This project has completed the development of two methods to target selected hazoudous air pollutants (HAPs) in ambient air: 1. on-line thermal desorption mass spectrometry (on-line TD-GC/MS), 2. diffusive sampling.
The new method of on-line TD-GC/MS is derived from NIEA A715.15B which is an off-line method. By adopting the previously established experimental parameters and QA/QC requirements, the hourly filed monitoring data from on-line TD-GC/MS were compared with the caninster data based on NIEA A715.15B, as well as the real-time data of PTR-MS and PAMS to validate reliability and accuracy of the new method. .
During the monitoring period, pollution events were also effectively captured. We observed two pollution incidents. In the first field campaign, vinyl chloride and 1,2-dichloroethane exhibited similar concentration variations to suggest the emissions may come from the PVC industry upwind. In the third campaign, we also found that acetone, 2-butanone and toluene showed similar trends to suggest a common source of a plastic factory such as polyurethane (PU). These field campaigns have sussessfully demonstrated the effectiveness of using on-line TD-GC/MS to detect pollution events and facilitate tracing emission sources.
The success of the on-line TD-GC/MS technique came from constant optimization of various instrumental parameters and conditions. Over the course of optimization, constant revising operating procedures, adjusting the sample and internal standard injection volumes, dewatering and pre-concentration temperatures, the TD split ratio, the tuning method, etc., have all achived the desired improvements. However, the most remarkable one was the change of the diameter of the drawout plate from 3.0 mm to 6.0 mm which dramatically improved the data quality by 30%.
The mothod of online TD-GC/MS not only can measure target HAPs with high accuracy at sub-ppbv level, but also can identify unknown organic pollutants in the ambient air. By validating this new method with three other differnet methods, we found comparable results to credit the overall reliablility and applicability of the new on-line method.
In the diffusive sampling part, the main objective is to use diffusion sampling tubes to monitor the concentrations of 21 target compounds to obtain average concentrations over a relatively long period for the target HAPs. This studied method is largely based on the methods of U.S. EPA Method 325A/B, ASTM, ISO, as well as a number of published journal papers. The analytical technique is also based on TD-GC/MS, except that the method is used in an off-line mode.
The most challenging work in the project is to design and set up an exposure chamber to provide constant temperature, relative humidity, and concentrations for establishing the uptake rates for target compounds in different environmental conditions. We found that the uptake rates in our experiment were in high agreement with those determined by the U.S. EPA Method 325B using Carbopack X as the sampling medium. However, in our experiment we adopted two kinds of commercialized adsorption tubes, Carbopack X and Carboxen 569, and found that for some target compounds the overall performance of Carboxen 569 was even superior to that of the Carbopack X.
The correlation coefficients (r) range from 0.996 to 0.999. In terms of the method detection limits, the Carbopack X sampling tube ranges from 0.20 to 0.45 ppb for 12 target compounds, and the Carboxen 569 sampling tube ranges from 0.06 to 1.01 ppb for the other 9 target compounds. The stability of sample storage in 6oC for 7 days shows that the recoveries of the Carbopack X sampling tube are between 81.00% and 125.22% for 12 target compounds, whereas the recoveries of the Carboxen 569 sampling tube are between 73.89% and 101.14% for the other 9 target compounds.
This project has successfully completed the two methods of on-line TD-GC/MS and diffusive sampling to effectively monitor selected organic HAPs in ambient air. The on-line TD-GC/MS method allows hourly measurements to capture pollution events and facilitate source investigation, whereas the diffusive sampling provides a low cost method to better assess health risk by simulating exposure over time.