The Lower Atmospheric Regions and Their Composition

Even though the air at the top of Mt. Everest is still about 21% oxygen, the fact that the air there is only about one-third as dense as that at sea level means that a mountain climber would have to take in three times the normal 

The region of Earth s atmosphere closest to Earth, which extends to an altitude of roughly 10 km ( 6 mi), is known as the troposphere (Eigure 4.1). This is the region in which our weather occurs. Propeller and jet aircraft and balloons routinely travel in the troposphere. Denver sits at about 1.6 km above sea level, while Mt. Everest reaches almost 9 km into the troposphere. At the upper level of the troposphere the temperature of the air is about — 60°G ( 76°E), much colder than Earth s average surface temperature of 15°C (59°E). Most of the pollution discussed in this chapter occurs near the surface of Earth. We will refer to this as ground-level air pollution even though it may extend for several kilometers into our atmosphere. 

Above the troposphere is the stratosphere, which extends to about 50 km ( 30 mi). The temperature of the stratosphere rises gradually to about 0°C (32°E) at 50 km. 

Before we consider atmospheric pollutants, let s examine the normal composition of dry air in more detail. As shown in Table 4.1, nitrogen and oxygen make up about 99% of Earth s atmosphere. There are lesser amounts of many other gaseous substances. 

The concentration values in Table 4.1 are listed as percentages or as parts per million (ppm). All of us are familiar with percentages. For instance, in 

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