Flux And Source Function Of Sea Spray Aerosol Over The South China Sea

Sea spray aerosol is an important medium of material and energy exchange between the ocean and the atmosphere, and it plays a crucial role in global climate change. Sea spray generation functions or source functions were applied in multitudinous numerical models to estimate global sea spray aerosol productions or over some oceans. Therefore, the source function is the basis in evaluation of the marine climate effect of aerosol. The existing source functions are obtained under different experiments and the research areas mostly concentrated in the Atlantic Ocean. The researches and studies of sea apray aerosol in the low latitude sea area are still scarce at present.Sea spray aerosol generation fluxes over the South China Sea were calculated by the whitecap method and equilibrium method in this study. Preliminary source functions were proposed using the observed data and combing with the previous source functions. The source functions were tested by observed data of the South China Sea in another cruise. At the same time, comparison of the results calculated by the two different methods was made to evaluate the two methods reasonably. And compared the new source function with that of Gong(2003) which is applicative in the northeast Atlantic Ocean.This study is based on field experiment which was carried out in the South China Sea during August 25,2013 to October 11,2013 by the scientific research vessel "Dongfanghong 2". The number concentrations in the radius range of 0.06 μm-5μm of aerosol particles, meteorological parameters and whitecap coverage data were measured on board. In the whole radius range, the particle number concentration in the South China Sea was in agreement with that in the Bay of Bengal which is similarly surrounded by land on three sides. The results showed that the experimental data was authentic.Effective fluxes in the reference height of 10 m were estimated by steady state dry deposition method and the results showed that flux distributions of aerosol particles in different sizes are different. In the range of radius of less than 0.3μm, the size distributions of the fluxes were fitted with sum of log-normal and power-law distribution. However, the derived flux source function is only applicable to the particles of radius below 0.3μm due to the limited data.Effective fluxes were calculated by whitecap method and the widely used source function Monahan et al. (1980) was modified. The relationship between the production flux of unit whitecap area and wind speed was fitted by the use of the least square method. And the calculation functions of whitecap coverage were developed based on the observed data. The relationship between the flux and the wind speed in the source function is included in the above two aspects, and this is hardly involved in the previous studies.The flux results calculated by two kinds of methods and that in other studies at the same wind speed were compared. The maximum difference between the two methods in number was up to an order of magnitude. And the difference in shape of spectrum distribution was more obvious in larger super micron range. It indicates that the shape calculated by whitecap method was similar to the general form of sea spray aerosol generation. So we think that the flux calculated by whitecap method in the particle size range of this experiment was more accurate and reasonable.To verify the applicability and representativeness of the new source function, number concentrations of sea spray aerosol were observed in another experiment in which the instrument and parameter settings were consistent with the previous mentioned above. It was found that the source function simulated the production of aerosols very well at medium wind speed (5 ms-1) and overestimated the flux at lower wind speeds, underestimated when wind speeds were higher than 15 ms-1.In this paper, the production and source function of the sea spray aerosol in the tropical area were calculated, represented by the South China Sea. It provided reference for the numerical simulation of the global flux of sea spray aerosol. In addition, the generation of sea sprays under high oceanic conditions, the characteristics of coarse particles and the parameterization of whitecap coverage also need further exploration.