A Preliminary Study Of Influences Of Dust Events On Chemical Properties Of Precipitation And TSP Pollution

Dust event is a disastrous weather with much attention, which happens frequently in the arid, semiarid regions and its surrounding areas. The dust events not only cause serious direct destroy but also bring lagged, persistent and long-term harmful effects on climate, environment, ecosystem and human health. Whereas the indirect effects are still uncertain, which would be more serious. So it is important to investigate and research the environmental effects of the dust events.The acid-rain data from 1993 to 2001 of 87 stations over China and the dust-event data of 88 northwest stations are collected and systemicly analyzed in this paper. Additionally the rainwater and TSP samples are collected continuously in Lanzhou University and analyzed in the laboratory from April 2004. The effect of dust events on the chemical properties of precipitation and the air quality has been statistically analyzed and investigated in this paper. Some new and valuable results are gained as follow:(1) There is marked regional difference in the distribution of acid rain in China. The value of pH is less than 5.6 in the south of the Qinling Mountains, the south of the Huaihe River and the east of Qinghai-Tibet Plateau, as it gradually decreases from the north to the south. The zone of acid rain is extending in the north of China. In the period from 1993 to 2001, the acid rain was the most serious from 1993 to 1996, moderate from 1997 to 1999, but it increased again in 2000. The change is close to the emission of atmospheric pollutant. In the south, the zone of acid rain is larger in summer and autumn than in winter and spring, but the acidification degree is more serious in winter and spring. In the north, the pH of precipitation is higher in winter and spring than in summer and autumn. The seasonal changes of pH are different in arid regions, semi-arid regions, semi-humid regions and humid regions. The marked changes occur in the semi-arid regions and the humid regions. In the humid regions, the minimum of pH occurs in winter and spring, and the maximum in summer and autumn. As the opposite, the maximum of pH occurs in winter and spring, the minimum in summer and autumn, in the semi-arid regions. Moreover, of the value of pH remarkably rise in spring in the semi-arid regions. The changes are relative to the climate conditions and the chemical properties of soil in the regions. Especially in semi-arid regions, the dust events have significant effects on the increase of the precipitations pH in spring.(2) The average pH of precipitation is markedly higher in the dust events than others in Lanzhou, Xifeng, Geermu and Minqin City where dust events are frequent in spring. For example, the average pH of all 82 rainwater samples is 6.18 in Lanzhou City, which the average pH of 54 dust samples, 6.53, is higher than the average pH of other 28 no-dust samples, 6.00. The difference between two average pH has passed two-tailed significant test at 0.01 level which indicates that the difference is great. It can be concluded that the dust events increase the pH and restrain the acidification of precipitation in Lanzhou City, which is the same in other cities.(3) The chemical composition of precipitation can represent the atmospheric environment. 106 rainwater samples were collected from April 2004 to May 2005 in Lanzhou City which is the key region influenced by the continent dust events. The samples have been analyzed for major ions (Ca²⁺, HCO₃^-, SO₄^2-, NH₄⁺, Mg²⁺, Cl^-, Na⁺, K⁺ and NO₃^-) and pH. The rainwater is alkalescent. The equal concentration of Ca²⁺ is the highest, SO₄^2- is the next. The acid components in precipitation give priority to the SO₄^2- which comes from the burning of coal, which the ratio of mol concentrations between SO₄^2- and NO₃^- is 5.96. The atmospheric pollution is heavy at Lanzhou City. There are many soil particles, dust aerosols and acid pollutants in the air. In the process of precipitation, there is strong neutralization of the acidification by the soil particles and dust aerosols. The concentration of the whole ions is the highest in spring, the next in autumn and the lowest in summer. The proportion of Ca²⁺ in whole ions is the highest in spring, the next in autumn and the lowest in summer.(4) The pollution of TSP is serious in Lanzhou City. The annual average value of the mass concentration is 0.432mg/m³ which is over the national standard(III) badly. The mass concentration is the highest in spring (0.615 mg/m³), and the next in winter (0.464 mg/m³), the third in autumn (0.428 mg/m³) and the lowest in summer (0.219 mg/m³). The dust events have increased the concentration of TSP in spring. The average value of concentration of the 9 water-soluble ions (F^-,Cl^-,NO₃^-,SO₄^2-,Na⁺,NH₄⁺,K⁺,Mg²⁺ and Ca²⁺) of TSP is 80.39μg/m³, in which five ions: SO₄^2-,Ca²⁺,Na⁺,NO₃^- and Cl^- are main components which account for 90% of the concentration. There are obvious seasonal changes of the hydrophilic ions concentration, the highest in winter, the next in spring, the third in autumn and the lowest in summer. The concentration of SO₄^2- is the highest in winter, the next in spring and autumn and the lowest in summer, which shows the influence of the burning of coal for warming in winter. The concentration of Ca²⁺ is the highest in spring, the next in winter and autumn and the lowest in summer, which explaines that the dust events have increased the content of Ca²⁺ in spring. The minimum concentration of most ions appears in summer which can prove that the air quality is the best in summer. The sources of the pollutants have been deduced by analyzing the correlation coefficient and the mass concentration of the 9 ions. The sources of pollutants are relatively complex at Lanzhou City, which the anthropogenic sources and natural sources occupy great proportion. In winter, the hydrophilic ions mainly come from the burning of coal; the sources are multiform in summer and autumn; the emission of soil and dust is very high in spring.