2017, Vol. 38
2. 北京市朝阳区环境保护监测站, 北京 100125
2. Beijing Chaoyang District Environmental Protection Monitoring Station, Beijing 100125, China
多氯代二苯并-对-二
多介质归趋模型模拟研究表明大气传输是二
北京作为国家经济金融的决策中心和管理中心，随着经济的高速持续增长，城市规模的不断扩大和人民生活水平的迅速提高，城市生活垃圾的产生量急剧增加.除卫生填埋外，垃圾焚烧已逐渐发展成为一种重要的垃圾处理方式.考虑到垃圾焚烧产生的二次污染物如二
生活垃圾焚烧发电厂设计垃圾处理规模为1 600 t ·d-1，安装有800 t ·d-1×2炉排炉以及烟气处理系统 (脱酸塔+活性炭喷射+布袋除尘器).以厂区 (图 1中梯形区域所示) 为中心，采用同心圆布点法力求覆盖各方位的原则，并根据周边居民分布情况，在周边6 km范围内不同方位选取6个居民聚集区 (S1、S3~S7) 和1个厂界区 (S2) 进行环境空气二
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梯形区域为厂区 图 1 环境空气采样布点示意 Fig. 1 Ambient air sampling sites around the MSWI |
由于城市大气环境中二
样品的前处理和分析方法参照文献[21]. QFF和PUF均使用加速溶剂萃取仪 (DIONEX，ASE-300) 提取，前者使用甲苯作为萃取溶剂，后者使用二氯甲烷/正己烷 (体积比为1:1) 作为萃取溶剂.抽提物浓缩后依次进行硫酸处理、多层硅胶柱和活性炭硅胶柱等净化过程，得到待测组分后使用高分辨气相色谱-高分辨质谱进行分析 (Agilent 6890N/Waters Autospec Ultimate NT，美国).样品进多层硅胶柱净化前加入5 μL净化内标 (EPA 1613 LCS，Wellington Laboratories Inc.，加拿大)；进仪器前加入5 μL进样内标 (EPA 1613 ISS，Wellington Laboratories Inc.，加拿大).有机溶剂丙酮、二氯甲烷、甲苯、正己烷、癸烷均为农残级 (J. T. Baker，美国).多层硅胶柱为日本Wako公司进口成品柱，活性炭，浓硫酸为国产优级纯，其他辅助材料如无水硫酸钠、玻璃棉等均为进口产品 (日本Wako).色谱柱为DB-5MS (60 m×0.25 mm×0.25 μm)，采用不分流进样，进样量为1μL.进样口温度280℃，载气流量1.0 mL ·min-1，仪器初始温度设定为140℃，而后遵循以下升温程序140℃ (1 min) 20℃ ·min-1 200℃ (1 min) 5℃ ·min-1 220℃ (16 min) 5℃ ·min-1 235℃ (7 min) 5℃ ·min-1 310℃ (10 min).气相色谱-质谱接口温度290℃，离子源温度280℃，离子化电流650 μA，离子加速电压8 kV，采用选择离子模式检测，分析过程中动态分辨率大于10 000.
实验过程严格遵守HJ 77-2008标准方法进行质量保证/质量控制 (QA/QC)，每一组样品 (7个) 做一个实验室全程序空白.空白加标的检出结果均低于检出限，所有样品在分析过程添加的13 C内标物各异构体的回收率均处于33%~113%之间，符合HJ 77.2-2008的质量控制要求.在数据处理过程中，低于检出限的目标污染物质量浓度以0计算.
2 结果与讨论 2.1 大气二本研究7个采样点五期监测得到的大气PCDD/Fs浓度如表 1所示，其4~8氯代PCDD/Fs同族体总浓度范围为8.9~140 pg ·m-3，各采样点季平均值为28.1~46.8 pg ·m-3，17种2, 3, 7, 8位取代的PCDD/Fs毒性当量浓度范围为0.11~1.8 pg ·m-3，季平均值为0.32~0.61 pg ·m-3.除冬季和秋季雾-霾天气监测数据外，其余各采样点监测结果基本与之前报道的城市环境空气PCDD/Fs的TEQ浓度水平相当：北京市区 (2006年0.27 pg ·m-3；2011~2012年0.25 pg ·m-3)[22, 23]、广州市区 (0.36 pg ·m-3)[24]及上海市区 (0.27 pg ·m-3)[25]，均在Lohmann等[17]提出的城市环境空气中二
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表 1 各采样点大气中PCDD/Fs总质量浓度及毒性当量浓度 Table 1 Mass and TEQ concentrations of PCDD/Fs in the samples at each sampling site |
从分布情况可知，近源采样点S2( < 1 000 m) 在各采样期二英浓度水平与其他采样点相当，各采样点整体浓度水平与距污染源距离远近没有显著相关性.风向是影响排放源二英扩散的最基本因素.本研究中，由于采样时段涵盖一年四季，北京冬季主导风向为西北方向；夏季主导风向为东南方向；春秋季多为西北风，各期采样期间气象条件：风力小于三级，风向为无持续风向天气，天气以晴或晴间多云为主 (www.bjmb.gov.cn).因此，各采样点在主导风向上、下风向的浓度差异不明显.张振全等[28]对华南某工业区垃圾焚烧设施周边环境空气中PCDD/Fs含量研究发现点源烟气的影响存在“灯下黑”的特点，即下风向最大落地浓度点位置 (距离源一般1 km左右) 附近浓度较高.本研究中S1采样点夏季位于主导风向的下风向且与污染源距离接近最大落地浓度点位置，但其在夏季浓度却略高于其他采样点.另外，冬季西北风主导风向的下风向S4采样点的空气样品中二英毒性当量略低于上风向S1采样点，进一步说明该焚烧设施烟气排放通过扩散迁移对环境空气中的二英贡献较小；冬季较高浓度的二英来源较为复杂，并非受该生活垃圾焚烧设施烟气排放的主导影响.
PCDD/Fs气-粒分配行为研究表明其主要分布在颗粒相上[22, 29]，为了解环境空气中PCDD/Fs浓度水平与大气颗粒物浓度的相关性，将各个采样点每期监测结果与该监测时段内PM10和PM2.5的平均浓度水平及各个采样点每期二
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图 2 各点位总毒性当量与采样期间PM10和PM2.5相关性 Fig. 2 Correlation between I-TEQ in the samples at each sampling site and average PM10 and PM2.5 concentrations in the atmosphere |
从表 1可见，4个采样点最高值出现在冬季，3个采样点最高值出现在秋季雾-霾天气.最低值基本出现在夏季，其次是春季和秋季晴好天气.各点位不同季节PCDD/Fs平均浓度高低顺序为冬季 (91.6 pg ·m-3)>秋季雾-霾 (70.0 pg ·m-3)>秋季晴好 (24.1 pg ·m-3)>春季 (22.0 pg ·m-3)>夏季 (13.5 pg ·m-3)；二
由于本地排放源相对稳定，所以该区域大气中二
比较该研究区域2013~2014年监测结果平均值[33]与本文报道的结果 (2014~2015年) 如图 3所示.大气中PCDD/Fs浓度水平的年际间季节分布特征相似，即采暖季浓度水平显著高于非采暖季.春夏秋三季浓度水平年际间基本持平，而冬季浓度水平有所下降.
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图 3 不同季节监测区域大气平均PCDD/Fs浓度水平年际对比 Fig. 3 Annual comparison of average atmospheric concentrations of PCDD/Fs in different seasons in the studied area |
PCDD/Fs的17种毒性异构体分布特征是重要的指纹特性之一，常用作最基本的溯源分析，可以采用单体对总质量浓度的贡献率和单体对总毒性当量浓度的贡献率进行表述.
图 4为各采样点四季二
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图 4 四季17种2, 3, 7, 8-位取代PCDD/Fs单体占总质量浓度的相对丰度平均值 Fig. 4 Average relative abundance of 2, 3, 7, 8-substituted PCDD/Fs congeners in the samples from all sampling sites in each season |
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图 5 四季17种2, 3, 7, 8-位取代PCDD/Fs单体占总毒性当量的相对丰度平均值 Fig. 5 Average relative I-TEQ abundance of 2, 3, 7, 8-substituted PCDD/Fs congeners in the samples from all sampling sites in each season |
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图 6 焚烧厂烟气2, 3, 7, 8-PCDD/Fs异构体质量浓度、毒性当量浓度及4~8氯代同族体质量浓度百分比 Fig. 6 Relative contributions of individual congener and homologue to the total mass and TEQ of PCDD/Fs |
同族体包括4~8氯代所有二
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图 7 各点位PCDD/Fs同族体分布特征谱图 Fig. 7 PCDD/Fs homologue profiles of the samples from all sampling sites |
为进一步了解焚烧设施周边环境空气中PCDD/Fs与垃圾焚烧源的关系，应用主成分分析方法 (PCA) 对典型排放源烟气和空气样品之间PCDD/Fs的17种2, 3, 7, 8-位取代二
图 8显示了以不同季节采样点样品为变量的主成分分析结果，提取2个主成分因子，因子1和因子2分别解释42.1%和21.9%的总方差.二
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采样点名称指代季节+点位 图 8 排放源与环境空气中PCDD/Fs相关性分析 Fig. 8 Correlation of PCDD/Fs profiles in emission sources and air samples |
二
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式中，IPad/ch指经呼吸二
经计算，研究区域的成人对二
需要强调的是，由于冬季采暖期大气中颗粒物浓度水平较高，因此二
(1) 生活垃圾焚烧厂周边大气中PCDD/Fs质量浓度和毒性当量浓度的变化范围分别为8.9~140 pg ·m-3和0.11~1.8 pg ·m-3，其中1, 2, 3, 4, 6, 7, 8-HpCDF和OCDD是四季环境空气中PCDD/Fs质量浓度的主要贡献单体, 2, 3, 4, 7, 8-PeCDF是总TEQ贡献最大的单体.
(2) 各采样点浓度水平在焚烧设施不同主导风向上下风向的空间分布差异性不明显且与其距离远近没有显著相关性.
(3) PCDD/Fs在大气颗粒物中所占的百分比较稳定且未呈现出明显的季节变化，表明大气颗粒物浓度水平越高则PCDD/Fs的污染状况越重.
(4) 各采样点四季环境空气PCDD/Fs的组成呈现出热源影响的特征，但与焚烧源烟气分布特征不同，且可能受交通源影响较大的采样点在APEC限行期间特征单体贡献率下降幅度更显著，与PCA分析的结论较为一致即研究区域可能主要受到无组织燃煤和机动车等典型排放源共同影响，而非该焚烧设施烟气排放的主导影响.
(5) 对焚烧厂周边区域人群的PCDD/Fs呼吸暴露剂量的估算结果表明其处于较为安全的水平，但大气颗粒物重污染季节颗粒物浓度水平不降低则二
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