, and brought them to a laboratory to test them under controlled conditions. Using a highly sensitive instrument called a quartz crystal microbalance, which can detect changes in mass as small as a few nanograms, they watched how the particles grew as humidity increased and how they shrank when exposed to ultraviolet light.

The results revealed a stark contrast between the two environments. The particles from the clean Himalayan air behaved in a very predictable, crystalline way. As the humidity rose, they remained solid until the air reached a specific point, around 68 percent humidity. At that exact moment, they suddenly absorbed a burst of water and dissolved into liquid droplets, a process known as deliquescence. This sharp transition is typical of particles made largely of salts and highly soluble organic compounds. In fact, these Himalayan particles were quite eager to grab water, showing a strong tendency to grow as the air became more humid. They also contained components that were easily broken down by sunlight. When the researchers shone ultraviolet light on them, a significant portion of the particle mass evaporated, suggesting that the organic matter in the clean mountain air was chemically active and prone to fading under the sun.

In sharp contrast, the particles from the polluted city of Delhi refused to behave in the same orderly fashion. Even as the humidity climbed to 86 percent, the highest level the experiment could safely reach, these particles never showed that sudden, sharp transition from solid to liquid. Instead, they seemed to remain in a sticky, amorphous state, absorbing water very slowly and gradually. They were far less eager to take up water than their Himalayan counterparts. The researchers found that the Delhi particles were likely coated with complex, less soluble organic compounds, possibly from vehicle exhaust and the burning of solid fuels, which prevented them from dissolving cleanly. Furthermore, these urban particles were much more resistant to sunlight. When exposed to the same ultraviolet radiation, they lost far less mass than the mountain particles, indicating that their chemical makeup was more stable and harder to break down.

These findings highlight that the source of the pollution matters just as much as the amount of pollution. The air in the Himalayas, though distant from major cities, carries aged particles that have traveled long distances and transformed into a mix of soluble salts and reactive organics. The air in Delhi, however, is dominated by fresh, locally generated emissions that create a sticky, water-repelling coating on the particles. This difference in chemical composition changes how the particles interact with the atmosphere. The clean mountain particles are more likely to swell with water and potentially seed clouds, while the dirty city particles might stay smaller and drier, altering their impact on sunlight and rainfall. The study also underscores that the history of the air, including how long the particles have been exposed to sunlight and humidity, shapes their physical properties.

By measuring these behaviors directly, the researchers provided a clearer picture of how aerosols function in two critical regions of the Indian subcontinent. The data suggests that models used to predict climate and weather need to account for these specific chemical differences. Treating all particles as if they react the same way to humidity or sunlight would lead to inaccurate forecasts. The study confirms that the organic fraction of the air, whether it comes from natural sources in the mountains or human activity in the cities, plays a decisive role in determining whether a particle will grow, shrink, or dissolve. As the climate changes and pollution patterns shift, understanding these microscopic interactions becomes increasingly vital for grasping the future of our atmosphere.