Low molecular weight (LMW) organic acids are important and ubiquitous chemical constituents in the atmosphere.A comprehensive study of the chemical composition of precipitation was carried out from June 2007 to June 2008 at a rural site in Anshun,in the west of Guizhou Province,China.During this period,118 rainwater samples were collected and the main LMW carboxylic acids were determined using ion chromatography.The average pH of rainwater was 4.89 which is a typical acidic value.The most abundant carboxylic acids were formic acid (volume weight mean concentration:8.77 μmol L-1) and acetic acid (6.90 μmol L-1),followed by oxalic acid (2.05 μmol L-1).The seasonal variation of concentrations and wet deposition fluxes of organic acids indicated that direct vegetation emissions were the main sources of the organic acids.Highest concentrations were observed in winter and were ascribed to the low winter rainfall and the contribution of other air pollution sources northeast of the study area.The ratio of formic and acetic acids in the precipitation ([F/A] T) was proposed as an indicator of pollution source.This suggested that the pollution resulted from direct emissions from natural or anthropogenic sources.Comparison with acid precipitation in other urban and rural areas in Guizhou showed that there was a decreasing contribution of LMW organic acids to free acidity and all anions in rainwater from urban to remote rural areas.Consequently,it is necessary to control emissions of organic acids to reduce the frequency of acid rain,especially in rural and remote areas.
采用离子色谱法测定了贵阳市区大气降水中甲酸和甲醛含量并分析了其浓度的变化规律及其来源.结果表明,两者的年体积平均浓度(volume mean weight average,VMWA)分别为(13.27±25.92)μmol/L和(7.89±8.92)μmol/L,并且夏半年甲醛和甲酸浓度低于冬半年的浓度,这可能与该地区冬季干燥少雨、降雨的同时易伴随干沉降有关.甲醛和甲酸的昼夜变化(白天>夜晚)则反映了人为活动和大气光化学反应对其浓度的影响.相关性分析发现,甲醛和甲酸是在云下(below-cloud)过程进入大气降水的,随降雨的进行,雨水中甲酸和甲醛浓度逐渐减小,降雨起到稀释作用.同时,雨水中甲酸和甲醛存在显著相关关系(r=0.68,p<0.001)说明两者存在类似的来源或反应.甲醛液相氧化反应生成的甲酸不能很好地解释所检测到甲酸数据,说明甲醛氧化反应不是贵阳地区甲酸的主要来源,植物或人类活动产生的不饱和碳氢化合物的氧化可能是甲醛和甲酸的主要来源.
建立了一种利用Dionex ICS-90型离子色谱仪同时测定雪冰中低分子有机酸和无机阴离子的新方法.采用Ion Pac AS11-HC 4mm高容量阴离子分离柱,进样量为800μl,RFC-30型试剂控制器控制EGC淋洗液发生器在线产生不同浓度的KOH淋洗液作梯度淋洗,在31min内可同时分离检测出F-、CH3CH(OH)COO-、CH3COO-、HCOO-、Cl-、NO2-、NO3-、SO42-、(COO)22-、PO43-等十余种组分.对低分子有机酸的检测限为0.7-22.8 ng g-1,对无机阴离子的检测限为0.2-22.4 ng g-1.应用此方法对天山雪冰样品中低分子有机酸和无机阴离子进行了测定.
