6.3.5 Ambient Measurements of Chemical Composition - VOCs NO_x, NO_yMeasurements  6.3 Measurements of Chemical Composition  6.3.3 Tunable Diode Laser Absorption Spectroscopy

6.3.4 Resonance and Laser-Induced Fluorescence

These two techniques involve the absorption of radiation of a very precise wavelength to provide very specific probes of atmospheric species. In contrast to absorption methods, in fluorescence techniques the re-emitted light is monitored. In the case of resonance fluorescence a lamp is used in which the species of interest is excited electronically, and emits light at some characteristic wavelength. For laser-induced fluorescence the light is generated using a laser tuned to the wavelength of interest. Both methods are used for in-situ monitoring and are based on well-established laboratory techniques. Unlike absorption measurements, a long path length is not required in order to get high sensitivity, since photon counting techniques are very effective at low light levels. The major applications are in the measurement of atomic chlorine and measurement of the OH radical. In both cases, the sample is drawn continuously through a detection chamber at reduced pressure and irradiated with radiation which is unique to the species to be detected.

In the case of chlorine atoms, resonance radiation is provided by a lamp consisting of a microwave discharge plasma excited in a dilute flow of Cl₂ in an inert gas. Excited Cl atoms produced in the discharge emit resonance radiation in the vacuum ultraviolet, which is absorbed by the Cl atoms in the sample, and re-emitted. The technique of resonance fluorescence has been employed in laboratory kinetics experiments since the late 1960s for both atomic (Cl, Br, H, O) and molecular (OH) species. Particularly, the use of chlorine resonance lamps was exploited by Clyne and coworkers in the early 1970s. These laboratory studies of chlorine atom reactions laid much of the groundwork for understanding the chemistry of stratospheric ozone depletion, which was proposed by Molina and Rowland (1974). The principal reactions are:

$$Cl + O_3 \rightarrow ClO + O_2$$ (6.20)

 

$$O + ClO \rightarrow O_2 + Cl$$ (6.21)

 

In the atmosphere, Cl atom concentrations have been measured using instrument packages flown on rockets, balloons and airplanes. A variation on the technique involves the conversion of ClO to Cl by addition of excess NO to the detection chamber:

$$ClO + NO \rightarrow Cl + NO_2$$ (6.22)

 This reaction is actually responsible for converting ClO back to Cl in the stratosphere. Since this is a relatively slow process, ClO is usually present in large excess over Cl, thus enabling the measurement of both species. The technique was demonstrated very dramatically in the measurements of ClO in the Antarctic Polar vortex made by Anderson and co-workers. These measurements clearly showed an anticorrelation between ClO and O₃, providing evidence for the involvement of chlorine catalysis in the development of the Antarctic Ozone Hole.

The laser-induced fluorescence spectrum of OH was likewise utilized for many years in laboratory measurements of rate coefficients, before it was used for field measurements. The absorption spectrum of the OH radical lies in the ultraviolet near 300 nm. The generation of laser radiation in this region usually involves frequency doubling from the visible using pulsed lasers with high repetition rates. A state-of-the-art system uses a copper vapor laser to pump a dye laser, the output of which is doubled to 282 nm. The laser radiation is tuned to a single rotational line in the OH spectrum, to enable specific detection of OH.

The detection of OH in the stratosphere has been accomplished by Anderson and co-workers between 20 and 40 km using a balloon-borne instrument package (Stimpfle et al., 1990). The addition of excess NO allows the detection of HO₂ also:

$$HO_2 + NO\rightarrow OH + NO_2$$ (6.23)

 The concentration of O₃ in the lower stratosphere is governed largely by OH and HO₂:

$$OH + O_3 \rightarrow HO_2 + O_2$$ (6.24)

 

$$HO_2 + O_3 \rightarrow OH + 2O_2$$ (6.25)

 Simultaneous measurement of both these species provides a stringent check on our understanding of the chemistry.

Laser-induced fluorescence has been employed recently to detect OH in the troposphere. The oxidation of most molecules emanating from the biosphere is initiated by OH attack, and measurements of ambient levels of OH are vitally important. The mixing ratios of OH encountered in the troposphere (typically 0.1 pptv) lead to problems with sensitivity, and the elevated pressure causes sampling problems. However, the importance of these measurements ensures that progress will continue to be made. 

  
 6.3.5 Ambient Measurements of Chemical Composition - VOCs NO_x, NO_yMeasurements  6.3 Measurements of Chemical Composition  6.3.3 Tunable Diode Laser Absorption Spectroscopy 
http://www.asp.ucar.edu