Spectral dependency of light scattering/absorption and hygroscopicity of pollution and dust aerosols in Northeast Asia

Ground-based in-situ measurements were performed to investigate the spectral optical properties and hygroscopicity of light scattering and absorbing aerosols in pollution and dust plumes in Northeast Asia. The scattering Ångström exponents of pollution plumes are high, ranging from 0.8 to 1.8, because light scattering aerosols consist of relatively fine particles such as sulfate, nitrate, ammonium and carbonaceous particles. Pollution case having high organic carbon (OC) exhibits a relatively high absorption Ångström exponent (AAE), in the range of 1.0–1.5 due to the absorption by refractory OC at near-UV spectral region. Among the different pollution cases, the AAE is the highest in a polluted dust plume; this can be attributed to the strong wavelength dependence of light absorption by dust particles. The mass absorption cross section (MAC), which represents the ability of light-absorbing aerosols to absorb a photon, is 4.2–8.3 m2 g−1, and it is high when black carbon is mixed with OC and dust particles. The MAC decreases with increasing relative humidity, especially for pollution plume with a high OC content and for dust plume. Under humid conditions, hygroscopic chemical species can absorb water vapor, thus growing in size and enhancing aerosol light scattering, but the MAC decreases because of the possible shielding effects of absorbing aerosols with water uptake.