Tunable diode laser systems for measuring trace gases in tropospheric air A discussion of their use and the sampling and calibration procedures for NO , N 0 2 , and HNO 3

Photochemical smog and acid de­ position are two environmental prob­ lems in which the nitrogen oxides, NO, NO2, and HNO3 play a central role. Measurements of these constituents in the troposphere are therefore essential for understanding the chemistry of these problems. N O , emitted mainly from automo­ bile exhausts and thermal power plants, is the major anthropogenic source of the nitrogen oxides. It is ox­ idized in tropospheric air to NO2 by reactions with O3, HO2, and organic peroxy radicals. NO2 is an essential ingredient in the formation of photooxidants. It is rapidly photolyzed by solar radiation at wavelengths less than 420 nm to produce atomic oxygen, which combines rapidly with oxygen to form ozone. N 0 2 is removed mainly by reaction with H O radicals to form HNO3. It also reacts with organic radicals to form strong oxidants and irritants such as peroxy-acetyl nitrate (PAN). HNO3, the major sink for the nitrogen oxides is, in turn, removed mainly by wet or dry deposition, often in the form of aerosol nitrate. The ox­ ides of nitrogen, therefore, not only contribute directly to the production of photooxidants, but also control the concentrations of H O radicals, which are the major initiators of chain reac­ tions involving hydrocarbons. Wet chemical and filter methods have been used for many years to measure these compounds in the at­ mosphere. The reliability of these methods has not been high, and long collection times are required. For these reasons they have been largely sup­ planted by optical methods for mea­ surement of N O and N 0 2 . Only indi­ rect chemiluminescence methods have been reported for measuring HNO3 with the sensitivity and the short re­ sponse time required for tropospheric modeling (1-4). Infrared absorption in the 2-15-μτη region offers a number of advantages for atmospheric trace gas measure­ ments; it is a passive technique, well adapted to in situ, real-time measure­ ments. While virtually every minor or trace constituent absorbs in this spec­ tral region the major gases, Ν 2 and O2, do not. In fact, the absorption spectra for the trace gases are so rich that high resolution comparable to the line width, typically 2 X 10~ cm" 1 (60 MHz) , is usually required to avoid mutual interferences. Such resolution can be obtained with a Fourier trans­ form infrared (FTIR) spectrometer using a broadband light source. But because of its low sensitivity, path lengths of several kilometers are needed to detect constituents at the ppbv level. Recently, tunable diode lasers have been developed that provide a wave­ length-tunable source of extremely narrow line width. Techniques have been developed for measuring very small absorbances with these diodes. Reid and co-workers (5-8) have de­ scribed optical systems for detecting tropospheric gases, but not the cali­ bration methods or techniques for sampling real air into the system. This paper describes the use of tunable diode laser absorption spectrometers (TDLAS) for measuring nitrogen ox­ ides in clean and polluted tropospheric air. We have used this spectrometer in a mobile system capable of in situ measurements of the oxides of nitrogen and other trace atmospheric gases.