其他摘要 | Mercury (Hg) is a global pollutant and a toxic heavy metal that is non-essential for human beings. The cycling and transformation of mercury on the global scale is one of the hotspot fields in environmental science research at present. The behaviors of mercury in the atmosphere play a significant controlling role in global mercury biogeochemical cycles, so researching on the cycling and transformation of atmospheric mercury has been significant in the territory of mercury science. In the air, the main three forms of Hg are: gaseous elemental mercury (GEM), reactive gaseous mercury (RGM) and total particulate mercury (TPM). The atmospheric behaviors of different mercury species are obviously different due to their chemical and physical properties. When evaluating the global transport of environmentally cycled mercury, a critical data is the atmospheric distribution of mercury among various forms. China is regarded as one of the largest anthropogenic Hg emission countries over the world, and the atmospheric Hg pollution problem in metropolitan cities which are the center district of human activity may be very serious. Thus, the researches on atmospheric speciation have important theoretical meaning and application value for forecasting and assessing Hg pollution characteristics in urban areas and for understanding the atmospheric mercury cycling at local, regional and global scale.
The sampling sites were located in downtown Guiyang. Guiyang (26º11´~27º22´ N, 106º07´~107º17´ E), the capital of Guizhou Province, is located in southwestern China and the Circum-Pacific Global Mercuriferous Belt. SO2 and particulate matter emitted mainly from coal combustion, which is the main energy source in Guiyang, are the main airborne pollutants in Guiyang. In this thesis, the three main research were performed. Firstly, the methods for sampling and analysis of total gaseous mercury (TGM), GEM, RGM and TPM were developed, and the different mercury species concentrations in the air and precipitation were determined, and the atmospheric mercury dry and wet deposition fluxes were estimated in the Institute of Geochemistry, Chinese Academy of Sciences (IGCAS) from April to December, 2004. Secondly, three seasonal measurement campaigns of spring, summer and winter were carried out to determine GEM, RGM and TPM in ambient air of four study districts including a residential district, a business district, an ordinary industrial district and a tourism district in central urban area and one contrast site in rural area of Guiyang from April 2005 to January 2006. The atmospheric mercury species dry deposition fluxes were estimated, and qualitative analyses of the particulate matters were performed by a high-resolution transmission electron microscope (HR–TEM) in order to elucidate the source of TPM in winter in downtown Guiyang. Furthermore, the distribution characteristics of total mercury (THg) in surficial soil and some plants at all sampling sites were investigated to evaluate the amount of atmospheric mercury deposition and subsequent watershed contamination. Through the researches mentioned above, some main conclusions were obtained as follows:
1.Based on the systematic literature review of the overseas and domestic studies, the procedures to determine TGM and TPM in ambient air by a gold trap coupled with cold vapour atomic fluorescence spectrometry (CVAFS) and a miniaturized trap coupled with CVAFS were developed, and the synchronous collection of RGM and GEM in ambient air was performed using a KCl coated tubular denuder and a gold trap. The detection limit of each method was in the order of pg•m-3. For the first time in urban areas of China, the simultaneous measurements of GEM, RGM and TPM in ambient air were carried out.
2.The yearly average GEM, RGM and TPM concentrations in central urban Guiyang in the year of 2005 - 2006 reached up to 9.11 ng•m-3, 132.4 pg•m-3 and 1.02 ng•m-3, respectively, which were 1.5 times of those at the contrast site and significantly elevated compared to the respective global background: 1.5 ~ 2.0 ng·m-3, < 10 pg•m-3 and 1 ~ 86 pg•m-3. The seasonal, diurnal and spatial distribution patterns of GEM, RGM and TPM concentrations were observed as follows: ⑴The seasonal mean GEM concentrations were: winter > summer > spring. The high GEM concentrations in winter could be attributed to household heating using coal. The nighttime GEM concentrations were generally larger than that of daytime in spring and summer seasons, while in winter the opposite GEM diurnal distribution pattern was observed. The diurnal patterns of GEM in spring and summer seasons were mainly influenced by sources and their emission form, its chemical and physical characteristics, meteorological conditions, whereas the winter exhibited patterns that could be attributed to household heating using coal in the daytime. The yearly mean GEM concentrations were: an ordinary industrial district > a residential district > a business district > a tourism district > a rural district. ⑵The seasonal mean RGM concentrations were: spring > summer > winter. RGM concentrations were likely to be highly variable and highly related to meteorological conditions. The daytime RGM concentrations were generally larger than that of nighttime in all seasons. The diurnal patterns of RGM depended upon many factors: daytime sources, its chemical and physical characteristics, oxidant levels, and meteorological conditions. The yearly mean RGM concentrations were: a business district > an ordinary industrial district, a residential district > a tourism district, a rural district. ⑶The seasonal mean TPM concentrations were: winter > summer > spring, and The high TPM concentrations observed in winter could be attributed to the household heating using coal. The nighttime TPM concentrations were generally larger than that of daytime in all seasons. The factors of sources and their emission form, its chemical and physical characteristics, meteorological conditions impacted on the diurnal patterns of TPM. The yearly mean TPM concentrations were: an ordinary industrial district > a residential district > a business district > a rural district > a tourism district. Local emission sources contributed significantly to TPM contamination, and coal combustion in winter posed influences on the TPM concentrations at all sampling sites.
3.On the average, the atmospheric mercury apportioned in the central urban air in the year of 2005 - 2006 as follows: GEM (89.8 %) > TPM (8.8 %) > RGM (1.5 %). Measured (RGM + TPM)/TAM ratios were fewer than the contrast site of 10.0 % and greater than the global background of 1 ~ 5 % and therefore would lead to even greater amounts of deposition of mercury to the local environment. Despite both RGM and TPM constitutes a small percentage of TAM, they have chemical and physical properties which make them likely to govern a substantial part of the total deposition of atmospheric mercury.
4.The fact that most of the correlation between every two atmospheric mercury species daily average concentrations was not significant on the basis of the whole sampling periods observation in the year of 2005 - 2006 suggested the presence of a multitude of sources for these species in both the central district and the contrast district of Guiyang.
5.The mean GEM, RGM and TPM dry deposition fluxes in central urban Guiyang in the year of 2005 - 2006 reached up to 28.7 μg•m-2•yr-1, 10.4 μg•m-2•yr-1 and 160.9 μg•m-2•yr-1, respectively, which were 1.5 times of those at the contrast site. TPM governed dry deposition of atmospheric mercury to the local environment. The mean TAM dry deposition fluxes were 200.1 μg•m-2•yr-1, while its seasonal and spatial distribution patterns was observed as follows: winter > spring > summer and an ordinary industrial district > a residential district > a business district > a rural district > a tourism district.
6.The total dry and wet deposition fluxes of TAM in central urban Guiyang were estimated to be 54.7 kg•yr-1 in the year of 2005 - 2006, which only constituted 16.4 % of the annual Hg emission from coal combustion to the ambient air in 2003 which was calculated to be 334 kg. Most of atmospheric mercury still resided in the air, and was likely to be transported over very long distances from the central urban district of Guiyang.
7.We observed TGM concentrations in the ambient air of IGCAS with years from 2000 to 2006. TAM dry deposition fluxes to ecosystems of IGCAS were 16.5 ng•m-2•h-1 in 2004, higher than 12.2 ng•m-2•h-1 of wet deposition fluxes.
8.The geometric mean THg concentrations in surficial soil from the central district and the contrast district of Guiyang were 0.370 and 0.276 mg•kg -1, which were much elevated compared to the Guiyang’s background of 0.201 mg•kg -1. In Guiyang, natural emissions from soil contribute significantly to the elevated TGM concentrations in the ambient air, and soil Hg concentrations were used to scale up Hg emission from soil. So we should take strong measures against the Hg pollution in soil for protecting human health in Guiyang.
9.The mean of THg concentrations in moss from the central urban area of Guiyang was 0.248 mg•kg -1, which was about 1.5 times of that in the contrast site. THg concentrations in leaves of the central urban woody plants ranged from 0.068 to 0.181 mg•kg -1. Mercury uptake by leaves was in the order of: deciduous plants > evergreen plants. THg in both naturally growing mosses and leaves of Frimiana simplex were controlled by Hg concentrations in the ambient air, and were able to reflect the present pollution level and the spatial distribution pattern of regional atmospheric mercury. |
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