关键词: 连续监测, 可凝结颗粒物, 前体物, 固定污染源

Abstract: Significance　Condensable Particulate Matter (CPM) in the flue gas of stationary pollution sources may be the major component of primary particulate matter. However, current control measures, such as ultra-low emission control for stationary pollution sources, have limited effectiveness in mitigating CPM. As a result, CPM has gradually become a key contributor to excessive atmospheric fine particulate matter (PM2.5). The main precursors of CPM in flue gas include HCl, NH3, SO3, and volatile organic compounds (VOCs) such as alkanes and esters. To comprehensively and deeply explore the formation process and key influencing factors of CPM formation from stationary pollution sources, and clarify the emission status and characteristics of the aforementioned precursors, thus to support the effective control of CPM and its precursors, there is an urgent need to carry out continuous monitoring of HCl, NH3, SO3, and VOCs in the flue gas of stationary pollution sources. For this purpose, it is necessary to review the current status of continuous monitoring technologies for these CPM precursors, summarize monitoring difficulties, explore key quality control methods, and promote the establishment of a systematic, accurate, and reliable monitoring technology system to support CPM-related research and management. Progress　Continuous monitoring methods for CPM precursors from stationary pollution sources include in-situ monitoring and extractive monitoring. In-situ monitoring can avoid chemical reactions and loss of target compounds that often occur in extraction monitoring methods, but it is easily affected by dust and vibration in flue gas, and its calibration and maintenance are also challenging. The core challenge of extractive monitoring lies in minimizing the adsorption, reaction, and loss of target components in the transmission pipeline. This can be addressed by optimizing pipeline design, selecting appropriate pipe materials, adding filters to remove dust, increasing heating temperature, or adopting Nafion tube dehumidification. Previous studies have shown that the heating temperature of pipelines is crucial for monitoring accuracy in extractive monitoring: it needs to be above 120 ℃ for VOCs monitoring, above 180 ℃ for HCl and NH3 monitoring, and above 280 ℃ for SO3 monitoring. Optical methods such as tunable diode laser absorption spectroscopy (TDLAS) and Fourier transform infrared spectroscopy (FTIR), as well as chemiluminescence, online isopropanol absorption method, and gas chromatography, are also widely used for continuous monitoring of CPM precursors. Among them, TDLAS and FTIR are suitable for monitoring HCl, NH3, and SO3. TDLAS has high selectivity and sensitivity, but it requires to compensate for the effects of flue gas temperature. FTIR can simultaneously determine multiple components, but it is susceptible to crossinterference and the influence of moisture in flue gas. Dilution sampling-chemiluminescence can be used for NH3 monitoring, but its results are susceptible to various factors, and its sensitivity may be insuffient to meet the requirements of ammonia slip monitoring. The online isopropanol absorption method is applicable for SO3 monitoring with a response time of approximately 8 minutes. Besides, continuous monitoring equipment for SO3 is mainly calibrated using SO3 standard generators, and the accuracy of measurement results depends on the reliability of the generators. Gas chromatography coupled with flame ionization detector (FID) or mass spectrometry (MS) can be used for VOCs monitoring. Considering that some volatile organic compounds which can serve as CPM precursors are present at low emission concentrations in stationary sources, attention should be paid to whether the detection limit of the equipment meets the requirements prior to monitoring. Conclusions and Prospects　To ensure the accuracy and reliability of monitoring results, the following quality control points should be emphasized in the continuous monitoring of HCl, NH3, SO3, and VOCs from stationary pollution sources: (1) For the extractive method, optimize pipeline design to reduce component loss, and install a filtering device at the sampling probe to eliminate the influence of fly ash; (2) Select heating temperature according to the target components to avoid water vapor interference; (3) Eliminate the influence of interfering components through spectral line selection and algorithm optimization; (4) When monitoring based on methods such as TDLAS, it is necessary to correct the effects of flue gas temperature and pressure; (5) When monitoring VOCs by gas chromatography, it is necessary to verify whether the equipment sensitivity meets the standards and perform zero-point checks to reduce blank values. Future recommendations include: continuous monitoring of CPM precursors from stationary pollution sources; conducting in-depth research on the quality control methods during the monitoring process; comparing the monitoring results of different principles and equipment to ensure consistency and comparability; and formulating monitoring standards related to continuous monitoring of HCl, NH3, SO3, and VOCs.

Key words: continuous monitoring, condensable particulate matter, precursor, stationary source