Spectral behavior of imaginary part of aerosol refractive index obtained from ground-based sun-sky radiometer measurements in Beijing, China

Black carbon, organic matter, and mineral dust are the main absorption components in atmospheric aerosols and their fractions determine the wavelength dependence of absorption. We analyze the spectral measurements of imaginary refractive indices (k) from 440 to 1020 nm obtained from ground-based AErosol RObotic NETwork sun-sky radiometer located in Beijing. The results show that the k spectra in Beijing have a wavelength dependent characteristic, with k at 440 nm significantly higher than that at the other three bands. Hence, the ratio of k(440 nm) to k(670 nm) is established to express the k spectral behavior. A significant seasonal variations of k(440 nm)/k(670 nm) is found in Beijing, with maximum value of about 3.5 appeared in March, and minimum value of about 0.9 in August. This is consistent with the variation trends of absorption angstrom exponent, indicating that k(440 nm)/k(670 nm) is also a useful tool in identifying aerosol composition.