Annular Denuders and Filter Packs Designed to Measure Ambient Levels of Acidic and Basic Air Pollutants

The GC-MS data files for three field samples were also analyzed [3]. The identities of the target compounds were determined by using both GC retention times and a combination of forward and reverse spectral matching techniques with stringent matching parameters. The identification of other compounds not on the target list was based on a Finnigan search technique. The application of the pattern recognition scheme to the transformed data for the target compounds resulted in 88% correct classification. The compound identification results were 85% accurate. There were 75 different nontarget compounds identified in 120 occurrences in the three samples. The classification results agreed very well for the two class I and class 2 spectra. However, a very large number of alkanes and alkenes were incorrectly classified as chlorocompounds. Further details of this study are given in references [2], [3], and [4]. Although the research described in this article has been funded by the U.S. Environmental Protection Agency under Cooperative Agreement CR-811617 with the University of Illinois at Chicago, it has not been subjected to Agency review. The mention of commercial products does not constitute endorsement or recommendation for use.

compounds not on the target list was based on a Finnigan search technique. The application of the pattern recognition scheme to the transformed data for the target compounds resulted in 88% correct classification. The compound identification results were 85% accurate.
There were 75 different nontarget compounds identified in 120 occurrences in the three samples. The classification results agreed very well for the two class I and class 2 spectra. However, a very large number of alkanes and alkenes were incorrectly classified as chlorocompounds. Further details of this study are given in references [2], [3], and [4].
Although the research described in this article has been funded by the U.S. Environmental Protection Agency under Cooperative Agreement CR-811617 with the University of Illinois at Chicago, it has not been subjected to Agency review. The mention of commercial products does not constitute endorsement or recommendation for use.
of an annular denuder system which removes reactive gases (e.g., HNO 3 , S0 2 ) from air samples an order of magnitude more efficiently per unit length and at lower Reynolds number than open tubular denuder designs. Recently we developed an improved version of the annular denuder that incorporates several important features to minimize losses of key species during sampling and reduce the possibility of leaks in the components that join the various parts of the system.
To demonstrate the applicability of this improved design of the annular denuder, a series of field studies were conducted in Research Triangle Park, NC during the fall of 1986 and winter of 1987. An annular denuder system ( fig. 1) is composed of four components: an inlet (to remove large particles), coated denuders (to collect the acidic and basic gases), filter pack (to collect fine particles and HNO 3 that may evaporate from the filter), and a flow controller-pump assembly. In this study a new glass Teflon-coated impactor inlet was designed with a short tube extending below the impactor to prevent large particles and rain droplets from entering the impactor. With the exception of the impactor surface, the entire inlet is coated with Teflon. The impactor surface is a sintered glass disc coated with silicone grease to prevent particle bounce. At 16.7 L/min, a 50% DAE= 2 . 64 jLkm cutpoint was obtained with an impactor nozzle jet diameter of 4.0 mm (Baxter and Lane [2]).
The first two denuder tubes were coated with a 1% solution of glycerine and Na 2 CO 3 . Anions collected on the second denuder are used to correct for any particle deposition that may have occurred in the denuders during sampling. Both denuders were extracted with ion chromatography eluent and analyzed for NOf (HNO 2 ), NOj (HNO,), and (SO 2 ). The two filters in the filter pack were extracted with 20 mL deionized water and analyzed for SO4 and NOr content. For some portions of the study, a third annular denuder coated with citric acid was used to collect NH 3 . As a result of these experiments, we have demonstrated that a relatively inexpensive ($150.00) Teflon-coated impactor will quantitatively transmit acidic gases to an annular denuder.
In addition, modifications to the annular denuder itself resulted in a reliable system for field investigations. Paired samples were run to compare the new denuder assemblies and the average percent differences between sampler results with different inlet configurations for SO 2 , HNO 3 , HNO 2 . The differences were 3.4, 6.4, and 8.3%, respectively. During the second phase of the testing, an NH 3 denuder was incorporated into the assembly. The average % difference between two identical annular denuder systems for SO 2 , HNO 2 , HNO 3 , NH 3 , SO4 and NOT were 1.8, 5.5, 15, 16, 4.6 and 3.6%, respectively. Experiments by Appel et al. [3] indicated that HNO 3 was not retained in the Tefloncoated glass inlets.
In our study, intercomparison of denuder assemblies showed that ratios of HN0 3 to particle nitrate tended to decrease with decreasing ambient temperature and increasing humidity. This is qualitatively consistent with previous theoretical phase equilibrium calculations of NH 4 NO 3 .
Disclaimer. Although the research described in this article has been funded wholly by the U.S. Environmental Protection Agency, it has not been subjected to review and therefore does not necessarily reflect the views of the agency and no official endorsement should be inferred.