The effect of biomass burning in increasing the atmospheric aerosol and in turn the atmospheric warming through light absorption has been clearly highlighted in a study carried out by a team of researchers from the Indian Institute of Technology (IIT) Kanpur. While the role of black carbon produced by biomass burning in increasing the atmospheric warming has already been well established, the latest study highlights the role of the less known role of brown carbon.

Brown carbon produced by biomass burning has higher light absorption capacity and, therefore, higher capability to increase atmospheric warming. Compared with earlier studies carried out in the U.S, light absorption at 365 nm was found to be five times higher in Kanpur, which has a high biomass burning area. Also, brown carbon accounts for about 30 per cent of light absorption in Kanpur. The results were published on November 24 in the journal Scientific Reports.

“What is seen in Kanpur can be generalised for the entire Indo-Gangetic Plain because the sources of aerosol remain the same throughout the region,” says P.M. Shamjad from the Department of Civil Engineering, IIT Kanpur and the first author of the paper.

“Based on 50 days of measurement (December 23, 2014 to February 24, 2015) we were able to clearly apportion the amount of light absorption by different carbonaceous aerosols. Though brown carbon is 10 times more than black carbon in terms of mass, the absorption capacity of black carbon is 50 times more than brown carbon,” says Prof. S.N. Tripathi from the Department of Civil Engineering, IIT Kanpur and the corresponding author of the paper.

As a result, up to about 70 per cent of light absorption during 24 hours is by black carbon. Brown carbon (when present independently) has nearly 15 per cent potential to warm the atmosphere by absorbing light. Additionally, depending on the spectrum of light, the light absorption capacity of brown carbon is 15-30 per cent when present as a coating (shell) over a black carbon core. “This is because the brown carbon coating behaves like a lens and focuses light towards the black carbon core,” says Shamjad.

The lensing (concentration of light on the core) is dependent on three parameters — ratio of the diameter of the total to the diameter of the core, wavelength of light and the scattering or absorbing property of the coating. “When you have an absorbing coating less light reaches the core. But when the coating is non-absorbent, light gets scattered and more light reaches the core. This leads to more overall light absorption and, in turn, more atmospheric warming,” says Prof. Tripathi.

Unlike at 405 nm (near UV), at 781 nm (near infrared), the lensing becomes predominant as the absorption capacity of the brown carbon coating is non-existent. This is because during daytime, photo bleaching of the brown carbon by sunlight creates a new compound that is no longer able to absorb sunlight.

“The contribution of lensing to light absorption goes up to nearly 35 per cent. This has very serious implications on atmospheric warming,” Prof. Tripathi says.

Published in The Hindu on November 26, 2016