東沙海域大氣懸浮微粒中水溶性離子之化學特性研究

Abstract

本研究分析了2007年4月至2008年12月在南海北部的東沙島上採集的550組左右的大氣懸浮微粒雙道粒徑分離標本，這些標本經處理後以離子層析儀分析其所含的水溶性離子Na+、NH4+、K+、Mg2+、Ca2+、Cl-、NO3-和SO42-。研究的目的是要了解東沙海域大氣懸浮微粒中水溶性離子的濃度、時序變化，並鑑別這些成份的來源，了解這些成份在大氣中的作用，並估算這些成份的大氣沉降通量。 研究結果顯示東沙海域大氣懸浮微粒水溶性離子的濃度呈現明顯的季節變化，其中濃度高值主要出現在東北風盛行的春、秋、冬三季，濃度低值主要出現在西南風盛行的夏季。各水溶性離子間相關性的研究及主成份分析之結果顯示Na+、Mg2+和Cl-主要為海水來源，NH4+、nss-SO42-及大多數的K+和NO3-為汙染來源，而Ca2+及部分的NO3-為地殼來源。粒徑的分析結果顯示NH4+、nss-SO42-及K+主要分布在細顆粒中，Na+、Cl-、Mg2+、Ca2+及NO3-主要分布在粗顆粒中。大氣中Cl-/ Na+比值分析結果顯示東沙海域大氣懸浮微粒中呈現明顯的氯虧損現象，此可能與這個海域受到較多酸性污染物介入之影響有關。大氣中NH4+/nss-SO42-的莫耳濃度比值的分析結果顯示東沙海域大氣中NH4+對酸性成份(如nss-SO42-及NO3-)的中和能力在夏季較佳，在其他季節則較差。在一般情形下NH4+會優先形成一系列的硫酸銨化合物如NH4HSO4、(NH4)3H(SO4)2和(NH4)2SO4等，但只有在夏季才會有多餘的NH4+和NO3-作用形成NH4NO3之化合物。風向族群的分析結果顯示東沙海域大氣中的水溶性離子有80%左右的量是盛行的東北季風所帶來，其來源主要是來自中國大陸。通量的估算結果顯示東沙海域大氣中nss-SO42－之乾沉降通量為671mg/m2/yr，Ｎ的乾沉降通量為337 mgN/m2/yr。

About five hundred and fifty pairs of dichotomous size- fractionated aerosol samples were collected at Dongsha Atoll, in northern South China Sea, from April 2007 to December 2008. The samples were analyzed by ion chromatography for soluble ions Na+, Mg2+, K+, Ca2+, NH4+, Cl-, NO3-, and SO42-. The objectives of the study are to determine soluble ion concentrations in marine aerosols over the study area, to examine time series ion variability, to identify ion sources, to investigate the chemical processes of ions in marine atmosphere and to estimate the atmospheric deposition fluxes of ions to the study area. The results indicate that there are strong seasonal variations in soluble ion concentrations in marine aerosols over the Dongsha Atoll. The high concentrations of ions are found in northeast monsoon seasons (autumn to spring), and the low concentrations in southwest monsoon season (summer). The investigations of correction relationships between ions and the performances of principal component analysis on chemical data reveal that Na+, Mg2+ and Cl- are mainly derived from sea water sources, NH4+, nss-SO42- and the majority of K+ and NO3- are from anthropogenic sources, and Ca2+ and part of NO3- are from crustal sources. The examinations of chemical data from size-fractionated samples suggest that NH4+, nss-SO42- and K+ are predominantly associated with fine aerosols and Na+, Cl-, Mg2+, Ca2+ and NO3- are associated with coarse aerosols. The evaluation of Cl-/Na+ ratios in aerosols indicates strong chloride deficient. This suggests that the atmosphere over the study area suffer from heavy acidic ion species pollution. The calculations of molar ratio between NH4+ and nss-SO42- reveal that the capability of NH4+ to neutralize the available acidic ion species, like nss-SO42- and NO3-, is strong in summer, but weak in other seasons; and in general cases NH4+ is prior to form a series of ammonium sulfate compounds, like NH4HSO4, (NH4)3H(SO4)2 and (NH4)2SO4, and will react with NO3- to form NH4NO3 compound only when the excess NH4+ exists, this could be occurred in summer. Wind factor analyses suggest that about 80% of soluble ions in marine atmosphere over the study area are carried by prevailing northeast monsoon wind, and the main source area is mainland China. The estimated atmospheric dry deposition fluxes of nss-SO42- and N to the Dongsha area were 671 mg/m2/yr and 337 mgN/m2/yr, respectively.