Before beginning the lab, please watch the short video below. Mila is going to introduce the topic, show you one of her favorite videos, and then end her video by reminding you of the three main questions you should be able to answer at the end of the lab.
This lab has 21 short-answer questions you will answer prior to the three big questions (i.e., research questions) Mila has noted above.
The first lab among the suite of climate-literacy labs had us standing on the Earth’s surface, basking in the warmth of the sun, and trying to understand how the solar radiation that gives us that warmth changed over time (the seasons) and space (latitude). In the second lab, we journeyed out into space, stopping at the layer of the atmosphere that exists between 20 and 50 kilometers out: the stratosphere. While there we were interested in what happened to a key component of that solar radiation – the part making up the ultraviolet portion of the electromagnetic spectrum – and how a change in the composition of a critical gas in the stratosphere – ozone – was affecting that, as well as affecting life down on the Earth’s surface. In today’s lab, our journey through the atmosphere takes us back down to the layer in which we live – the troposphere – and, for at least part of the time, we will be interested in ozone again. However, we will see that it plays a different role in this different layer, and what that role is will be important to understanding the quality of the air we breathe. After that, we will look more broadly at the substances that make up the troposphere and, sticking with one of our key themes in this course, consider changes in the concentrations of those substances across space – both vertically (as you move up in the troposphere) and horizontally (as you move across the surface of our planet). The lab ends with an examination of how the concentrations of pollutants in the troposphere over the United States have changed over the past several decades.
By the end of this lab, you should be able to answer the following research questions:
How does the troposphere differ from the stratosphere?
How does the composition of the troposphere change with a change in altitude?
How does the composition of the troposphere change with a change in latitude?
Entering with the right mindset
Throughout this lab you will be asked to answer some questions. Those questions will come in three different varieties:
Fact-based question →This will be a question with a rather clear-cut answer. That answer will be based on information (1) presented by your instructor, (2) found in background sections, or (3) determined by you from data, graphs, pictures, etc. There is more of an expectation of you providing a certain answer for a question of this type as compared to questions of the other types.
Synthesis-based question → This will be a question that will require you to pull together ideas from different places in order to give a complete answer. There is still an expectation that your answer will match up to a certain response, but you should feel comfortable in expressing your understanding of how these different ideas fit together.
Hypothesis-based question → This will be a question which will require you to stretch your mind little bit. A question like this will ask you to speculate about why something is the way it is, for instance. There is not one certain answer to a question of this type. This is a more open- ended question where we will be more interested in the ideas that you propose and the justification (‘I think this because . . .’) that you provide.
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.
Q1: What are the two most abundant gases in both the stratosphere and troposphere?
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.
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 shows clouds covering a portion of the Earth).
Q2: Compared to the other layers of the atmosphere, of what does the troposphere have high concentrations?
The Troposphere has the potential to prevent a large percentage of incoming solar radiation from reaching the surface. Back in the Solar Radiation and Seasons lab, you explored changes in solar radiation across latitudes and seasons, but one thing we intentionally left out of that lab was the fact that the atmosphere absorbs, scatters, and reflects solar radiation. You were quickly introduced to the concept of the atmospheric absorption of solar radiation in the Stratospheric Ozone lab. As you just learned, the troposphere is comprised of gases, particles, and clouds, and these substances can both further absorb solar radiation as well as scatter and reflect it. For the Earth as a whole, only about half the solar radiation present at the top of the atmosphere reaches Earth’s surface. The left-side portion of the image below shows the absorption and reflection of solar radiation by the atmosphere and Earth’s surface. Note that the incoming solar radiation is 341.3 W m-2, rather than the solar constant value of 1365.2 W m-2. The solar constant is averaged across the entire surface of Earth; therefore, the value is divided by four. The 341.3 W m-2 value is the total solar radiation reaching the top of the atmosphere per second (174,226,942,644,300,000 W) divided by the surface area of Earth (510,000,000,000,000 m2).
Q3: For the Earth as a whole, what percentage of the incoming solar radiation is reflected by clouds and other parts of the atmosphere? You need to examine the figure above to find the numbers you will use when calculating the percentage. A percentage is a proportion multiplied by 100.
Q4: For the Earth as a whole, what percentage of the incoming solar radiation is absorbed by the atmosphere? You need to examine the figure above to find the numbers you will use when calculating the percentage. A percentage is a proportion multiplied by 100. Also, you examined a major component of this absorption in the Stratospheric Ozone lab.
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, compared to kicked footballs 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.
Q5: What percent of the global atmosphere is carbon dioxide (CO2)? Lab 5 and other subsequent labs have an emphasis on CO2.
Q6: How do atmospheric pressure and density change with an increase in altitude?
Q7: Why do kicked footballs tend to travel farther at stadiums located at higher elevations?
You are going to obtain data to determine characteristics of three levels of the troposphere: the surface, the middle troposphere, and the upper troposphere. Weather data will be obtained from measurements made via a weather balloon launched at 7 A.M. or 8 A.M. at an airport (most likely). The weather instrument attached to the weather balloon is known as a radiosonde. Another balloon is launched at 7 P.M. or 8 P.M. There are approximately 800 launch sites globally; therefore, approximately 1,600 weather balloons with radiosondes are launched each day. A picture of a weather balloon and radiosonde ready to be launched are shown below. The balloon, which is either rubber or latex, is filled with either helium or hydrogen, and the maximum altitude to which the balloon ascends is determined by the diameter and thickness of the balloon. The decrease in pressure with an increase in altitude causes the balloon to expand as it ascends. The balloon eventually “pops,” and the radiosonde, which has a parachute attached to it, falls back to the surface. Less than 20% of the approximately 75,000 radiosondes released by the U.S. National Weather Service (NWS) each year are found and returned to the NWS for reconditioning. Check out this site to see what radiosondes look like and to learn what to do if you find one.
Click Troposphere_Data to open the file in Microsoft® Excel. Instructions for obtaining data from a radiosonde launch are provided on the Altitudes sheet. Input your data in the spreadsheet. Proceed through the sheets in the following order: (1) Altitudes; (2) Weather Data; (3) Carbon Dioxide; and (4) Density and Mass, and (5) Sea Level Pressure. If you have access to instruments that measure pressure, temperature, relative humidity, and CO2 concentrations, then we recommend you take a break from staring at this screen and go outside to make some surface measurements.
• All yellow cells need values.
• Values are calculated for all non-yellow cells that do not have either “0” or “#VALUE!”.
Q8: Why do you think temperatures are higher at the surface than at the middle and upper portions of the troposphere?
Q9: How would you describe the change in air density with an increase in altitude and how does that compare to what you learned in the previous sections?
Q10: Which gas had the largest proportional increase in mass when moving from the upper troposphere to the surface? Why?
The fifth sheet (Sea Level Pressure) converts the surface pressure you obtained or measured and converts it to an equivalent pressure at sea level. If you are at sea level, then no changes have been made to the surface pressure. If your location is above sea level, then the sea level pressure (SLP) for your location is higher than the surface pressure. If your location is below sea level — which is not very common, then the sea level pressure (SLP) for your location is lower than the surface pressure. The mean sea-level pressure (SLP) for the globe is 1013.2 hPa.
Q11: Why was your surface pressure higher or lower than the SLP of your location? If your location is Falcon Field, then remember that the surface pressure was measured at 245 meters above sea level.
In this section of The Troposphere lab, you will be exploring changes across space and time in water vapor, cloud coverage, and particulates in the troposphere.
Let’s begin with a key component of the atmosphere in terms of weather and climate: water vapor. There is a picture of the water (hydrologic) cycle below. Take a minute to examine it and consider all the different processes that are a part of that cycle, as well as the sources of water (in its various forms) for each of those processes.
Now think about these processes on a global scale. It should make sense that the dominant source of water vapor in the atmosphere – which comes from evaporation of water on the Earth’s surface – is the Earth’s oceans. Note that the amount of water vapor evaporated from the oceans (436,500 km3 yr-1) is nearly seven times greater than the amount of water vapor evaporated/transpired from the land (65,500 km3 yr-1). Next, think back to the Solar Radiation & Seasons lab. Based on what you learned in that lab, it should make sense to you that tropical and subtropical oceans are huge sources of water vapor. You will get a chance to show off what you learned in that lab in the questions below.
Because all of the different processes in the water cycle are constantly taking place and water is constantly move from the Earth’s surface to the troposphere and back again, water vapor has an average atmospheric lifetime of just ten days. As a result of this, it is valuable to track the movement of water vapor across space and time. The MODIS sensor on-board NASA’s Aqua satellite (see image below). This sensor provides the concentration of water vapor as the amount of precipitable water. Precipitable water is defined as the depth of water in a column of the atmosphere if all the water in that column were precipitated as rain; the unit is centimeters (cm).
Click the image below to see month-to-month changes in water vapor and focus on the area of highest water vapor concentration and how that area is distributed and moves across the globe during the course of the year.
Q12: Which region of Earth generally has the highest concentration of water vapor and which region generally has the lowest concentration?
Q13: Why would the distribution of water vapor change across the globe in the way that you identified in Q13? (i.e., Why would the region that had the highest concentration be expected to have the highest concentration and why would the region that had the lowest concentration be expected to have lowest concentration?)
Q14: How would you describe the movement of the area with the most water vapor over the course of a year?
Q15: How does the movement you just described relate to what you learned in the Solar Radiation & Seasons lab?
Through a visualization of water-vapor data, you have identified a pattern in the movement of the area of the highest water vapor concentration across the Earth over time. That area is known as the Intertropical Convergence Zone (ITCZ), and this feature will be examined again in this lab and also in Labs 8 and 9, which focus on climate change.
Next, let’s focus on clouds, which are visible masses of liquid droplets, or ice crystals, or both. Click the image below to see month-to-month changes in the amount of cloudiness. Cloudiness is provided as cloud fraction, and this is the proportion of a 1-km by 1-km grid cell that is covered by clouds. Cloud fraction was estimated from data collected by the MODIS sensor on-board NASA’s Terra satellite.
Q16: How would you describe the movement of the area with the most cloud cover throughout the year? The movement is exaggerated over Africa; therefore, it is best to look at the white areas over Africa and observe how they move throughout a year. NASA also provides the answer to this question in its text below the animation.
Q17: For subtropical areas that are mostly cloud-free, what would be the effect on the amount of solar radiation and also the temperature at the surface of that region from having such little cloud cover? Hint: You looked at a landmass in January at the end of Lab 1 (Solar Radiation & Seasons).
The system you just witnessed that took place in the tropics and subtropics is called the Hadley Circulation (see image below), and it is the large-scale movement of air in the troposphere with rising air at the ITCZ and sinking air outside the tropics in areas known as subtropical high-pressure cells. Humid air rises at the ITCZ, typically forms deep cumulonimbus clouds as it goes to the top of the troposphere, and then the air heads poleward and sinks in subtropical areas (i.e., subtropical highs). In the next lab (Air Pollution), the Hadley Circulation will be revisited and you will be introduced to the westerlies, which is the dominant circulation in the middle latitudes.
Let’s shift our focus from water to particulates, which are tiny solid or liquid particles suspended in the atmosphere. Particulates are also known as aerosols. There are a whole host of different kinds of particulates in the atmosphere: Soot, smoke, smog, frog (just checking), ash, dust, spores, pollen, and various allergens. Natural processes contribute to the levels of particulates; for instance, deserts in subtropical regions are sources of windblown dust. Human activity, of course, also adds to the particulate concentration: “slash-and-burn” agriculture – in which forests are cut and burned to create fields for agriculture, pasture for livestock, etc. – and the annual burning of grasslands (i.e., savannas) in the tropics produces smoke (see images below). Fossil-fuel combustion (from power plants, cars, etc.) is another source of particulates in industrialized regions of the Earth.
Since particulates are suspended in the air, they represent a heterogeneous (‘uneven’) mixture of solids/liquids in a gas. The general name for such mixtures is aerosols, so that term should only be applied to the particulate-air mixture. Sometimes, though, people just use aerosols to refer to the particulates themselves. One way to measure the particulate concentration is by determining the aerosol optical depth, which is the degree to which aerosols (i.e. particulates) prevent transmission of light by absorbing or scattering the light. Areas with high aerosol optical depth values have high concentrations of particulates. Aerosol optical depth was estimated from data collected by the MODIS sensor on-board NASA’s Terra satellite. When you look at this file, focus on the areas of high and low concentrations of aerosols (i.e. particulates). Click the image below to see month-to-month changes in aerosol optical depth.
Q18: What is the most likely source of the high concentration of particulates that appear off the coast of northwestern Africa?
Q19: During what months are there typically high concentration of particulates over central Africa just south of the equator (i.e., the southern portion of the Democratic Republic of the Congo) and over central South America (i.e., west/central Brazil)?
Q20: What is the most likely source of the high concentrations of particulates over central Africa and central South America?
Q21:What is the most likely source of the high concentrations of particulates over eastern China?
Before the next lab, write for yourself a one-sentence response to each of the following big questions of this lab.
How does the troposphere differ from the stratosphere?
How does the composition of the troposphere change with a change in altitude?
How does the composition of the troposphere change with a change in latitude?