(The 0.1%figure cited above for trace constituents is a global average.) Inthe stratosphere, the layer of the atmosphere where temperaturesincrease with height and vertical motions are weak, water vapor istypically a few parts water per million molecules of air by volume(ppmv). Near the surface, water vapor can be as high as 2-3% of thegaseous portion of the atmosphere in a warm ground fog. They arecalled trace gases because they exist in small amounts. These include water vapor, carbon dioxide, ozone,methane, various oxides of nitrogen, neon, and helium. ![]() The remaining 0.1% of the atmosphere consists of the traceconstituents. 2.2 The Minor (Trace) Constituents of the Atmosphere The constituent elementsare primarily nitrogen and oxygen, with a small amount of argon.Below a height of about 86 kilometers, the three main gaseouselements, which together account for about 99.9% of the totalatmosphere, exist in essentially constant proportion to the total:nitrogen comprises 78% of air by volume, oxygen comprises 21% of airby volume, and argon comprises another 0.9%. While Earth's atmospherehas no definitive ending point, but instead thins out to nearnothingness several hundred kilometers above the surface, virtuallyall of the atmosphere exists below100 km. Yet the air is actually acombination of gaseous elements that have a remarkable uniformity interms of their contribution to the whole. ![]() ![]() Its very uniformitymade the ancient Greeks classify it as one of the four basicelements: earth, water, fire, and air. Chapter 2 Section 2 2 - CHEMICAL COMPOSITION OF THE ATMOSPHERE 2.1 The Major Constituents of the Atmosphereįew things are as sure as the air: it is all around us, invisible,odorless, and essential to all life on Earth.
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