Actinic flux and photolysis frequencies

A straightforward approach to calculate actinic flux and photolysis frequencies at the Earth's surface was published by [Mayer et al., 1998]. In a model study, it has been shown that the cloud reflectivity measured by the TOMS instrument can be used to estimate actinic flux in a similar way as it has been done for irradiance, see e.g. [Eck et al., 1995].

The calculation of surface radiation quantities is comparatively easy because the vertical distribution of the clouds has only a small effect on the averaged surface radiation. The next step, which I am currently working on, is the calculation of altitude profiles of photolysis frequencies. This requires much more information than is provided by the TOMS instrument: in particular, the altitude profile of the cloud optical properties is needed. Part of this information is available from the International Satellite Cloud Climatology Project (ISCCP). However these data have to be complemented, e.g. with climatological cloud observations from the surface because satellites usually only provide cloud top height and cloud fractions as seen from above. The latter implies that the satellite only registers those fractions of the lower clouds which are not hidden by higher layers.

Droplet In a recent publication, [Mayer and Madronich, 2004] studied the actinic flux and photolysis in water droplets, using Mie calculations and geometrical optics. Photolysis of water-soluble components inside cloud droplets by ultraviolet/visible radiation may play an important role in atmospheric chemistry. Two earlier studies have suggested that the actinic flux and hence the photolysis frequency within spherical droplets is enhanced relative to that in the surrounding air, but have given different values for this enhancement. Here, we reconcile these discrepancies by noting slight errors in both studies that, when corrected, lead to consistent results. Madronich (1987) examined the geometric (large droplet) limit and concluded that refraction leads to an enhancement factor, averaged over all incident directions, of 1.56. However, the physically relevant quantity is the enhancement of the average actinic flux (rather than the average enhancement factor) which we show here to be 1.26 in the geometric limit. Ruggaber et al. (1997) used Mie theory to derive energy density enhancements slightly larger than 2 for typical droplet sizes, and applied these directly to the calculation of photolysis rates. However, the physically relevant quantity is the actinic flux (rather than the energy density) which is obtained by dividing the energy density by the refractive index of water, 1.33. Thus, the Mie-predicted enhancement for typical cloud droplet sizes is in the range 1.5, only coincidentally in agreement with the value originally given by Madronich. We also investigated the influence of resonances in the actinic flux enhancement. These narrow spikes which are resolved only by very high resolution calculations are orders of magnitude higher than the intermediate values but contribute only little to the actinic flux enhancement when averaged over droplet size distributions. Finally, a table is provided which may be used to obtain the actinic flux enhancement for the photolysis of any dissolved species.


Eck, T.F., P.K. Barthia, and J.B. Kerr.
Satellite Estimation of Spectral UVB irradiance using TOMS derived total ozone and UV reflectivity. GRL, 22, 611-614, 1995.