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.