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细颗粒物(PM2.5)是影响我国秋冬季空气质量的首要污染物,其化学组分包括有机物、水溶性离子、无机元素等。硫酸盐(
${\rm{SO}}_4^{2 - } $ )、硝酸盐(${\rm{NO}}_3^{ - } $ )和铵盐(${\rm{NH}}_4^{ + } $ )(合称SNA)是最主要的水溶性离子,在PM2.5中的占比超过一半[1-4]。以往水溶性离子的测量主要通过离线膜采样-离子色谱分析的方法,但该方法时间分辨率低,无法精准捕捉环境大气污染过程。为了提高颗粒物监测数据的时间分辨率,在线离子色谱仪被广泛应用于大气复合污染的研究中[5-10]。在线离子色谱仪能够同时测量颗粒物水溶性离子和气态污染物,主要利用颗粒物和气体在溶液中扩散系数和自身质量的差异而实现分离。为了保证在线离子色谱仪的准确性和可比性,已有研究开展了颗粒物膜采样/离线实验室分析和在线离子色谱仪的比对研究。Wittig等[11]发现,半连续的湿式在线离子色谱仪对
${\rm{SO}}_4^{2 - } $ 和${\rm{NO}}_3^{ - } $ 的测量偏高10%;袁超等[12]发现,在线URG-900B对${\rm{NH}}_4^{ + } $ 和${\rm{NO}}_3^{ - } $ 的监测结果较好,但对${\rm{SO}}_4^{2 - } $ 存在明显的高估;杨懂艳等[13]的研究显示,在线URG-900获得的离子总量高于滤膜测试结果,${\rm{SO}}_4^{2 - } $ 和${\rm{NH}}_4^{ + } $ 明显偏高,${\rm{NO}}_3^{ - } $ 的年均浓度差异不大。整体来看,在线离子色谱仪对颗粒物组分如${\rm{SO}}_4^{2 - } $ 和${\rm{NO}}_3^{ - } $ 均有不同程度的偏差[11-18]。然而现有研究主要集中于颗粒物组分间的比对,鲜有研究针对气态污染物开展比对工作,同时缺乏湿式旋转溶蚀器(WRD,在线离子色谱仪核心部件)对气态污染物吸收效率的研究,成为在线离子色谱仪测量研究的“盲点”。本研究利用Marga 1S和API 100E测量的2017年环境大气SO2监测数据,评估不同测量方法的差异性。同时,配制不同浓度的SO2标准气体,测量湿式旋转溶蚀器对不同浓度SO2的吸收效率,估算测量干扰,对提升在线离子色谱仪性能的认识存在积极意义,也为在线离子监测设备在大气复合污染精细化和科学化中的应用提供依据。
湿式旋转溶蚀器对二氧化硫吸收效率和测量干扰的评估
Evaluating the absorption efficiency and measurement interference of wet rotating denuder for sulfur dioxide
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摘要: 利用大气细颗粒物水溶性离子在线监测仪(Marga 1S)分别与API 100E和Thermo 43i同时测量环境空气和二氧化硫(SO2)标气,进而评估了湿式旋转溶蚀器对SO2吸收效率及其测量干扰。研究结果显示,基于API 100E和Marga 1S测得的2017年南京市环境空气SO2浓度分别为(17.1±7.7 ) μg·m-3和(9.6±5.9 ) μg·m-3,Marga 1S较API 100E低43.8%,当API 100E监测SO2浓度低于25 μg·m-3时,API 100E和Marga 1S的相对误差较大,秋、冬季Marga 1S测量结果与API 100E最为接近,夏季Marga 1S测量结果偏低;基于实验室研究发现,Marga 1S和Thermo 43i的相关系数r为0.999,相关性较好,Marga 1S的测量结果偏低,与环境空气结论一致。湿式旋转溶蚀器对SO2吸收效率为82.1%—91.7%,随着SO2浓度逐渐升高,湿式旋转溶蚀器的吸收效率逐渐升高,60 μg·m-3附近时吸收效率趋于稳定。高浓度SO2条件下,颗粒态中
${\rm{SO}}_4^{2 - } $ 残留率介于0.43%—1.34%之间,高浓度SO2对颗粒物${\rm{SO}}_4^{2 - } $ 组分监测影响较小。Abstract: Utilizing Marga 1S together with API 100E and Thermo 43i to simultaneously measure sulfur dioxide (SO2) concentration in ambient air and standard gas respectively, we evaluated the absorption efficiency of SO2 by wet rotating denuder and its measurement interference. Atmospheric SO2 concentration in Nanjing in 2017 measured by API 100E and Marga 1S were (17.1±7.7) μg·m-3 and (9.6±5.9) μg·m-3 respectively, with Marga 1S 43.8% lower than API 100E. When atmospheric SO2 concentration was below 25 μg·m-3 monitored by API 100E, Marga 1S had a large relative deviation from API 100E. Marga 1S measurenments in ambient air were closest to API 100E in autumn and winter, and lower in summer. While measuring standard gas, Marga 1S had a high correlation of 0.999 with Thermo 43i. However its measurement were still lower than Thermo 43i, as was stated by the measurements in ambient air. The absorption efficiency of wet rotating denuder for SO2 was 82.1%—91.7%, gradually.increased with SO2 concentration and tended to stabilize near 60 μg·m-3. At high SO2 concentration, the${\rm{SO}}_4^{2 - } $ residual ratio in the particulate state was 0.43%—1.34%. Therefore high SO2 concentration had little effect on the measurment of particulate${\rm{SO}}_4^{2 - } $ component.-
Key words:
- Marga 1S /
- wet rotating denuder /
- sulfur dioxide /
- absorption efficiency
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表 1 Marga和API 100E线性拟合参数
Table 1. Marga and API 100E linear fitting parameters
表 2 不同SO2浓度时
残留效率${\rm{SO}}_4^{2 - } $ Table 2.
residue efficiency at different SO2 concentrations${\rm{SO}}_4^{2 - } $ SO2浓度/ (μg·m-3) 14.4 26.8 34.2 54.4 73.8 浓度/ (μg·m-3)${\rm{SO}}_4^{2 - } $ 0.29 0.35 0.41 0.49 0.48 残留率/% 1.34 0.87 0.80 0.60 0.43 -
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