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(terpene-like compounds)

" Adapted from Spicer et al. (1994a).

(terpene-like compounds)

" Adapted from Spicer et al. (1994a).

(MCP) detectors, respectively. The latter are preferred in TOF instruments since the area of detection is much larger and space charge distortions within the separated ion packets can be minimized.

In short, mass spectrometry is a powerful analytical tool that has been used successfully for a number of years at high altitudes and is now seeing increasing use in the troposphere, including at the earth's surface. A number of different approaches have been developed, including systems that are designed to measure species such as OH, NO, and HN03. They are described in more detail in the sections on measurement techniques for the individual species.

3. Filters, Denuders, Transition Flow Reactors, Mist Chambers, and Scrubbers

A variety of methods have been used to collect gases for subsequent quantification by techniques such as ion chromatography or colorimetric or other (e.g., electrochemical) analyses. These include filter methods, denuders, transition flow reactors, and scrubbers. Sampling must be carried out for sufficient periods of time to collect measurable amounts of the species of interest. From the total volume of air sampled and the amount of the analyte measured, the average concentration of the species in air over the collection period can be calculated. These techniques do not provide real-time analyses, although collection periods as short as ~0.5 h provide sufficient sample for analysis in some cases.

a. Filters

Air is drawn through a filter that consists of, or is coated with, a substance that takes up the species of interest. The filter is then extracted and the ions of interest are measured. For most species of interest, care must be taken to avoid interference from particles. For example, as discussed later, filter collection of HN03 will have a contribution from particulate nitrate if particles are not removed first. Figure 11.22 shows a typical filter pack used to measure gaseous HN03 and NH3 (Anlauf et al., f 988).

As discussed below with respect to measurement techniques for individual compounds, the filter material is optimized for the compound of interest. For example, nylon has been found to be effective for the adsorption of gaseous HN03 (Spicer, 1977). However, care must be taken that the filter does not also remove other gases simultaneously that are measured downstream of the filter. For example, SOz has been shown to be taken up by nylon filters, forming sulfate, with the extent of conversion being quite variable (e.g., Chan et

Impregnated Whatman 41 filter for NH3

Nylon for HNO3

Impregnated Whatman 41 filter for NH3

Nylon for HNO3

Teflon 1 (im pore size for particles

Teflon

FIGURE 11.22 Schematic diagram of a typical filter pack used to measure gaseous HN03, particulate matter, and gaseous NH3 (adapted from Anlauf et al., 1988).

Teflon 1 (im pore size for particles

Air flow

Teflon

FIGURE 11.22 Schematic diagram of a typical filter pack used to measure gaseous HN03, particulate matter, and gaseous NH3 (adapted from Anlauf et al., 1988).

al., 1986). In addition, changes in the characteristics of what is ostensibly the same filter material with time have been reported in the literature; for example, Cadle and Mulawa (1987) reported that nylon manufactured prior to May f984 did not retain S02 to a significant extent (< 4%), whereas that manufactured at later times retained as much as 70% of gaseous S02.

In short, the performance of filter media must be carefully assessed prior to field deployment.

b. Denuders

Denuders (also known as diffusion denuders) are based on differences in the diffusion properties of gases compared to particles. The principle is illustrated in Fig. 11.23. A laminar flow of air is pulled through a tube. The inertia of the particles carries them through the tube, while the relatively high diffusivity of gases means that they will strike the walls of the tube a number of times while passing through it. If the walls are coated with a substance that will take up a particular compound, or group of compounds, from the gas phase, then these substances are removed from the gas stream. As discussed in detail by Durham et al. (1987), the depletion of the gas as a function of distance, x, as it travels along the tube is given by the Gormley-Kennedy equation:

This holds for values of Cx/C() less than 0.819, where the gas is collected with 100% efficiency. C0 and Cx are the mass concentrations of the gas at the entrance to

11. ANALYTICAL METHODS FOR GASES ANL) PARTICLES IN THE ATMOSPHERE

, Absorptive coating s

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