Transport And Deposition Of Atmospheric Nitrogen Into The East China Sea

Nitrogen compounds are very important nutrients for marine ecosystem.Since many anthropogenic source regions are close to coasts,there is a significant amounts of atmospheric pollutants containing nitrogen transported over and deposited into ocean.To understand the impacts of atmospheric nitrogen input into the East China sea,the magnitude and seasonality of atmospheric nitrogen compounds' transport flux,concentration and deposition flux has been studies by the field mesurements and numeric simulation.The major contents and conclusions in the study are shown below.Field measurements of aerosols were conducted at typical site in the East China Sea.Ion composition,size distribution,source and dry deposition of NO₃^--N and NH₄⁺-N of TSP were studied.NO₃^- and NH₄⁺ reached average concentrations of 10.4 and 5.1μg/m3,respectively,and as high as 38.5 and 22.8μg/m³ on a sea fog day. Size-distribution analysis showed evident shifting of nitrate and ammonium to coarse mode.That might have been the result of reactions with dust aerosols and sea salt aerosols by statistic analysis and pollution process simulation.Also the Micrometeorological Concentration Gradient Method(MCGM)was used to estimate the dry deposition flux of nitrogen.The mean dry deposition velocities of NO₃^- and NH₄⁺ were estimated to be 1.15 cm/s and 1.23 cm/s,respectively.For observed nitrogen deposition in the aerosol,N-NH₄⁺ deposition contributed about 70%.The air mass back-trajectory analysis showed the coastal sea region has been seriously influenced by terrestrially derived materials carried on the westerly terrestrial airflow.The meteorological conditions and pollutants concentrations have been simulated by the middle scale meteorological model(MM5)and chemical transport model (CMAQ).The atmospheric circulation and synoptic system in the winter and the autumn were in favor of the transport of nitrogen compounds from the terrestrial to East China Sea.As the primary pollutants,the peak transport fluxes for NO_x and NH₃ occurred in the Yangzte delta and the north part of East China corresponding to their source region.However,that is different for the secondary pollutants of HNO₃,NO₃^-and NH₄⁺,transport fluxes of which were more evenly distributed in the model domain.Lateral transport in different section planes showed the variation with fetch, which is larger over fetches of up between 50 km and 100 km.And the transport flux in July could reach up to higher vertical layer,1000 m higher than that in January. There also were obvious seasonal and spatial variations in the concentration of atmospheric nitrogen compounds.In January,NO_x,NH₃,NO₃^- and NH₄⁺ in the north part of East China Sea reached higher concentrations of 15-100nmol/m³,60-500 nmol/m³,80 nmol/m³ and 100 nmol/m³,respectively.And(NH₄)₂SO₄ were found to be suitable for transport to more remote sea area than NH₄NO₃.The impacts of the sea roughness length on the dry deposition velocities have been studies.The dry deposition velocities of NH₃ and HNO₃ have been influenced in the most degree than the other nitrogen compounds.Totally,the sea roughness length in model domain varied in 0.01 cm-0.06cm,the turbulence resistance R_a and sub-layer resistance R_b decreased above 10%and 5%,respectively.By statistics,the mean dry deposition velocities(V_d)of HNO₃ were 0.51 cm/s in January,0.58 in April,0.65 cm/s in July and 0.79 cm/s in October.The V_d for NH₃ was very close to the one of HNO₃.By comparison with the other studies and the measure result in Yangshan isel, the dry deposition velocity estimated considering dynamic sea roughness length was proved to be more precise.The atmospheric nitrogen loads to the East China sea have been estimated by modified regional model.The nitrogen deposition flux was the largest in January due to higher transport fluxes and concentrations of atmospheric nitrogen compounds.Dry deposition accounted for 20%of the total deposition in the full ocean area.However, there were regional gradient in the ratios of the dry to wet deposition.The dry deposition is 2.5 times as high as the wet deposition.Pollutants from the Yangzte river delta source region have contributed 44%of the total deposition roughly,which is comparable with that contribution by the region outside East China.The atmospheric nitrogen inputs into the China sea were shown to be comparable in size to the other terrestrial nitrogen inputs such as the riverine transport etc.Averagely,the deposition of atmospheric nitrogen has the potential to promote primary productivity of 100-200 m mol C/m²/year for the East China Sea.