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FIGURE 3.28 Vertical measurement of 7(0'D) using an upward-facing detector and the total /(O'D) measured using both upward- and downward-facing detectors (from Junkermann, 1994).

increased actinic flux is expected compared to the surrounding air. As discussed in detail by Madronich (1987), there are several different effects that must be taken into account when a light beam strikes a water droplet in air. Initially, of course, some of the light will be reflected from the surface and not enter the drop itself. The portion of the incident beam that enters the droplet is subject to absorption, depending on the droplet composition and the wavelength of light, which also effectively reduces the actinic flux. However, counterbalancing these effects is the possibility of multiple internal reflections occurring at the inner droplet surface, which redirects the light beam back into the droplet. In addition, refraction of the light beam at the air-water interface as it enters the drop leads to an increased path length through the drop itself, in effect increasing the probability of light absorption (Beer-Lambert law).

The net enhancement factor for a droplet consisting of pure water can be as much as f .6 (Madronich, 1987). Calculations by Ruggaber et al. (1997) suggest that the actinic flux inside cloud drops with a typical size distribution and dissolved particulate matter is more than a factor of two greater than in the cloud interstitial air. This effect of enhanced actinic flux inside droplets may be quite important for aqueous-phase photochemistry in fogs and clouds.

h. Comparison of Calculated Actinic Fluxes to Experimentally Measured Values

As described earlier, measurements of actinic fluxes are made using chemical actinometry, particularly the photolysis of NOz or 03, or using flat-plate or 2it radiometers. Intercomparisons of such measurements have been made by a number of investigators, as well as comparison with calculated photolysis rates using published actinic fluxes such as those in Table 3.7. In general, there is good agreement between results obtained with different radiometers and calculated values, with the largest uncertainties generally being at shorter wavelengths (<310 nm) and larger solar zenith angles (e.g., see Seckmeyer et al., 1995; Kato et al., 1997; and Halthore et al., 1997).

Figure 3.29, for example, shows measurements of the photolysis rate of 03, /(03), made at the Mauna Loa Observatory on two different days, compared to model calculations of the photolysis rate constant (Shetter et al., 1996). The two model calculations use different assumptions regarding the quantum yield for 03 photolysis in the absorption "tail" beyond 310 nm (see Chapter 4.B). The measurements are in excellent agreement for the second day but somewhat smaller than the model calculations on the first.

Similarly, Fig. 3.30 shows measurements of /(N02) at an altitude of 7-7.5 km as a function of solar zenith angle compared to a multidirectional model calculation (Volz-Thomas et al., 1996). The agreement in this case is generally good. However, this is not always the case. For example, Fig. 3.31 shows some measurements of /(N02) as a function of solar zenith angle made by different groups at different locations and using different techniques (Kraus and Hofzumahaus, 1998).

The reasons for discrepancies between various measurements and between the measured and model calculated values are not clear. Lantz et al. (1996) suggest that one factor that will affect instantaneous photolysis rates is cloud cover and that under some circumstances, the instantaneous photolysis rates may exceed

Time (HST)

FIGURE 3.29 Measured rates of 03 photolysis, J(03), shown as heavy solid line, at Mauna Loa Observatory on two days (October 2, 1991, and February 3, 1992) compared to model calculations using two different assumptions (shown by the lighter dotted and dashed lines, respectively) for the quantum yield for 03 photolysis at A > 310 nm. (Adapted from Shetter et al., 1996.)

Time (HST)

FIGURE 3.29 Measured rates of 03 photolysis, J(03), shown as heavy solid line, at Mauna Loa Observatory on two days (October 2, 1991, and February 3, 1992) compared to model calculations using two different assumptions (shown by the lighter dotted and dashed lines, respectively) for the quantum yield for 03 photolysis at A > 310 nm. (Adapted from Shetter et al., 1996.)

the clear-sky values. However, the average photolysis rate will not exceed the clear-sky value. This remains an area of active investigation.

i. Actinic Fluxes in the Stratosphere

Tables 3.15, 3.16, and 3.17 give Madronich's calculated actinic fluxes for altitudes of 15, 25, and 40 km, respectively. The reduction in actinic flux as the light travels through the atmosphere is very evident. Thus, at 40 km but not at 15 km, there is substantial light

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