Investigation Of Dry Deposition Flux Of Atmospheric Aerosol Particles Over The China Seas Under Dust Weather Conditions

Dust events frequently break out in the northwest of China in spring. Dust particles can be transported and deposited into the China Seas and even to the North Pacific Ocean, which have exert great effects on oceanic primary production and algae blooms. The concentration, chemical components and size distributions of atmospheric aerosol particles change a lot when comparing to non-dust periods, leading to a substantial increase of atmospheric deposition flux in the marine boundary layer. However, there is little study on the dry deposition flux of aerosols over the China adjacent seas under dust weather conditions.In this study, the contribution of Asian dust in the global budget of the dust deposition was compared with those of the dust from the North African. The concentrations, dry deposition velocities and fluxes of aerosols over the China adjacent seas are investigated in terms of the sampling data, meteorological data and air backward trajectories. Size-dependent mass and dry deposition fluxes of aerosols along dust transport routes are also estimated. In addition, the temporal and spatial variations of the concentration and dry deposition fluxes of aerosols and particulate inorganic nitrogen during a dust event were discussed using a WRF-Chem model. The major conclusions in this study are summarized as below.1) Using the ground-level observational data at Qingdao, China and Jeju, Korea during 2001^-2007, variations of PM₁₀ concentration at the western and eastern coast of the Yellow Sea were studied. Moreover, dry depositions velocities (Vd) and fluxes during dust events in spring over this sea area during the period from 2001 to 2007 were also estimated. The analysis result shows that (1) there is a similar seasonal trend of PM₁₀ concentrations between Qingdao and Jeju, e.g., a good correlation between the PM₁₀ concentrations at these two sites in spring when dusty days often occurred. The desert regions located in the Inner-Mongolia, China were found to be the most important source of dust affecting Qingdao and Jeju. (2) The number of dust events entering the China adjacent seas during 2001^-2007 fluctuated and had a decreased trend in general, which may be related with the atmospheric circulation feature in the East Asia. (3) The estimated Vd of PM₁₀ during dust events ranged from 0.19 cm s^-1 to 8.17 cm s^-1, with an average of 3.38 cm s^-1. The differences of physical and chemical properties of dust particles, which were resulted from the discrepancy of dust transport routes, distances and intensities between each dust events, could partly explain the differences of the estimated Vd of PM₁₀ over the Yellow Sea during dust events. (4) Dry deposition fluxes of PM₁₀ over the Yellow Sea ranged about 68.5^-2647.1 mg m^-2 d^-1, with an average of 545.4 mg m^-2 d^-1. The averaged deposition fluxes during dust events were 2^-10 times higher than those calculated by previous studies. During the years with a high frequency of dust events, the aerosol dry deposition during dust events could have a great contribution, as large as 100%, to the annual deposition flux of dust into the Yellow Sea estimated by the previous studies.2) The size distribution of the mass and dry deposition flux of atmospheric aerosols collected in Beijing and Qingdao in China and Fukuoka in Japan along dust transport routes were investigated. The mass size distribution of these aerosols during non-dust periods exhibits a bimodal pattern, i.e., a fine modes at Dp< 2.1μm and a coarse mode between 2.1μm and 11μm. On the other hand, the mass size distribution of these aerosols during dust periods exhibits a uni-modal pattern in the coarse particle size range. The coarse mode diameter decreased with the distance from the dust source zone to the western and eastern coast of the Yellow Sea during heavy dust periods. It was not case in the small and moderate dust periods. The dry deposition fluxes increased with increasing size in the coarse particle size range during non-dust and dust periods. No evident variations occurred in the fine mode.3) Temporal and spatial variations of the concentration and dry deposition fluxes of arosols and particulate inorganic nitrogen during a dust events occurring on March 19^-25, 2002 were simulated using a WRF-Chem model. The modeling result shows that the concentrations and dry deposition fluxes of PM₁₀ obviously increase during dust events. For example, the maximum dry deposition flux of PM₁₀ is about 8.4 and 5.5 times higher than that at the beginning of dust events, repectively. However, the variations of the concentrations and dry deposition fluxes of pariculate inorganic nitrogen between the Yellow Sea and the East China Sea were different. Over the Yellow Sea, there was a decreasing trend for the concentrations and dry deposition fluxes of NO₃^- and NH₄⁺. The minmun dry deposition fluxes of NO₃^- and NH₄⁺ during this dust event is about 62.0% and 16.2% of that at the initial stage of dust event, respectively. While, over the East China Sea, the concentrations and dry deposition fluxes of NO₃^- and NH₄⁺ has an increasing trend. The maximum dry deposition fluxes of of NO₃^- and NH₄⁺ is about 4.1 and 2.6 times higher than those at the initial stage of dust events, respectively. The dust plume air-mass arrived at the East China Sea, passing by the regions with a high concentration of air pollutants. The dust air-mass carried large nitrate and ammonium, which would increase the concentrations and dry deposition flux of particulate inorganic nitrogen over the East China Sea. While, the dust air-mass arrived at the Yellow Sea, passing by the regions with a low concentration of air pollutants, which could lead to low concentrations and low dry deposition flux of particulate inorganic nitrogen over the Yellow Sea.