Divergence of sun-rays by atmospheric refraction at large solar zenith angles

[摘要] For the determination of photolysis rates at large zenith anglesit has been demonstratedthat refraction by the earth's atmosphere must be taken into account.In fact, due to the modified optical paththe optical transmittance is thereby increased in most instances.Here we show that in addition the divergence of sun-rays,which is also caused by refractionbut which reduces the direct solar irradiance,should not be neglected.Our calculations are based on a spherically symmetric atmosphereand include extinctionby Rayleigh scattering, ozone, and background aerosol.For rays with 10km tangent altitudethe divergence yields a reductionof about 10% to 40%at solar zenith angles of 91° to 96°.Moreover, we find that the divergence effect can completely cancelthe relative enhancement caused by the increase of transmittance.

[学科分类] 大气科学