Changes in Air Pollutants in the United States
If someone asked you, “What is special about 2013?”, a lot things might come to mind, but one of them would probably not be, “It is the year we celebrate a very important piece of legislation passed 40 years ago.” It should be, though, because that piece of legislation, the Clean Air Act (http://www.epa.gov/air/caa/), has had a significant and positive impact on the world you live in. This impact is a result of the fact that it helped do what its name suggested: clean up the air. Initially, the law just supported research into what harmful effects could be clearly linked to the presence of certain pollutants in the atmosphere, and what levels of those pollutants represented reasonable risks. Then, in 1970, with clear results of that research in hand, politicians made decisions about regulating the levels of the pollutants. This legislation has been amended several times, with the last major amendment coming in 1990, when acid rain and ozone depletion were among the problems explicitly targeted (http://en.wikipedia.org/wiki/Clean_Air_Act_%28United_States%29).
In this part of the lab, we are going to investigate the effects that this act had on general pollutants [e.g., PM10 Emissions and Volatile Organic Compounds (VOCs)] and specific pollutants [e.g., SO2 and NOX], and then look at how those effect were related to changes in such things as the acidity of rain in the U.S. and the amount of harmful ozone in the troposphere.”
The first thing we will examine is changes in particulate emissions and concentrations for the entire country. Prior to 1987, total suspended particulates (TSP), was regulated; after the 1987 amendments to the Clean Air Act, PM10 (Particulate Matter, 10 micrometers or smaller), was regulated. The graph on the left below presents the emissions of PM10 and the concentrations of TSP (values are averages from approximately reporting 1,000 sites) from 1970 – 2010; the pie chart on the right below presents the sources of PM10 in 1970 and 2010, which will help you understand what lead to the changes in the graph.
Q15: Why do you think the level of PM10 emissions changed so rapidly in the first part of the graph above (left portion) and less rapidly in the remainder of the graph (right portion)?
Q16: What source had the largest decrease in PM10 emissions from 1970 to 2010?
Next, let’s turn our attention to nitrogen oxides (NOX) and sulfur dioxide (SO2) which are produced when fossil fuels (gasoline, coal) containing traces of nitrogen and sulfur are burned. SO2 is also released in volcanic eruptions (such as the 1991 Mount Pinatubo eruption that was mentioned in Day 1 of this lab). Below are images showing changes in NOX and SO2 emissions and concentrations for the United States. The NOX concentrations are averages from 81 sites and the SO2 concentrations are averages from 121 sites across the country where those concentrations are monitored.
Q17: When did SO2 emissions and concentrations first show a rapid decrease? When did NOx emissions and concentrations first show a rapid decrease?
Q18: During what time period did both SO2 and NOx emissions and concentrations show a rapid decrease?
Acid rain is produced when there is an abundance of either SO2 or NOx or both in the atmosphere, as these gases react with water vapor in the atmosphere to make sulfuric acid and nitric acid. View the picture on the left below to visualize this process. The pH (power of Hydrogen) scale can help us keep track of acid rain, but only if we understand two things about the scale. First, because the ‘p’ or ‘power’ part of ‘pH’ stands for the fact that these numbers are ‘power of 10’ values. This means that pH is a logarithmic scale, like the decibel scale for sound. What that tells us is that changing the pH value by 1 really means the level of acidity is changing by a factor of 10. Second, because of how pH values are calculated, a smaller number on the pH scale means a higher acidity. So, putting those two things together, if the pH measurement at a particular place changes from 5 to 4, that should be interpreted as the acidity has increased 10 times. Look at the picture of the pH scale on the right below to become more familiar with the meaning of pH values.
‘Clean’ rain has a pH between 5 and 6, which means it is slightly acidic (that’s because of CO2 which is naturally in the air and forms carbonic acid). Electricity generation (e.g., coal-burning power plants) contributes SO2 and NOx to the air, causing the pH of the rain to decrease (i.e., making the rain more acidic). An interesting thing about Title IV of the 1990 Clean Air Act (which focused on acid rain) is that it penalized power companies for causing increases in the acidity of water within a certain radius of the plant. The power companies responded by building taller emission stacks that sent the SO2 and NOx gases traveling farther from the plant. (The group REM wrote the song Fall on Me about this; http://www.youtube.com/watch?v=lf6vCjtaV1k.) Acid rain can kill aquatic life forms and destroy forests. Acid rain has eliminated insect life and some fish species, including the brook trout in some lakes, streams, and creeks in geographically sensitive areas, such as the Adirondack Mountains of the United States. Some high-altitude forests are often shrouded in acid clouds and fog; as a result, entire sections of forests have died.
Now you are going to look at a Google™ Earth file that was created to show the changes in pH in different parts of the country over the last 16 years. Click on conc_phlab_1994-2010 to view this animation.
Q19: What was the general trend from 1994 to 2010 in the size of the area with pH levels less than or equal to 4.5 (below which the environment is considered to be acidic)?
Q20: Across that entire time range, in what part of the country are the pH values the lowest? In what part of the country are the pH values the highest?
So the Google™ Earth animation showed you that there has been a change in the pH of rain over the last 20 years, but knowing that begs the question, ‘What caused that change?’ There are two pie charts below designed to help answer that question. The charts show the change in total nitrogen oxide (NOX) and sulfur dioxide (SO2) emissions from 1994 to 2010 through the size of the pies. They also show the change in the percentage of emissions from different sources between those two years. Use the charts to answer Q22 and Q23 below.
Q21: What source of NOX emissions was reduced the most between 1994 and 2010 and, as a result, helped to raise the pH level (i.e., decrease the acidity) of rain in this country?
Q22: What source of SO2 emissions was reduced the most between 1994 and 2010?
Below are links to two Google™ Earth files, one which shows data for the locations of key emitters of SO2 and one which shows the same data for NOX. Do the following with these files:
• Open the SO2_Emissions_Facilities. The size of a red circle is proportional to the emissions of SO2 from a facility.
Q23: What region of the country has the highest concentration of SO2 emitters? Why does this region have such a high concentration of SO2 emitters?
• Zoom in on 3 – 4 of the larger red circles to get a closer look at the main emitters of SO2 found in that region.
Q24: What did you learn about these emitters (their identity, the structure of the facility) by zooming in on them?
• Open the NOx_Emissions_Facilities. The size of a yellow circle is proportional to the emissions of NOX from a facility. The red circles for the SO2 emitters should still be present.
Q25: How does the region of highest concentration of NOX emitters compare to the region of highest concentration of SO2 emitters? Why do you think this is the case?
Q26: In Q20 you identified the parts of the country with the lowest and highest pH values. How does the information in these two Google™ Earth files explain which parts of the country have the lowest and highest values?
Finally, we want to bring this set of laboratory experiences on the troposphere full circle – back to our exploration of that Jekyll-and-Hyde substance ozone. Recall from the Day 1 lab that ozone in the troposphere is produced by reactions involving Nitrogen Oxides (NOX), Volatile Organic Compounds (VOCs), and ultraviolet radiation. There are graphs and pie charts below showing changes in the emissions and sources of NOX and VOCs. Use these to answer the last three questions on this page.
Q27: What source had the largest decrease in VOC emissions from 1970 to 2010?
Q28: When did VOC emissions begin to most significantly decrease? How does this compare to when NOX emissions began to decrease?
Q29: Why did O3 concentrations decrease from 1980 to 2010?
