Actinic flux (relative units)

FIGURE 3.24 Calculated relative actinic flux using best estimate albedos as a function of height above the earth's surface for solar zenith angles 0 of 20, 50, and 78°, respectively, at (a) 332.5, (b) 412.5, and (c) 575 nm [from Peterson (1976) and Demerjian et al. (1980)].

However, tropospheric ozone formed as an air pollutant by VOC-NOx chemistry discussed throughout this book can also impact solar radiation reaching the earth's surface. For example, Frederick et al. (1993) reported that measurements of broadband UV in Chicago had a marginally significant negative correlation to surface 03 concentrations under clear-sky conditions.

(3) Aerosol particles Table 3.13 shows the percentage change in the actinic flux calculated by Peterson (1976) and Demerjian et al. (1980) for two cases: (1) a particle concentration of zero, corresponding to a very clean atmosphere, and (2) a total particle concentration doubled compared to the base case. The actinic flux is predicted to increase if the total particle concentration is zero and decrease if it doubles (note, however, as discussed later, the sensitivity to the vertical distribution of particles and the relative importance of light scattering compared to absorption).

Figure 3.25 shows the results of one set of calculations of the effects of aerosol particles whose properties were judged to be characteristic of continental or urban situations, respectively, on the transmission of UV and visible radiation to the earth's surface (Erlick and Frederick, 1998). The ratio of the transmission with particles to that without is plotted in two wavelength regions, one in the UV and one in the visible. Two different relative humidity scenarios are shown. The "average summer relative humidity" was 70% RH in the boundary layer and 20% RH in the free troposphere. The high relative humidity case assumes 90% RH in the boundary layer and 30% in the free troposphere. (The RH in the stratosphere was taken to be 0% in both cases; see Chapter 12.)

The transmission of UV below ~ 320 nm is particularly impacted by aerosol particles. This is primarily due to multiple scattering caused by the aerosol particles, which enhances the light absorption by 03 in this region, since the effective absorption path length is increased (Erlick and Frederick, 1998). There is also a small contribution from assumed light absorption by aerosols (which, however, is highly uncertain; see Chapter 9). The increase in transmission with wavelength above 320 nm is due to decreased Mie light scattering by the particles, which depends on A (see Chapter 9). It is evident that aerosol particles, particularly at high RH (which affects particle size by water uptake), can have significant impacts on the actinic flux at the earth's surface.

Model studies that incorporate both scattering and absorption of light by particles have shown that the vertical distribution and the relative importance of scattering versus absorption are critical in determining not only the magnitude but also the sign of the effect of particles on the actinic flux in the boundary layer and the associated photolysis rates for gases. For example, Dickerson et al. (1997) have shown that particles in the boundary layer which primarily scatter UV light lead to decreased actinic fluxes at the earth's surface but increased fluxes a few hundred meters above the surface. This leads to increased rates of photolysis of such species as N02 in the boundary layer. On the other hand, for aerosols that absorb strongly, the opposite effect occurs, reducing the actinic flux and photolysis rates of gases such as N02 (e.g., see Jacobson (1998) and Krotkov et al. (1998)).

It is these contrasting effects of aerosol particles, combined with uncertainties in the contribution of absorption due to 03, that provide the largest uncertainties in calculations of actinic fluxes and photolysis rates in the boundary layer (e.g., Schwander et al., 1997). As a result, it is important to use the appropriate input

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