What is brown carbon?
Carbonaceous aerosols are formed by a mixture of substances with different chemical, physical, and optical properties. 

Certain organic substances are mostly transparent to sunlight and therefore do not contribute to Earth warming or cooling. Other substances are mostly reflective, therefore their presence in the atmosphere contributes to Earth's cooling. Yet there are substances that absorb sunlight thus warming the atmosphere. 

Light absorbing carbonaceous aerosols do not form a uniform group either. They include soot particles, smoke, organic compounds such as levoglucosan and the spectre of humic substances all with different optical properties. The figure below shows schematically the absorption spectrum of two compounds representative of different groups of light absorbing materials.



One compound labeled B1 has a more uniform distribution of absorption coefficient across the visible spectrum where the absorption coefficient at the red edge is about half of that at the blue edge. Another compound labeled B2 has a much steeper slope at the blue edge. It would have a yellowish or light brown color. Think of the yellowish fumes that can be seen when a piece of wood is ignited.

Assuming uniform absorption spectrum for light absorbing organic carbon can lead to significant overestimation of light absorption by atmospheric carbon.

On the other hand estimating the carbon content of the atmosphere via measurements in the green or red parts of the spectrum would lead to a significant underestimation of light absorption in the blue and ultraviolet parts of the spectrum.

To reduce the error caused by the non-uniform absorption spectrum of some of light absorbing carbon we could instead think of light absorbing carbonaceous aerosols as two separate groups namely Black Carbon with uniform light absorption spectrum and Brown Carbon with steep increase in light absorption towards the blue edge.

In our example B1 would be assigned to Black Carbon group and B2 would be assigned to the Brown Carbon group.