| Dustfall is a major component of particulate matter (aerosols) in the atmosphere, which plays an important role in the transformation and cycling of surface material in various spheres. Dust has an important impact on the climate, terrestrial and marine ecosystems through certain mechanisms such as radioactive forcing and a wide range of processes of transporting particles that containing different elements. As an important removal mechanism of atmospheric particulate matter, dustfall can visually reflect the basic atmospheric conditions of the study area, which has important reference value for urban atmospheric pollution management. Based on this consideration, we have collected dust samples at some places from March2011to February2012to analyze the sources of dustfall in LAN Zhou (Chengguan District) through the distribution characteristics of particle size and element combined with the statistical analysis model.The granularity test results show that particles of10-100μm constitute the main part of the annual size distribution of atmospheric dust in LAN Zhou whereas silt particles of10-50μm are the major component presenting monthly stable fluctuations. With granularity parameter scatter diagram and distribution characteristics of frequency curve of dustfall, we believe that the dustfall in different sampling points has similar source of supply, reflecting the homogeneity of the atmospheric pollution in LAN Zhou City. The comparison of the particle size of dust storm and non dust storm conditions shows that particles of30-62.5μm are the major component of sand and dust storms dustfall, which may be the long-range handling by strong winds. The amount of dustfall with diameter between3.9-7.8μm and7.8-15.6μm from non dust storm weather is higher than the dust weather condition, indicating that this particle size range is the main portion of non-dust storm weather. The primary air pollution particles in LAN Zhou is the dust with diameter less then10μm, namely respirable particles, which has the same diameter range as non dust storm weather, indicating that the dustfall of non-dust storm weather better reflect the actual atmospheric particulate pollution in LAN Zhou, while the occurrence of dust events may only have a greater impact on the periods of the spring dust events. The correlation analysis of contents of different particle sizes shows that the particles less than10μm present an absorption state or the state of aggregation when carrying from sources, further it can be seen as the state of absorption or aggregation between fine particles produced by human activities and soil dust (or other natural dust), constituting a main state of the aerosols in atmospheric environment.There exists a good correlation between the modal particle size and the median grain size (average diameter) and we find that the content of dust with diameter between50-100μm is closely related to the modal size through stepwise regression analysis, indicating that the content of dustfall with diameter between50-100μm can be used as a proxy of transporting capacity of urban dust in LAN Zhou.We get two major sources of dustfall in LAN Zhou through factor analysis of granularity and enrichment factor of dustfall elements i.e., fine-grained material sources of particles less than10μm (on behalf of the background of atmospheric environment) as well as short-range handling of coarse-grained suspended natural particles. The former has the impact of human activities or emission factors, while the latter basically from the topsoil of surrounding mountains in LAN Zhou. |
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