The effect of biomass burning in increasing atmospheric aerosols and in turn atmospheric warming through light absorption has been highlighted in a study 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 atmospheric warming has already been well established, this study highlights the lesser-known role of brown carbon.
Compared with earlier studies carried out in the U.S, absorption of light of 365 nanometre wavelength 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 in the winter of 2014-2015 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.
In mixed forms
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 Mr. Shamjad.
The lensing (concentration of light on the core) is dependent on three parameters — ratio of the diameter of the shell 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.
