Lab 3 Part 2

Background Information

The “bad nearby” ozone described in the Prezi presentation is known as tropospheric ozone, meaning that it is produced and exists in the lowest layer of the atmosphere known as the troposphere. We live in the troposphere, and similar to the stratosphere, it is comprised of gases, particulates, and droplets.  As can be seen in the comparison of the pie charts below, the major differences between the stratosphere and troposphere are the concentrations of ozone and water vapor: the stratosphere has much higher concentrations of ozone than does the troposphere, while the troposphere has much higher concentrations of water vapor than does the stratosphere.

The troposphere is not very thick: it ranges in depth from just 6 km at the poles to 20 km at the equator. The average thickness in the middle latitudes is approximately 17 km. The images below provide insights into how shallow the troposphere is. For instance, the Structure of the Atmosphere figure displays the relative thickness of each of the layers of the atmosphere. The image on the right – Cumulonimbus Cloud at the Top of the Troposphere – dramatically illustrates how the cloud ‘flattens out’ when it reaches the stratosphere since the stratosphere cannot maintain the concentrations of water vapor needed for cloud formation.

Compared to the other layers of the atmosphere, the troposphere has relatively high concentrations of water vapor and particulates. Particulates, which are also known as aerosols, are extremely tiny pieces of solid or liquid matter. The far-left image below shows a sand storm beginning in the Sahara desert; such wind-blown dust from natural sources, represent one of the largest particulates in the atmosphere. (It is worth pointing out that particulates from dust storms can travel as far as the Caribbean Sea.). The second image from the left shows particulates being ejected by Mt. Pinatubo in the Philippines. Very explosive volcanic eruptions, such as the Mt. Pinatubo eruption in 1991, can actually inject particulates into the stratosphere. Other major sources of particulates in the troposphere, besides deserts and volcanoes, are fossil-fuel combustion and biomass burning. Both water vapor and particulates are needed for clouds to form; therefore, nearly all clouds occur in the troposphere. One exception is polar stratospheric clouds, which you observed in the Stratospheric Ozone lab. Please examine the graph (the third picture from the left below) which shows the change in water vapor concentration with a change in altitude. From this graph, you should be able to see that water vapor concentrations in the troposphere decrease rapidly with an increase in altitude. This is because Earth’s oceans are the overwhelming source of water vapor. The decrease in concentrations of water vapor with an increase in altitude does not mean that the upper troposphere is less cloudy than is the lower troposphere. The formation of clouds is a complex process; at any one time, clouds cover 60% of the globe (see the right-hand image below which provides a picture of cloud formation across a large portion of the Earth from space).

In order to successfully complete the measurements and calculations in the next part of the lab, you have to understand the relationships among the concepts of percent, parts per million (ppm), and mass. You were shown percent information in the pie charts showing the gaseous compositions of the stratosphere and troposphere. For example, approximately 77.69% of the troposphere is nitrogen and 20.84% is oxygen. Out of a sample of one million molecules in the troposphere, 776,900 would be nitrogen and 208,400 would be oxygen. Therefore, the concentrations of nitrogen and oxygen are 776,900 ppm and 208,400 ppm, respectively. The table below shows conversions from percent to ppm. The concentration of water vapor decreases with an increase in altitude. The average concentration of water vapor for the troposphere is 0.46%, while the average concentration for the stratosphere is 0.0005%. The concentrations in ppm are 4,600 ppm and 5 ppm, respectively. If a gas has a low concentration, it is easier to express its concentration in ppm, rather than percent. In future exercises, you will encounter gases present in very low concentrations, with the concentrations expressed in parts per billion (ppb) and parts per trillion (ppt). For example, the concentrations of the ozone-depleting substances you examined in the Stratospheric Ozone lab are expressed in ppt; these gases are present in extremely low concentrations in both the troposphere and stratosphere.

Percent and ppm are relative terms; therefore, they do not provide information on exactly how much (e.g., mass) of a certain gas is within a given volume of air, such as one cubic meter of air. For example, in this lab, we will be exploring the density of air, which will be the mass in kilograms per one cubic meter of air; the units will be kg m-3. Atmospheric pressure is needed to calculate the mass of air and the various gases that comprise it. Atmospheric pressure is defined as the force per unit area (e.g., square meters) exerted into a surface by the weight of air above that surface. A graph showing the decrease in pressure with an increase in altitude is shown below (left-hand image). The average pressure at sea level is 1,012 hectopascals (hPa), while the pressure at the top of the troposphere is less than 100 hPa. Therefore, atmospheric pressure decreases rapidly with an increase in altitude. And as atmosphere pressure decreases so does the density of air. Examine the right-hand image below to see this change in density with change in altitude; you should note how it mirrors the left-hand graph. Less air means less resistance. This is why kicked footballs go farther at “Mile High” Stadium in Denver, which is located at 1,587 meters above sea level, than at any other professional football stadium. The reduced atmospheric density at Denver also causes athletes to have access to fewer oxygen molecules. The concentration (i.e. ppm or percent) of oxygen undergoes trivial changes throughout the troposphere, but the rapid decrease in pressure is associated with a rapid decrease in the number of oxygen molecules for a given volume of air. Consequently, there is nearly 20% less oxygen available at Denver compared to a location at sea level. Teams playing the Denver Broncos at “Mile High” Stadium have more trouble breathing than do the Broncos, because players on the visiting teams are not acclimated to the lower oxygen levels.

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