2.04676 2.01589 1.98594 1.95686 g value

FIGURE 11.54 Diurnal profile of average (H02 + R02) concentrations measured at Cape Grim, Tasmania (•), and at Mace Head, Ireland (■), under clean air conditions using a chemical amplification technique. (Adapted from Carpenter et al., 1997.)

FIGURE 11.55 Altitude profiles for H02 + R02 in the free troposphere over southern Germany determined by conversion to OH and measuring OH by the mass spectrometric derivatization technique (adapted from Reiner et al, 1998).

FIGURE 11.55 Altitude profiles for H02 + R02 in the free troposphere over southern Germany determined by conversion to OH and measuring OH by the mass spectrometric derivatization technique (adapted from Reiner et al, 1998).

Figure 11.55 shows an altitude profile for peroxy radicals measured above the boundary layer over southern Germany using chemical amplification with the mass spectrometric derivatization measurement of OH (Reiner et al., f998). Concentrations are again seen to be in the range of I08-f09 cm-3.

In summary, H02 and R02 radical concentrations are substantially greater than those of OH, typically by several orders of magnitude. There are several different approaches to measuring these peroxy radicals, and the results from these are in overall agreement as to the magnitude of the concentrations and their diurnal variation. However, there have not been a significant number of intercomparison studies of these methods, so evaluation of the absolute accuracies will require further work.

5. Generation of Standard Gas Mixtures

As seen throughout this discussion of the measure ment of gases in the atmosphere, a critical component is the accurate calibration of the technique for the gas(es) of interest. This clearly requires sources of such calibration gases, which however, vary depending on the particular gas.

In the simplest case, the gas of interest can be purchased in a gas cylinder with known concentration provided by the supplier. In the United States, NIST

has some mixtures relevant to atmospheric measurements. This approach has been used, for example, for simple hydrocarbons that are readily available and relatively stable. Preparation of standards in cylinders can also be carried out by the individual laboratory (e.g., see Apel et al., 1998a). Such standards are frequently at higher concentrations than those to be measured in air. In this case, dynamic dilution systems are used to dilute the cylinder mixtures to the desired concentration range.

Caution must be exercised in using cylinder gases in some cases, however. For example, NOz in air from cylinders commonly contains a few percent HNO-, as an impurity, and nickel carbonyls are present in CO stored in cylinders.

In other cases, the species cannot be preprepared as a mixture in air and hence flow systems must be used to generate them. For example, HN03 strongly adsorbs to surfaces and hence it is not possible to make a stable calibration mixture that can be stored. Some larger organics also do not have long-term stability when stored in gas cylinders.

In such cases, permeation tubes or diffusion cells are commonly used to generate the species in a flow of air, which can then be introduced into the measuring device. Permeation tubes are permeable tubes whose ends are sealed off and which contain the species of interest as a liquid in equilibrium with its vapor. The vapor permeates through the walls of the device at a rate that depends on temperature. The rate of permeation at a given temperature is normally supplied by the manufacturer and can be determined independently by weighing the permeation tube before and after use. From a knowledge of the flow rate of the gas passing over the tube, which entrains the vapor, the concentration of the species of interest in the air flow can be calculated. This approach is commonly used for species such as HN03, Cl2, and HC1.

A similar approach is the use of diffusion cells. In this case, the liquid is held in a container that has a capillary of fixed length and diameter through which the vapor over the liquid diffuses. The vapor exiting the capillary is swept into a flow of gas to provide the gas mixture; this approach has been used to prepare mixtures of terpenes in air, for example (Larsen et al., 1997). The concentration of the gas can be varied by using capillaries of varying internal diameter and length.

In some cases, the compound itself is sufficiently unstable that it cannot be purchased and must be synthesized. This is the case for compounds such as 03 and HONO. Ozone at ppb to ppm concentrations in air is generated either by photolyzing 02, e.g., using a low-pressure mercury lamp, or by a discharge in 02;

when discharges are used, care must be taken to exclude air from the discharge region to avoid the simultaneous formation of oxides of nitrogen. In the case of HONO, a flow of gaseous HC1 over NaN02 salt is often used to generate this compound in a flow system (Febo et al., 1995). For other more "exotic" species such as C10N02 and C1N02, synthesis of the compounds is more involved and the literature should be consulted for methods of synthesis.

0 0

Post a comment