Study of Elemental Mass Size Distributions of Aerosol in Lijiang, a Background Site in Southwest China

Elemental mass size distributions of atmospheric aerosols over Mount Yulongxue, Lijiang, Yunnan Province, China were monitored by an 8-stage cascade sampler and analyzed using PIXE method. The results show that crustal elements such as Mg, Al, Ca, Si, K, and Fe are the main compositions of atmospheric aerosols over the Mount Yulongxue Region and account for more than 82% of a total of 20 elements determined. The concentration of soil dust in Lijiang is very low with a value of 3.22 μg/m. Concentration of Pb, Se, and Br in atmospheric aerosol in Lijiang is much higher than that over Mount Qomolangma Region. High enrichment factors of S, As, Se, Br, and Pb in atmospheric aerosol in Lijiang were observed. These results indicate that the impact of human activity in Lijiang can’t be ignored.


INTRODUCTION
Lijiang, located in northwest Yunnan Province, is one of the famous historic and cultural cities in China.This region is where remnants of the ancient Dongba culture are preserved (Li et al., 2001) Human activity in the Lijiang area is relatively light, but the ecosystem is fragile.Recently, it has been observed that the snow-covered area has decreased rapidly, the snow line is higher and the glacier has been retreating (He and Zhang, 2004).These changes have been attributed to global climate change as well as increased human activity associated with the development of tourism in the area (He and Zhang, 2004). Very

Observation site
The observation site located at the top of a two-floor building of a power transformer The air flow of the sampler was measured in the beginning and at the end of the sampling.
The average of the flow data was set as the sample flow rate.

Element analysis
The aerosol samples collected in Lijiang were analyzed by a Particle Induced X-Ray

Emission (PIXE) technique at the Institute of
Low Energy Nuclear Physics, Beijing Normal University.The PIXE analyses were carried out using 2.5 MeV proton bombardments with a beam of 30-40 nA.The concentrations of 20 elements-Mg, Al, Si, P, S, Cl, K, Ca, Ti, V, Cr, Mn, Fe, Ni, Cu, Zn, As, Se, Br, and Pb-were determined for each sample (Zhu and Wang, 2000;Zhang et al., 2001Zhang et al., , 2003)).

Elemental concentration
Thirty-two samples were obtained during this study.The mean value averaged from the samples was set as the background concentration value in the Lijiang area.Table 1 presents the distribution of aerosol element mass concentration with aerosol size for the 20 listed elements.The total concentration of the 20 elements observed was 1.99 µg/m 3 , which is lower than that of 2.87 µg/m 3 over Mount Qomolangma (Zhang et al., 2001).The total mass concentration in Lijiang was exactly the same value as that observed in the summer of 1989 over Wudaoliang area, Qingzang plateau(35°17′N, 93°36′E, 4650 m ASL) (Yang et al., 1994)     The mass concentration of soil dust can be estimated by summing up the concentration of several elements predominantly associated with soil plus oxygen, assuming the compounds involved are the most common oxides (Malm et al., 1994).The formula to calculate the soil dust concentration is given as:

Size distributions of elements
C soil = 2.2C Al + 2.49C Si + 1.63C Ca + 2.42C Fe   was low and its size distribution showed a double peak mode.

Element enrichment analysis
An element enrichment analysis was used to identify the origins of elements from natural or anthropogenic sources.The enrichment factor is defined as (Winchester et al., 1981;Liu et al., 2006): few studies on the atmospheric environment in the Lijiang area have been published: Guo et al. (2004) investigated climate change and Liu and Hong (1996) analyzed the chemical composition of precipitation.Studies on the chemical composition of atmospheric aerosol in Lijiang have not been performed.Observation and analysis of changes in atmospheric chemical components could help evaluate the impact of human activities, so that the environment can be protected.Some atmospheric observation over mountain Yulongxushan has been performed with the support of the public welfare project "The Integrated Impact of the Asian Brown Cloud over China and Corresponding Strategy" by the China Ministry of Science and Technology, The current study presents the observational results on the aerosol chemical composition.The purpose of this study was to observe and analyze the background aerosol chemical composition over the Yunan-Guizhou Plateau in southwest China, and to assess potential impacts of human activity.

Fig. 1
Fig. 1 illustrates the aerosol size distribution of total concentration for the 20 elements in Lijiang and Mount Qomolangma.It shows that the difference in concentration between Lijiang

Fig. 1 .Fig. 2 .
Fig. 1.Aerosol size distribution of total concentration for the 20 aerosol elements in Lijiang and Mount Qomolangma.

Fig. 3 .
Fig. 3. Size distributions of elemental concentrations of Al, S, Se, and Pb.

Fig. 3
Fig. 3 shows the size distributions of elemental concentrations of Al, S, Se, and Pb: the distribution of Al was mainly in fine mode; while most S and Se mass were in fine mode as well, reflecting a possible coal-burning source.The concentration of Pb EF = (C element / C refercence ) air / (C element / C refercence ) crust (2) where C element is the concentration of any element, C refercence is the concentration of reference element.Generally, Al, Fe, or Si are chosen as reference elements, and in the case of this study, Si was chosen.The crustal concentrations were obtained from Winchester et al. (1981).

Table 1 .
Distribution of aerosol element composition with the aerosol size in Lijiang (ng/m 3 ).

Table 2 .
Ratios of aerosol elemental concentrations in Lijiang to Beijing.

Table 3 .
Enrichment Factors of aerosols in Lijiang.

Table 3
presents the element enrichment factors in Lijiang.Enrichment factors of crustal elements, such as Al, Ca, Fe, and Ti, were not very large; however, the element enrichment factors of S, As, Se, Br, and Pb, which are closely related with human activities, were very high.The aerosol element S in the