New article in Sciences Advances by Sanjeev Dasari (ACES)

This new study has shown how brown carbon loses its light-absorption during transport in the South Asian environment. This finding will help in modeling its warming effect in the region.

Atmospheric particles, or aerosols, are tiny particles which contribute to cooling Earth’s climate by scattering the sunlight back into space, and through aerosol-cloud interactions. They are short-lived (few days to few weeks), have several natural and manmade sources (incl. transportation, industry, and agriculture) and can be transported over thousands of kilometers. Although most aerosols reflect light, some also absorb it. A fraction of the aerosols are organic (contain carbon), while the rest are inorganic (such as sulfate, sea-salt). In addition to climate effects, aerosols play a major part in causing air pollution with severe health effects, even death. The World Health Organization (WHO) reports that over 3 million people die annually from air pollution, which places air pollution as the world's largest single environmental health risk factor. Currently, developing countries like China and India are severely affected by air pollution, threatening over 2 billion people.

Black Carbon aerosols are a component of incomplete combustion from the burning of fossil fuel and biomass (e.g., wood), and have been known for some time to absorb sunlight and contribute to the warming of the atmosphere. In several countries, measures to control the emissions of black carbon aerosols have been put into place to reduce their effects on the environment.

Brown carbon study and implications:
Despite having an overall climate cooling effect, there are also specific aerosol components that contribute to climate warming, by absorbing rather than scattering the sunlight. One such component is light absorbing organic carbon aerosols – brown carbon aerosols – which has attracted much attention in the scientific community lately.
Brown carbon aerosols, named after their brownish appearance, have attracted interest as a possible contributor to global warming and climate change. Like black carbon, brown carbon can also be produced from human activities, such as forest fires, fossil fuel combustion, biomass burning, and agriculture. Laboratory and field studies have shown that as BrC particles absorb sunlight, the brownness of BrC fades with time in a process known as bleaching. Bleaching can affect the warming capacity of BrC. However, the time-scale and rate at which the bleaching happens in the atmosphere is still uncertain.

With the help of a network of atmospheric-climate observatories in India, Bangladesh and the Maldives, along with a team of international collaborators in the South Asian Pollution Experiment (SAPOEX-16), our team was able to track the plume of air pollution containing BrC from source regions in the Indo-Gangetic Plain to the remote region of the Indian Ocean, some 6000 km away from the continent. By observing how fast the bleaching of BrC particles occurred, we could infer the bleaching half-life of these particles in the atmosphere. 

Our findings have two-fold implications:
1. We show that the light-absorption capacity of BrC decreases by an order in magnitude during atmospheric transport, implying previous modeling studies relying on “set” values of BrC light absorption might have overestimated the warming effect. This is and can be said in general for BrC everywhere
2. For the South Asian atmosphere, we found that BrC bleaches with a half-life of 3.6 days, which is significantly longer than what has been previously reported from laboratory and field studies ~9 to 15 hours in other regions of the world. This could have a significant implication on the warming effect of BrC and thereby on the regional climate. The “true” warming effect of BrC, yet to be modeled, thus remains at large, specially in polluted environments like South Asia.
Our findings reveal that that BrC not only occupies near-source regions but can also spread to remote locations away from the sources. For example, in regions like the Artic and the Himalayas where localized sources of pollution are minimal, BrC emitted from continental locations far away could contribute to accelerated regional warming.

Further information:
Dasari et al., 2019, Science Advances


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