Case study of Asian dust optical and deposition properties over the Yellow Sea of China by shipboard and ground-based photometers, along with Satellite remote sensing

Aerosol optical properties were measured by a POM-01 MarkII sun/sky photometer onboard the Dongfanghong No. 2 Research Ship on the Yellow Sea of China, during the passage of a cold front surrounded by airborne dust that originated in Mongolia between 21 and 24 April 2006. The aerosol size distributions in clean marine environment were dominated by an accumulate mode with radius of 0.15 μm and a coarse mode with radius of 4.5 μm, and the mean AOD and Ångström exponent were 0.26 and 1.26 respectively. As the frontal system approached on 22 April, the mean AOD (SD) and Ångström exponent (SD) changed to 0.36 (0.02) and 1.20 (0.07). The concentration of the nucleation mode with radius of 0.03 μm increased, which was attributed to the prefrontal continental air mass moving to the Yellow Sea with the frontal system. On 23 April, in the frontal zone, the aerosol size distribution was dominated by an accumulate mode with radius of 0.25 μm and two coarse modes with radii of 1.69 and 7.73μm, and the AOD and Ångström exponent were 2.46 and 0.84, respectively. After the front passed, on 24 April, a large region of weakly absorbing nonfrontal dust appeared over the Yellow Sea. In the nonfrontal dust conditions, the concentration of coarse modes with radii of 2.5 μm increased to a maximum of 0.3 μm3 μm-2, and the mean AOD and Ångström exponent were 0.70 and 0.30 respectively. AERONET observations combined with shipboard measurements reveal the decreasing concentration of dust aerosol during it was transported from continent to Japan. The spatial distribution of dust aerosol was studied using the Aqua/MODIS and Aura/OMI products. On 22 April, for frontal dust, their AOD and UVAI increased with decreasing distance to the frontal line, peaked with values of 4.36 and 5.21 in the frontal zone, and decreased rapidly with increasing distance off the frontal line. On 23 April, nonfrontal dust showed the lower AOD and UVAI with peak values of 2.0 and 2.7. The total dust column and the dry deposition flux were calculated based on the optical properties without cloud contamination on 24 April. The total dust column was calculated by using an integration method of the particle size distribution; the mean value was 1.42 ± 0.30 g m-2. A linear correlation between the total dust column and the aerosol optical depth (AOD) with a linear factor of 2.7 g m-2 over the Sahara was applied to calculate the total dust column in this study; the results were lower than these calculated by the integration method. A reasonable factor of 3.2 g m-2 was achieved by minimizing the standard deviation (SD) of the two methods. The two layers model, which includes the deposition processes of turbulent transfer, Brownian diffusion, impaction and gravitational settling over the sea’s surface, was used to calculate the dry deposition flux; the mean value was 5.05 ± 2.49 μg m-2 s-1.