贵阳市区大气降水中甲酸和甲醛含量的季节变化特征

Seasonal Variations of HCOOH and HCHO in Precipitation in Guiyang

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.

Formic acid and formaldehyde are the important and ubiquitous chemical constituents in the atmosphere. Precipitation in Guiyang, the capital city of Guizhou province, was analyzed with ICS-90 ion chromatography for their concentration changes in a year. The volume mean weight average concentration （ VMWA ） is （ 13.27 ±25.92 ） μmol/L and （ 7.89 ± 8.92 ） μmol/L for formic acid and formaldehyde, respectively. Both species demonstrated higher concentration during winter-half-year than the summer-half-year. Day-time concentration is higher than the night-time, the diurnal variations for beth chemical constituents indicate the sources from human activities and photochemical reactions. Regression analysis of concentration vs precipitation volume and deposition amount vs precipitation volume reveals that dilution and scavenging of precipitation in the below-cloud process are responsible for the changes of the concentrations. The significant correlation between formic acid and formaldehyde suggests similar sources or reaction relationship. The known aqueous oxidation of formaldehyde, however, can not be used to explain the link of the two compounds, and thus should not be the major source of formic acid in Guiyang, Instead, photochemical reactions of unsaturated hydrocarbons （natural or anthropological） were the major sources for formic acid and formaldehyde.