Observational Study On The Influence Of Wet Deposition Of Aerosols During Summer Precipiation At Shangdianzi Station

The wet deposition of Aerosol by precipitation is not only an important sink ofatmospheric aerosol, but also one of significant processes which influence the chemicalcharacteristics of precipitation. In reality, it can affectively change the spatial and temporaldistribution of aerosols, thus bringing about uncertainties in the estimation of radiationforcing of aerosols. It also plays an important role in the formation of cloud drops andraindrops, and changes their chemical properties. In order to study wet deposition of aerosolsnear-surface and its influence on the chemical characteristics of rainwater, a field studyprogram is developed in this paper. According to the program, a two-month continuousobservation on mass concentration and ions concentrations of aerosols near-surface, gasesconcentration as well as the pH value, electrical conductivity （EC） and ions concentrations inthe different stages of rain events can be made in Shangdianzi Regional BackgroundMonitoring Station, Beijing. On the basis of the data from observation, the variationcharacteristics of aerosol mass concentration before， after and during precipitation aregained. Combined with the ions concentration of aerosols in different size, the rough estimateon influence of aerosol near-surface on the chemical characteristics is made.According to the results, the mass concentrations of aerosol near to surface changesignificantly before and after rain events, varying from one rain event to another. It showsthat the mass concentrations of aerosol near-surface at the end of rain decrease by from-8.0%to81.4%（minus value means that the mass concentration of aerosol increase at the end ofrain）. The results also shows that the difference in the mass concentration of aerosolnear-surface between the beginning and end of rain events are the combined results of severalcontributing factors, such as air masses changes over Shangdianzi Station and precipitationscavenging progress, and that former is the more important one. Like the aerosol mass concentration variation during rain, the ions concentrations of aerosol decrease in the processof rain, but the rate of different ions varies considerably. The data obtained in two rainevents shows that the sequence of decrease ratio of different ions ranked from large to smallis that NH₄⁺>NO₃^-> SO₄^2-> K⁺>Cl^-and NO₃^-> NH₄⁺> SO₄^2->K⁺>Cl^-. The observationaldata shows that aerosol mass concentrations in different size do not change at the same rateduring the rain, either.The observational data of precipitation shows that change of pH values, EC and ionsconcentrations vary from one rain event to another. For the rain events which last for shortperiod of time, the precipitation pH values, EC and ions concentration in different rain eventschange similarly: high in the beginning, then decreasing gradually as the rain goes on. Sincethe time of rainwater sampling varies due to different precipitation rate, the pH values, EC,and ions concentrations are less likely to decrease smoothly. For the rain events which last forlong time, the chemical characteristics of cloud water during rain are likely to changesignificantly, making changes of chemical characteristics of rain water more complex. Thecomparison between the mass concentrations of aerosol near-surface during rain and pHvalues, EC and ions concentrations in rain water indicates that EC and ions concentrations inrain water are generally in positive relation with aerosol mass concentration, but pH values inrain water have in a weak relation with aerosol mass concentration. The reason why a generalpositive relashionship between ions and aerosol mass concentration is that particles collectedby raindrops will dissolute and ionize cations and inons, which increase ions concentrationsand EC. The weak relationship between aerosol mass concentrations and precipitation pHvalues is because the influence of aerosol on pH values in rain water involves a series ofphysical and chemical processes which are complicated and H⁺concentration is not aconserved quantity. In addition to aerosol near to the surface, there are other factors whichcan affect chemical characteristics of precipitation, such as the chemical properties of cloudwater, the vertical distributions of water-soluble ions and mass concentrations of below-cloudaerosols. The wet removal simulation results of aerosol show that the scavenged amount ofcoarse particles in a10m air column by rain is higher than that of fine particles. The particlecollected by rain can change the concentrations of different ions to the extent of which rangefrom10^-5to10^-1. The results also indicate that the contributions to the concentration of Ca²⁺,Mg²⁺by coarse particles are much higher than those by fine particles, and that thecontributions to NH₄⁺、 SO₄^2- by coarse particles are close to those by fine particles.The comparison of pH values and EC between Mt. Tai Meterological Station and TainanMeterological Station suggests that the pH values and EC of Mt. Tai Station are lower thanthose of Taian Station. The comparision of pH values and non-hydrogenic conductivities（NHC） between both stations when they rained at the same time indicates that the pH values and NHC of Taian Station are higher than those of Mt. Tai Station. If we take thegeographical locations of both sations into consideration, a conclusion can be safely reachedthat despite that the wet deposition of aerosol near to surface and in the atmosphericboundoury layer by rain might be very little; the influence of aersol taken into rain drops canaffect the chemical characterisitics of rain water significantly.