short-lived that quenching does not totally dominate (see Problem 2). As discussed in more detail later, this technique is also used to measure NO as well as the OH free radical in air.

c. Infrared Spectroscopy (IR)

Infrared spectroscopy has been applied to ambient air measurements since the mid-1950s (Stephens, 1958). Indeed, PAN was first identified in laboratory systems by its infrared absorptions and dubbed "compound X" because its identity was not known (Stephens et al., f956a, 1956b). It was subsequently measured in ambient air (Scott et al., 1957). Since then, IR has been applied in many areas and has provided unequivocal and artifact-free measurements of a number of compounds. Because of its specificity, it has often been used as a "standard" for intercomparison studies (e.g., for HN03; see later).

Other infrared absorption techniques are also used in ambient air measurements, including tunable diode laser spectroscopy (TDLS), nondispersive infrared (NDIR) spectroscopy, and matrix isolation spectroscopy. These are discussed in more detail later.

A major advantage of infrared absorption spectroscopy derives from the characteristic "fingerprints" associated with infrared-active molecules. On the other hand, interferences from common atmospheric components such as C02 and H20 are significant, so that the sensitivity and detection limits that can be obtained are useful primarily for polluted urban air situations. For atmospheric work, long optical path lengths are needed.

To obtain these, multiple-pass cells are commonly used. Such cells are also often used in UV-visible spectroscopic measurements in air, discussed in Section A.l.d.

(1) Multipass cells There are several different configurations of multipass cells in use. The most common approach is a three-mirror multiple-pass cell (Fig. 11.1a) known as a White cell after the individual who first put forth the basic design (White, f942). The light is first focused on the entrance to the cell. The beam diverges and falls on spherical mirror Ml, which reflects the image and refocuses it onto mirror M2, known as the field mirror. The diverging beam from M2 is reflected to spherical mirror M3, which, like Ml, reflects and refocuses the image at the opposite end of the cell. If the mirrors are adjusted so that this image is at the exit aperture of the cell, the light beam leaves and strikes a detector. A total of four passes of the cell has therefore been made and the effective path length for absorption is L = 4a, where a is the length of the cell. However, the mirrors may be adjusted so that the reflected image from M3 falls on mirror M2 and is again reflected to Mf at a small angle to the original input light beam, leading to another set of four passes along the length of the cell.

For example, in the spot pattern shown in Fig. 11.1b, the beam enters the cell at the gap marked "0" in the field mirror and, after multiple reflections, exits at the gap in the field mirror on the opposite side, marked "28." A total of 28 passes (or 2n + 2, where n is the


Field mirror M2


Field mirror M2


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