dc.description.abstract | Hazardous Air Pollutants (HAPs) are closely associated with public health. This issue has received much attention in recent years. In 2017, the Environmental Protection Administration (EPA) of Taiwan announced the draft of “Emission Standards for Hazardous Air Pollutants from Stationary Pollution” to control 72 HAPs. In order to immediately control the source of HAPs emissions and effectively access ambient concentrations of HAPs, the development of a method to continuously monitor HAPs in atmosphere with gas chromatography mass spectrometry (GC/MS) to provide hourly continuous data of HAPs has undergone intensive research by our laboratory funded by Taiwan EPA.
In this research, we used the thermal desorption (TD) technique for pre-concentration to couple with GC/MS. Due to the perennial high humidity on the island, water removal prior to TD-GC/MS analysis is critical. Our under-developed on-line TD-GC/MS method for HAPs is mainly based on the existing NIEA A715.15B method in terms of the requirements of quality control/assurance for target HAPs. These requirements include concentration calibration, as well as the assessment of accuracy (recovery), precision and method detection limits (MDL). Our QA results show that the relative standard deviations (RSD) range from 2.59% to 27.3% The linearity (R2) ranges 0.993 ~ 1.000. The MDL range 0.11 ~ 1.76 ppb. The accuracy (recovery) precision (relative standard deviations %) is 72.40% ~ 130.50% and 1.42% ~ 16.94%.
After the completion of QA assessment in the laboratory, a month-long field measurement was conducted from October 5 to November 7, 2019 in Taoyuan city and March 17 to April 29, 2020 in Kaohsiung city. Both were targeting industrial emissions and had detected industrial related species on several occasions. The field measurement was inter-compared with other methods of PTR-QMS and GC-FID with comparable results observed. The average concentration of all species summed during the first field measurement period was 10.0 ppb compared to 71.1 ppb for the second. The much lower level in the first period was due to the fact that measurement was conducted during the northeasterly monsoon season and the monitoring site was at the upwind of the target industrial zone. The second period observed higher levels of pollutants as well as more local emission events due to the favorable location and meteorology.
In order to ensure the credibility of the on-line TD-GC/MS data, we also conducted canister sampling at both sites. It was found that carbon tetrachloride which was banned by the Montreal Protocol, but detected by both the canister sampling and TD-GC/MS on October 28, 2019 with comparable results of 2.3 ppb vs. 2.7 ppb, respectively, which effectively validated the online TD-GC/MS method. This online method not only can provide the temporal trends and identify unknown compounds, but also capture the unexpected emission episodes.
Although the mass spectrometer has strong qualitative and quantitative capabilities, a major problem with the GC/MS method was found in the on-line mode. The rapid formation of oxides on the filament of the ion source in MS deteriorates the sensitivity over time. Based on the signals of the internal standards the sensitivity dropped 80% within the first 4-5 days. In the study, we also used chlorofluorocarbons, whose concentration are stable in the atmosphere as the "intrinsic" internal standards. The signal of CFC-11 (CCl3F) was compared with those of the originally added internal standards, and the same downward trend as those of the original internal standards also appeared, confirming that the decrease in sensitivity is owing to the MS detection. Frequent cleaning the ion source every 7-10 days was adopted as a tentative solution before a more permanent one developed by the MS industry becomes available. | en_US |