Actinic flux and photolysis in water droplets: Mie calculations and geometrical optics limit B. Mayer1,* and S. Madronich2 1Deutsches Zentrum für Luft- und Raumfahrt (DLR), Oberpfaffenhofen, Germany 2National Center for Atmospheric Research (NCAR), Boulder, CO, USA *during this research at: National Center for Atmospheric Research (NCAR), Boulder, CO, USA/
Abstract. 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.
Citation: Mayer, B. and Madronich, S.: Actinic flux and photolysis in water droplets: Mie calculations and geometrical optics limit, Atmos. Chem. Phys., 4, 2241-2250, doi:10.5194/acp-4-2241-2004, 2004.