贵州省遵义市降水中低分子有机酸对自由酸度的贡献

Contributions of low-molecular-weight organic acids to free acidity in precipitation in Zunyi City,Guizhou Province

贵州省遵义市是我国酸雨高发地区,研究遵义降水中有机酸对自由酸度的贡献是认识遵义市有机酸地球化学循环和酸雨机制的重要内容。在遵义市中心城区进行了一年的降水采集,并利用离子色谱法和原子吸收光谱法对主要阴离子(包括无机和有机阴离子)和阳离子进行了测定。结果显示,年均pH值为4.11(范围3.12～6.04),pH<5.6的样品占93.1%,说明遵义市酸雨污染仍然严重;单因素方差分析显示四季的pH值存在显著差异(P<0.05),表明遵义市降水酸化程度具有显著的季节性特征;甲酸和乙酸的雨量加权平均值分别为9.29μmol/L和6.47μmol/L,检出率分别为98.68%和93.42%,表明有机酸在降水中广泛存在;应用热力学理论计算了有机酸对降水自由酸度的贡献量,发现在pH<5的遵义降水中甲酸和乙酸对自由酸度的平均贡献量分别为14.79%和3.66%,表明有机酸对降水自由酸度具有重要贡献;对比显示,遵义市的有机酸贡献量低于边远地区,这与遵义市降水的低pH值和高无机酸度有关;四季的有机酸贡献量大小为:春季(31.95%)>夏季(26.16%)>冬季(11.17%)>秋季(8.02%),可见有机酸对自由酸度的贡献量存在明显的季节性变化。在低酸度和低水温的春季降水中,由于人类活动或有机物质的燃烧所释放的大量有机酸(如汽车尾气的排放、燃煤的燃烧),促使有机酸对降水自由酸度的贡献量最高;其次是在低酸度和强降雨的夏季;而在低酸度的秋季和冬季,有机酸对酸度的贡献量相对最低。

As a city with severe acid precipitation, Zunyi in north Guizhou Province, provides a typical background for understanding organic acidity in a heavily polluted region. Studies on the contributions of organic acid to free acidity of precipitation in Zunyi help shed light on the geochemical cycles of organic acids, as well as the formation of acid precipitation. We collected precipitation samples in the center of Zunyi City for a whole year and measured inorganic and organic anions by ion chromatograph and cations by atomic absorption spectroscopy. The annual average pH was 4. 11, with 93.1% of all the samples being acidic （pH 〈 5.6）, indicating the acid atmosphere. One-way analysis of variance showed pH was significantly different among seasons. This result suggests seasonal variation in environmental acidification. The volume-weighted average concentrations of formic and acetic acids were 9.29 umol/L and 6. 47 umol/L respectively, with occurrence percentage of 98.68% and 93.42%. This suggests formic and acetic acids are ubiquitous in precipitation. Based on the thermodynamic equilibrium calculation method, the contributions of formic and acetic acids to free acidity in samples with pH≤ 5 were 14.79% and 3.66%, respectively. Organic contributions in seasons followed in the order： Spring （31.95%） 〉 Summer （26. 16% ） 〉 Winter （11.17%） 〉 Autumn （8.02%）, a clear indicator of their importance to precipitation acidity. The organic contribution in Zunyi is lower than that in remote regions, which is attributed to low pH value and high inorganic acidity. Furthermore, organic contributions showed seasonal variations. The highest organic contribution in Spring is influenced by abundant organic acids, low temperature and high pH value. The second was that in Summer due to the high pH value and intensive rainfall. The contributions in Autumn and Winter were the lowest because of the low pH in precipitation.