Water Vapor in the Atmosphere
Of the variable substances in the atmosphere, water vapor (H2O) is the most variable with concentrations ranging from 0-4% by volume. Most water vapor enters the atmosphere via evaporation and transpiration. Evaporation occurs when a single water molecule on a liquid water surface gains enough kinetic energy (often by solar radiation) to break the bond which holds the molecules together. If the energized molecule happens to be heading in the right direction, it will escape into the atmosphere as a single water vapor molecule. Transpiration is better explained in terms of vapor pressure. During the day, plant leaf pores called stomata open as a response to the sunlight. If the water vapor pressure inside the cell of the leaves exceeds the vapor pressure in the atmosphere, the water vapor molecules will travel from areas of high pressure (inside the leaf) to areas of lower pressure (outside the leaf). This process is referred to as transpiration. Evaporation and transpiration are collectively known as evapotranspiration.
Nearly all of the water vapor in the atmosphere resides in the lower portion of the atmosphere known as the troposphere. The cold temperatures at the top of the troposphere prevent all but a few water vapor molecules from escaping into the stratosphere where temperatures are warmer. Water vapor molecules that do manage their way into the higher regions of the atmosphere become disassociated by energy from the sun and can then participate in other chemical reactions. The troposphere actually conserves water on earth.
Water content in the atmosphere is often expressed as relative humidity (RH). The relative humidity is the ratio (usually expressed as a percent) of the actual amount of water in a sample of air to the amount of water in the same volume of saturated air at the same temperature. Relative humidity is only one of many expressions of moisture content. Others will be covered in depth in a later session.
Though the concentrations of all other variable substances might seem so minute as to be insignificant, many of these numbers are on the increase. For instance, air quality professionals have long been concerned with the amount of CFCs already present in the atmosphere because of the destruction of ozone in the stratosphere that occurs because of these chemicals. Stratospheric ozone protects us from the sun's ultraviolet rays which can cause skin cancer. Note in the table on Page 3 that the concentration of CFCs is but a mere 0.0001 part per million.That tiny fraction is already at work doing damage to the atmosphere which in turn has long term effects for us. In the next section, we will be looking more closely at some of the substances with which air quality professionals are particularly concerned.You can investigate how temperature has an effect on the properties of water vapor by going to the water properties calculator. If you enter the temperature into the input box, various important values of water vapor will be shown.
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