This project is purposed to develop monitoring techniques for ambient organic Hazardous Air Pollutants (HAPs), i.e., 1. on-line in-field gas chromatography mass spectrometry (GC/MS), 2. diffusive sampling.
The on-line GC/MS technique is aimed to analyze a large variety of ambient organic HAPs including carbon tetrachloride. The method is derived from NIEA A715.16B employing thermal desorption (TD) coupled with GC/MS to form an on-line technique, denoted as TD-GC/MS which is capable of measuring 31 target organic HAPs along with other numerous hazardous compounds. This technique had been deployed in the field for over 45 days of continuous monitoring with hourly resolution.
For method validation multiple existing techniques, such as NIEA A715.16B and PTR-MS, were employed in the project to demonstrate the robustness of the TD-GC/MS method. The comparison showed extremely high agreement between the three techniques.
During the 45 days of continuous monitoring elevated concentrations called episodes or spikes for selected target HAPs were frequently observed. For instance, vinyl chloride and 1,2-dichloroethane were found to exhibit similar variability. Other target HAPs such as 1,3-butadiene, acrylonitrile, and styrene also showed frequent concentration spikes, though not as high as vinyl chloride. These spikes are thought to have strong linkage to the plastic related factories in the neighboring industrial parks.
Our past efforts in extending the lifetime of the ion source in the GC/MS have resulted in great success. These efforts include the test of optimal sampling volume, the quality control based on internal standards, the selection of ion source type, MS tuning recipes, the aperture of the draw out plate, etc. We found that reducing water vapor plays a critical role in extending the lifetime of the ion source and thus allows continuous monitoring for at least two weeks.
With regard to the development of diffusive sampling technique, it is based on multiple international methods, such as U.S. EPA Method 325A/B, ASTM and ISO, as well as renowned journal papers from Atmospheric Environment and Science of the Total Environment. These research papers have been reviewed and summarized in this report.
The diffusive samples are analyzed by TD-GC/MS using the mode of Total Ion Chromatogram (TIC). The selection of the sorption tubes was successfully assessed by this project to include the use of Carbopack X, Carboxen 569 and Tenax GR to cover a wide range of target HAPs as required by NIEA. The core technique of the diffusive sampling lies in the test of an exposure chamber to evaluate the uptake rates for individual target HAPs at different temperatures and humidities.
The correlation coefficients for the target HAPs ranged from 0.990 to 0.999. In terms of the method detection limits, the Carbopack X sampling ranged from 0.20 to 0.45 ppb for 12 target species. The Carboxen 569 ranged from 0.06 to 1.01 ppb for 9 target species, and Tenax GR added another five target species between 0.16 to 0.57 ppb. The test of sample storage suggests that Carpack X for the 12 target species showed recoveries between 81- 125%, Carboxen showed 74 - 101% for 9 target species. The samples should be stored at 6℃ and analyzed within a week.
These two techniques, i.e., on-line TD-GC/MS and diffusive sampling, developed in this project allow precise capture of the concentration spikes of organic HAPs, as well as their long-term exposure in a given environment, based on which more accurate and realistic risk assessment can be performed. By coupling spikes and chemical composition with local wind data, emission source investigation becomes much more straightforward. These two techniques have been successfully executed and validated over the past three years and are deemed ready to serve as compendium methods of NIEA.