Transport of Solar Radiation through the Atmosphere: Aspects Relevant for Health

The spectral distribution of the solar radiation reaching the Earth's surface depends on the irradiance emitted by the Sun, the Earth-Sun distance, and the transmission properties of the atmosphere. The changing Earth-Sun distance due to an eccentric orbit implies that the irradiance received at the top of the atmosphere varies with an annual cycle, being 6.9% above and below the yearly mean. The bulk of the Earth's atmosphere (99% by mass) consists of molecular nitrogen and oxygen, which are radiatively inactive homo-nuclear, diatomic molecules. Trace amounts of polyatomic molecules including ozone are responsible for atmospheric absorption of solar radiation. In recent years ozone loss has been tied to the release of man-made trace gases, mainly chlorofluorocarbons used in the refrigeration industry and as propellants in spray cans. Since ozone provides an effective shield against harmful ultraviolet (UV) solar radiation, a thinning of the ozone layer could have serious biological ramifications. This paper provides an overview of how UV radiation affects human health, how UV radiation is measured, and how the penetration of UV radiation is affected by the ozone layer as well as by molecules and suspended particles (aerosols and clouds) in the atmosphere.