Study On Aerosol Number Concentration, Size Distribution And The Particle Formation And Growth Process

The number concentration and size distribution of the atmospheric particle varies with different photochemiscal process,air mass characteristics and emission type.To know about the size distribution and the origination of the particles in different size can help us have a deep understanding about the effects of atmospheric aerosol on the atmospheric radiation balance,atmospheric chemical process and even climate change.The particle number concentration and size distribution in the size range of 3 nm to 10μm were measured over north China and East China region. The sites were established in the down wind direction of mega cities,urban downtown area and on the summit of mountain.In this thesis,we discussed the pollution level and size distribution characteristics of ultrafine particles and particle formation and growth process over the sites.Not only the higher number concentration,but also the higher percentages in PM_2.5counts of ultrafine particles have been observed in Shanghai and Beijing area, than in Europe and North America sites.It has been found that the number concentrations were dominated by nuclei mode particles in most of the site (Shanghai,Beijing,summer in Ji'nan).It suggested that the sites may have experienced strong particle formation process during the summer season.But for Mt. Tai,it was dominated by particles in the size range of 20-90 nm,e.g.Aitken mode particles,which inferred that the site was frequently influenced by aged air came from the boundary layer and the free troposphere.And the newly formatted particles have been grown to larger size before their getting to the mountain top site.The origination of ultrafine particles in different environment was studied based on the analysis of temporal variation of particle number concentrations in different size range.It is obviously that the ultrafine particles were brought by new particle formation and growth process in most of the site,e.g.downwind site of Shanghai and Beijing.For Ji'nan,we believe the new particle formation contributed most of the ultrafine particles in summer while the contribution from vehicle will be a little more in winter.Few nuclei mode particles were observed in Mt.Tai and the high concentration of uitrafine particles were thought to be caused by particle growth and transport form the boundary layer.Particle formation and growth can shape the particle size distribution and also can increase the number concentration of dry particles and CCN.The particle formation and growth process were also analyzed based on the observation in Shanghai,Beijing,Ji'nan and Mt.Tai.A series of parameters have been calculated to evaluate the characteristic of these events,including the particle formation rate, growth rate,the net increase of nuclei mode particles,the condensational sink and gas phase H₂SO₄ in ambient air.The grow rate of particles in Mt.Tai is 3.2 nm h^-1, and 6.0 nm h^-1in shanghai and 4.4 nmh^-1in Beijing,which is comparable with other sites world wide.The particle formation rate and number concentration net increase rate were calculated to evaluate the strength of the particle formation events in Shanghai,Beijing and Ji'nan.And they were found to be much higher than that in comparable sites of Europe or North America.This may suggest the strong particle formation events happened in the urban area of north and east of China region.Here we also studied the relationship between the source(gas phase H₂SO₄ in ambient air and SO₂ mixing ratio)and the sink(particle condensational sink)and found nearly all the sites can be characterized by high concentration of precursors with high sink. For Beijing,the new particle formation events were observed under very high condensational sink and moderate precursor,suggesting the possibility of participant of condensation vapors beside of H₂SO₄.It is interesting that the frequency of particle formation and growth events in Mt.Tai(65%)was much(2-4 times)higher than any recently reported results in world scale.Both the strong events happened in boundary layer and high frequency of the events in the upper boundary layer may present the characteristic of particles over the large north and east China region. Knowing that the strong emission of precursors of particles over the region,north and east China maybe taken as a great source of ultrafine particles and may be contribute a lot to the world climate change.The particle density of PM_2.5and PM₁₀were also calculated base on the result of particle volume concentration measured by particle counter and mass concentration measured by filter,in summer of Shanghai and Beijing respectively. And the values are 1.51g-cm^-3for PM_2.5and 1.50 g·cm^-3for PM₁₀in Beijing,and 1.71 g·cm^-3for PM_2.5and 1.66 g·cm^-3for PM₁₀in Shanghai,respectively.