A radiation, with wavelengths longer than 0.32 μ m. But radiation in this band is relatively innocuous, except perhaps when it is used to illuminate velvet blacklight posters of Elvis. There remains a small sliver of the UV-B band between about 0.31 and 0.32 μ m that manages to reach the surface without com- plete absorption; it is precisely this narrow sliver that is primarily responsible for sunburn. Lately, there has been considerable con- cern over observed declines in ozone layer density. If the decline continues, then the resulting widening of this narrow UV-B window could have serious consequences for life on Earth. In the very process of absorbing harmful shortwave UV ra- diation, the ozone layer influences our environment in another very important way. The solar energy that is absorbed by ozone warms the atmosphere at those levels to a much higher temperature than would be the case without the presence of ozone. Have you ever wondered why temperature increases with height in the strato- sphere, reaching a maximum at the stratopause before decreasing again in the mesophere? The ozone layer is responsible! In an atmosphere without free oxygen, and therefore without ozone, the temperature structure would be much simpler: we’d have a very deep troposphere (temperature generally decreasing with height) transitioning directly to the thermosphere. The strato- sphere and mesosphere would be missing. This is in fact what you find on Mars, whose atmosphere consists mainly of CO 2 . On Earth, the temperature structure of the lower stratosphere serves as a very important “lid” on tropospheric convection and other circulations. If you already have some background in atmospheric dynamics, try to imagine how different our weather might be if the tropopause were near 50 km altitude rather than its present 5-15 km! Photochemical Smog We have just surveyed the role of oxygen and ozone in the middle atmosphere (stratosphere and mesosphere) in absorbing UV-C and UV-B radiation. The UV-C, we saw, is mostly absorbed by photodis- sociation of O 2 ; the UV-B was then mostly absorbed by the resulting O 3 . This leaves mainly UV-A radiation to reach the troposphere.
Applications 71 Although less energetic than UV-B and UV-C radiation, UV-A radiation is a key player in tropospheric chemistry. Among other things, photochemical reactions involving organic molecules (e.g. unburned fuel vapors) and nitrogen oxides (produced by high tem- peratures in automobile engines) can lead to the formation of ozone in surface air. Although ozone in the stratosphere is highly desirable because of its UV-blocking characteristics, it is considered a serious pollutant in near-surface air where we live, because it is a strong chemical oxidant which attacks most organic substances, including the lining of your lungs. Ozone is thus one of the main ingredients of photochemical smog.
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