The amount and type of pollutants entering the air varies by time and location. Bad air days can occur in the winter, when there is a buildup of wood smoke or a temperature inversion, or in the summer, when photochemical smog is at its worst. The air quality of a location also depends on external features, such as winds, temperature inversions, and the local topography (the ups-and-downs of the landscape). Winds move polluted air away from a region or bring in fresh air to dilute it. On days when winds are strong, the air is rapidly cleansed. When there is little or no wind, the air stagnates, with little input of clean air and little chance for output of pollutants.
Temperature inversions trap cool air beneath warm air so that the cool air cannot rise. This puts a lid on the atmosphere so that anything that enters that volume of air is captured in it. The severity of the pollution depends on the height of the inversion. If the inversion is low, there is little air for mixing and pollution can reach dangerous levels. If the inversion is high, there is more air for mixing and the pollutants
Photochemical smog trapped beneath an inversion over Los Angeles, 1999. The view is northward toward smog-enshrouded downtown Los Angeles, with the Hollywood Hills in the background. (Steve Smith /SuperStock)
are diluted. Virtually any city can experience a temperature inversion; Los Angeles and New York are just two of them.
The topography of a region can contribute to its air quality. For example, a city located in a valley collects more pollutants than one that is located on a flat plain. Topography can also aid in changing pollution conditions both daily and seasonally. On cold winter nights, air sinks into valleys, forming an inversion and collecting pollutants. In the daytime, including during the summer season, the hills around valleys are heated by the Sun. During these periods, the air rises and pollutants cannot collect. This is why inversions are common in mountain valleys in the winter but not in the summer. Mountains also block winds, so air on the leeward side of a mountain range may stagnate and collect pollutants. Los Angeles is a good example of a city in which the topography couples with the large quantity of pollutant emissions to create a very smoggy city.
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Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.