城市与区域生态国家重点实验室知识库

      由自然源和人为源等活动排放到大气中的挥发性有机物（VOCs）对大气环境中起着十分重要的作用，这些VOCs 在大气中发生降解反应生成一系列复杂的二次产物，如氧化剂过氧乙酰硝酸酯、臭氧及二次气溶胶等。研究实际大气环境中具有不同反应活性VOCs 物种的浓度水平及主要来源对控制大气环境的二次污染具有非常重要的作用。为此，本文研发出一套无液氮-多毛细管色谱柱联用色谱分离系统在线分析VOCs 的方法，对比分析了我国北方地区冬季居民使用新型煤炉和传统煤炉在取暖做饭等燃烧过程排放VOCs 的特征，研究了华北地区夏季大气中VOCs 的污染水平、特征及来源，并进一步分析了华北农村地区及城市地区大气臭氧生成敏感性，取得以下主要研究成果：
      1、研发出一种无液氮-多根色谱柱联用的气相色谱分离系统在线对C2-C12 碳氢化合物进行分析，该系统主要包括低温制冷单元（双冷头制冷）、采样单元（低温富集）和色谱柱分离（三色谱柱）单元。其中，制冷单元提供冷源可以满足低温采样和低温分离的温度要求。采样单元包括采样泵、Nafion 除水管、采样流量计和不锈钢富集管。分离单元包括一个与制冷机相连的小型柱箱，柱箱内缠绕毛细管色谱柱。OV-1 和PLOT 柱分离单元联用后通过设定的程序进行升温操作可使目标化合物达到良好的分离效果。所建方法色谱工作站获得的峰面积和采样体积具有很好的线性相关性（相对标准偏差小于5%）。
      2、 研究了居民广泛使用的一种传统煤炉与新型共轭煤炉燃烧时排放VOCs 的特征。结果表明，烟气中VOCs 的浓度水平在传统煤炉加煤后迅速抬升，并在加煤后的几分钟达到峰值水平50ppm，而在新型共轭煤炉燃烧过程中，仅在初始瞬间出现了较低浓度的排放峰值。在燃烧过程排放的VOCs 中烷烃所占比重最大 (46.57-48.13%)，随之依次为烯烃、苯系物、乙炔及支链烷烃。传统煤炉得到的TVOCs 的排放因子比共轭煤炉燃烧VOCs排放高一个数量级，表明新型共轭煤炉可以极大减少VOCs 的排放。
      3、研究了夏季华北农村地区大气中NMHCs 的污染水平、变化特征和臭氧生成敏感性：观测期间，河北保定农村地区臭氧前体物TNMHCs 和NOx 浓度的最大值分别为52.71ppbv、174.99ppbv ；VOCs 组分比例的研究结果表明河北保定农村地区大气VOCs 中烷烃体积分数最高（57.72%），芳香烃（17.67%）及烯烃（15.74%）次之，炔烃最低；乙烷为河北保定农村地区大气NMHCs中浓度最高的物种；臭氧污染日大气臭氧浓度及TNMHCs/NOx 比值具有显著的午后峰值，且白天TNMHCs/NOx 比值基本都大于8，最高可达16，表明河北保定农村地区臭氧的生成处于NOx 敏感区；大气OH 自由基的消耗速率在TNMHCs 低于20ppbv 时主要以异戊二烯的贡献为主，且异戊二烯对OH 自由基消耗速率的贡献比例随着TNMHCs 浓度的增加显著减少，烯烃、烷烃和苯系物的贡献则随TNMHCs 浓度的增加呈现上升趋势；河北保定农村地区大气NMHCs 五种组分对OFP 贡献的顺序为苯系物>烯烃>烷烃>异戊二烯>炔烃。
      4、研究了夏季北京大兴区大气中VOCs 的污染水平、变化特征和臭氧生成敏感性：观测期间平均浓度水平最高的前五种VOCs 为氯代二溴甲烷、乙烷、壬烷、三氯甲烷和甲苯；VOCs 六大组分质量浓度顺序：卤代烃> 烷烃> 苯系物> OVOCs> 烯烃> 炔烃；基于特征比值法和EKMA 曲线法分析，认为大兴区观测期间臭氧以VOCs 控制为主；北京夏季大兴区大气中VOCs 的主要来源为溶剂挥发（占总来源的32.1%）和柴油车尾气排放（占总来源的20.7%）；柴油车尾气、植物排放、喷涂和印刷、溶剂挥发和汽油车排放的VOCs 对臭氧生成势的贡献分别为26.9  4.1%、24.4  8.5%、19.5  6.5%、16.7  2.5%和12.6、1.5%。
 

      Volatile organic compounds (VOCs) emitted from both biogenic and anthropogenic sources play very important roles in the tropospheric chemistry. In the atmosphere, their degradation can form a series of harmful secondary pollutants. To develop effective control measures for the secondary pollutants, the concentrations and sources of VOCs is needed to study. In this study, we developed a Liquid Nitrogen-free/ capillary columns system to research C2-C12 NMHCs in the atmosphere. In addition, volatile organic compounds emitted from conjugated-domestic coal combustion and the atmospheric VOCs in North China Plain during the summertime were sampled to thoroughly investigate their characterestics; and also the sensitive chemistry of ozone formation in city and rural area of North China Plain during the summertime were studied.
      1. A LN2-free GC-FID system (equipped with three capillary columns) has been developed for measuring atmospheric C2-C12 hydrocarbons. The LN2-free GC-FID system is consisted of a sampling unit (3mm I.D. stainless tube), a cooling unit (two cooler) and three separation units. The cooling unit which included two cooler head is used to meet the low temperature needs of the sampling unit (SS tube) and the separation unit (capillary column). The sampling unit includes a pump, a Nafion-dehydration tube, a mass flower controller (MFC) and a stainless tube. The separation unit attached on the cooling column, and OV-1 column (30 m × 0.32 mm I.D.) and PLOT column (10m× 0.32 mm I.D.)  were used to separate the C2-C12 hydrocarbons. The program temperature of the OV-1 amd PLOT columns were achieved to separate C2-C12 NMHCs. The signals of hydrocarbons obtained by VOCs analyzer and the enrichment amount of hydrocarbons had good linear correlations.
      2. Volatile organic compounds (VOCs) emitted from a prevailing domestic stove and new conjugated domestic stove were studied. In the traditional stove, the concentrations of the VOCs quickly increased after the coal loading and achieved their peak values in a few minutes. Whereas, only low concentrations of VOCs were found at the beginning of the burning processes in the conjugated dometstic stove. Alkanes (46.57-48.13%) accounted for the largest proportion under the combustion, followed by aromatics, alkenes, acetylene and branched alkanes. The emission factors of the total VOCs from the traditional domestic stove were nearly one magnitude greater than those from conjugated domestic stove.
      3. Atmospheric non-methane hydrocarbon compounds (NMHCs) were measured at sampling site in rural area of North Plain to recognize their variation characteristics, pollution levels and the O3 formation sensitivity. The proportions of alkanes, alkenes, aromatics and acetylene to the total NMHCs were 57.72%, 15.74%, 17.67% and 8.87%, respectively. Ethane were the most abundant species. The peak concentrations of ozone and TNMHCs/NOx ratio at the rural site in diurnal profiles both appeared in the period from 11:00 am to 15:00 pm. The average TNMHCs/NOx ratio was greater than 8 during the daytime and thus proved that the ozone generation in this period was limited by NOx. Isoprene was the largest contributor with the TNMHCs < 20 ppbv, and the contribution of isoprene gradually decreased to approximately 10% under TNMHCs > 30 ppbv, whereas the change of the contributions of alkanes, alkenes and aromatics showed the opposite trend. The order of the contribution of NMHC groups to the OFP was aromatics > alkenes > alkanes > isoprene > acetylene.
      4. Atmospheric volatile organic compounds (VOCs) were measured at sampling sites in Daxing district of Beijing to recognize their pollution levels, variation characteristics and the O3 formation sensitivity. Dibromochloromethane, ethane,nonane, chloroform and toluene were found to be the top five VOCs during the summertime. The proportions order of VOCs was: halohydrocarbons > alkanes >aromatics > ovocs > alkenes > acetylene. Based on the results of the typical ratio and EKMA, the ozone generation in this cite was limited by VOCs. For the atmospheric VOCs in Beijing during the summertime, five emission sources were obtained by positive matrix factorization (PMF), including gasoline exhaust, biogenic, solvent evaportaion, desiel exhaust and painting and printing. Solvent evaportaion and desiel exhaust were found to make the greatest contribution (20.7~32.1%) to atmospheric NMHCs. The propotions of the five emission sources identified by PMF to O3 formation were 26.9  4.1%(diesel exhaust),24.4  8.5% (biogenic), 19.5  6.5% (painting and printing), 16.7  2.5% (solvent evaporation) and 12.6  1.5% (gasoline exhaust), respectively.