Brown and black carbons in megacity smogs

Resumo

AERONET data are used to analyze variations in aerosol radiative characteristics caused by variations in the content of brown and black carbons in smogs of megacities by the example of Beijing, São Paulo, Santiago, and Mexico City. It is shown that the contribution of brown carbon to the imaginary part of refractive index at a wavelength of 440 nm in smogs can exceed the contribution of black carbon by a factor of three. In megacity smogs the spectral dependences of the aerosol absorption optical depth can be approximated by power-law functions with satisfactory accuracy, except in certain Beijing smogs at a wavelength of 400 nm for large relative concentrations of brown carbon. It is shown that the contribution of the coarse aerosol fraction to the total aerosol extinction optical depth in megacity smogs can be from 6 to 20% for wavelengths of 440 and 1020 nm, respectively, and that the contribution to aerosol absorption optical depth can be from 22% for a wavelength of 440 nm to 38% for a wavelength of 1020 nm. The aerosol in megacity smogs differs from the smoke aerosol from large-scale fires in the boreal forests of Russia by increased contribution of the coarse aerosol fraction to the total aerosol volume and by a relatively large imaginary part of the refractive index, and, hence, by a larger absorptance.