A team of researchers from Indian Institute of Technology (IIT) Kanpur in their study highlighted that carbonaceous aerosols such as black carbon (BC) and certain organic carbon species called brown carbon (BrC) have the potential to warm the atmosphere by absorbing light.
The study on brown carbon, whose role was lesser-known, is based on their experiment in Kanpur, Uttar Pradesh. In order to better characterise brown carbon, the team presented experimental and modelled absorption properties of submicron aerosols measured in Kanpur. Their study highlighted that brown carbon is found to be fivefold more absorbing at 365 nm wavelength compared to previous studies.
Black Carbon (BC) absorbs light over the entire solar spectrum. On the other hand, Brown Carbon (BrC) absorbs near UV wavelengths and to a lesser extent, visible light.
The study conducted by PM Shamjad, SN Tripathi, Navaneeth M Thamban & Heidi Vreeland was published in the journal Scientific Reports on 24 November 2016. The study presents submicron (PM1) carbonaceous aerosol light absorption properties by using offline and online measurements.
The study says, in developing countries, such as India, where combustion sources are prolific, the influence of brown carbon on absorption may be significant.
The team studied the light absorption characteristics of carbonaceous aerosols for 50days in the winter of 2014-15.
Kanpur was chosen for the study because the city and its surroundings are known for arsenal loadings, which is caused due to open biomass and trash burning. This activity helps in accumulation of large-scale particulate matter, especially during winter.
Results of the study in Kanpur
Results suggested ~30% of total absorption in Kanpur is attributed to brown carbon, with primary organic aerosols contributing more than secondary organics.
Brown carbon in Kanpur is highly absorbing in nature and that the mixing state plays an important role in light absorption from volatile species.
The study revealed, the mass concentration of PM2.5 (particulate matter less than 2.5 μm in diameter) were 187 ± 90 μg m−3 (mean ± standard deviation), which is 7 times higher than World Health Organization (WHO) standards.
It also highlighted that in terms of mass, brown carbon is 10 times more than black carbon. The absorption capacity of black carbon is 50 times more than brown carbon. It suggests that Black carbon was able to absorb about 70 percent of light in 24 hours, while brown carbon – independent in nature – had nearly 15 percent potential to warm the atmosphere by absorbing light.
Difference between Brown and Black Carbon
The black and brown carbons are distinguished by their optical (scattering, absorption and others) and physical (colour, volatility and other) properties. Both these gases are major absorbing carbonaceous aerosols in the atmosphere.
Brown Carbon (BrC)
Black Carbon (BC)