: Introduction to Environmental Science

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: Introduction to Environmental Science

NAMES: GEOL 0840: Introduction to Environmental Science Exercise 10: Air Pollution Spring 2018 Mandi Lyon & Tyler Rohan

Exercise 10: How Can We Reduce Air Pollution from Automobiles?

This week in recitation we’ll be exploring air pollution from automobiles. As they burn fuel, automobiles emit compounds that are hazardous and that react in air to form other substances that degrade air quality. Fuels for automobiles include both hydrocarbon fuels such as gasoline and oxygenated fuels such as ethanol. In this recitation we will analyze how fuels combust in a car engine, how pollution is generated in an engine, and how engine conditions affect pollution and fuel economy. Note that throughout the recitation, the term “automobiles” will be used to refer to all types of gasoline-powered vehicles, whether cars, vans, trucks or motorcycles. This week’s recitation is adapted from an activity created by Howard Drossman, Wayne Tikkanen, and Sandra Laursen in 2003 for ChemConnections titled “How Can We Reduce Air Pollution from Automobiles?”.

Background While new options for powering vehicles are becoming technologically feasible and may, within the next century, become everyday alternatives, it will be a while before our society completely abandons the convenience and low cost of the gasoline engine. Meanwhile, consumers, gasoline producers, and car manufacturers are all taking steps to reduce one of the serious hazards of gasoline, the toxic combustion products that are released into the air we breathe. Gasoline produces energy by reacting chemically with oxygen in a process called combustion. As you will learn, one of the toxic byproducts, carbon monoxide, is produced when there is a shortage of oxygen in the reaction mixture. Catalytic converters are one solution to this problem, reacting the carbon monoxide as it emerges from the engine but before it exits the tailpipe. Converters have helped greatly to lower air pollution over the last three decades. Another solution is to add more oxygen to the reaction mixture by incorporating it directly into the fuel. Fuel additives such as ethanol contain oxygen and burn as fuels themselves. The resulting mixture is called oxygenated fuel.

The fuel we call gasoline is distilled from petroleum. Distillation separates groups of chemical compounds from petroleum, a complex mixture, according to their boiling points. As the mixture is heated, the most volatile, or easily evaporated, components, with the lowest boiling points, distill first and are collected in a separate container. The least volatile, with high boiling points, distill last. Gasoline is the portion of petroleum that separates in the temperature range 40-100°C. It may contain over two thousand compounds, with the composition varying markedly with manufacturer, location, and season.

The two major products from combustion of automobile fuel are CO2 and H2O. However, we will focus on compounds that are emitted in much lower quantity, but have known human health effects. These compounds include nitrogen oxides, hydrocarbons (VOC’s), and carbon monoxide. These compounds have health effects of their own and can also combine in the atmosphere to produce other hazardous gases, such as ozone (O3), a component of the unpleasant

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NAMES: GEOL 0840: Introduction to Environmental Science Exercise 10: Air Pollution Spring 2018 Mandi Lyon & Tyler Rohan

pollution mixture known as smog. We will explore how combusting fuels in automobiles produces these compounds and the tradeoffs involved in reducing the emissions of each.

While there are natural sources of nitrogen oxides, hydrocarbons (VOC’s), and carbon monoxide, anthropogenic, or human-generated, sources tend to be emitted in a concentrated space and time. Pollutants emitted by automobiles are concentrated in areas of high population and traffic density. There is not enough time for the pollutants to disperse before more pollutants are added daily, keeping the average level in the local atmosphere high. For example, more than 95% of the carbon monoxide in the atmosphere of metropolitan areas results from human activities. Local concentrations of carbon monoxide can be fifty to one hundred times greater than global average concentrations.

At high concentrations, those above 100 ppm (parts per million), carbon monoxide is lethal. At lower concentrations, adverse human health effects are less well documented but still serious. Carbon monoxide binds to the hemoglobin in blood very efficiently, thus inhibiting binding of oxygen. The reduced blood capacity for oxygen leads to acute effects of CO exposure such as headache, fatigue, and dizziness.

Nitrogen oxides are produced in engines as a side effect of fuel combustion, the reaction at high temperature of the nitrogen and oxygen both naturally present in the air needed to combust the fuel. Because air is needed to react the fuel itself, there is no way to avoid this additional reaction. For a chemist, the nitrogen oxides include NO, NO2, and N2O. Atmospheric scientists, however, are most concerned by the toxic and highly reactive compounds NO and NO2, and refer to these collectively as “NOx”, where x can be equal to 1 or 2. These two compounds can be grouped because they rapidly interconvert in the atmosphere and thus have a joint role in producing smog. Atmospheric oxidation of NO leads to higher oxides of nitrogen and eventually nitric acid, a significant source of acid rain, second only to the sulfur oxides. The sunlight- catalyzed reaction in the atmosphere between hydrocarbon molecules (VOC’s) and NOx leads to photochemical smog, the brown haze often seen above large cities. One product of these reactions is ozone, a pollutant in the lower atmosphere that is a respiratory irritant.

Hydrocarbons are a large class of compounds formed from carbon and hydrogen, and thus are sometimes collectively designated “CHx”. The term “volatile organic compound,” or VOC, is often used in referring to hydrocarbon emissions. This term is broader and recognizes the fact that partially burnt hydrocarbons are not, strictly speaking, hydrocarbons, because they incorporate some oxygen in their structures. Examples of this group include formaldehyde (HCHO) and acetaldehyde (CH3CHO). Thus the term “VOC” includes both the oxygen- containing compounds and the original hydrocarbons. While some VOC’s are carcinogens or respiratory irritants, many compounds formed by atmospheric oxidation of hydrocarbons, including the aldehydes and irritating smog components such as peroxyacetylnitrate (PAN) and ozone, exhibit their own adverse health effects.

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NAMES: GEOL 0840: Introduction to Environmental Science Exercise 10: Air Pollution Spring 2018 Mandi Lyon & Tyler Rohan

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NAMES: GEOL 0840: Introduction to Environmental Science Exercise 10: Air Pollution Spring 2018 Mandi Lyon & Tyler Rohan

1) The figure below (Figure 1) shows carbon monoxide (CO) concentrations and traffic patterns throughout a typical weekday in New York City. (½ a point each)

! Figure from Johnson, K. L.; Dworetzky, L. H.; Heller, A. N. Science 1968, 160, 67.

a. Describe the pattern of hourly average carbon monoxide concentration.

b. Describe the pattern of hourly traffic count.

c. How does Figure 1 provide evidence for automobiles as a source of carbon monoxide?

d. What might account for the time lag between the traffic increase and the CO increase in the graph?

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NAMES: GEOL 0840: Introduction to Environmental Science Exercise 10: Air Pollution Spring 2018 Mandi Lyon & Tyler Rohan

e. How would you expect the pattern of traffic and carbon monoxide to differ on a weekend day or holiday?

f. The Clean Air Act of 1970 required automobile manufacturers to reduce pollutants resulting from fuel combustion. One very effective response to these requirements has been the introduction of catalytic converters in the US since 1975. What differences would you expect to see in the graph above now that catalytic converters are required in automobiles?

2) Figure 2 below shows the daily variation in the concentration of certain smog-related pollutants in a city. (½ point each)

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NAMES: GEOL 0840: Introduction to Environmental Science Exercise 10: Air Pollution Spring 2018 Mandi Lyon & Tyler Rohan

! a. Assuming traffic patterns similar to New York City in Figure 1 and recalling that

automobiles are a major source of NOx to the atmosphere, during what times of day would you expect NOx concentrations to be highest in a city?

b. How does this graph help explain the role of NOx in formation of ozone?

Oxygenated fuel additives are combustible compounds that contain oxygen in their molecular structure. Most oxygenated fuel additives are also octane enhancers, providing higher octane ratings when mixed with gasoline. Oxygenates were first introduced to the US as octane enhancers and as domestic substitutes for oil during the Arab oil embargo of the early 1970s. The

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NAMES: GEOL 0840: Introduction to Environmental Science Exercise 10: Air Pollution Spring 2018 Mandi Lyon & Tyler Rohan

EPA regulations state the total mass of oxygen that must be present in a given mass of a fuel. Ethanol is one of the most commonly used oxygenates. Methanol has properties similar to ethanol but is toxic. MTBE, methyl tert-butyl ether, was used regularly in the past, but because it is a carcinogen and has recently appeared in drinking water, it is being phased out.

The EPA mandates that oxygenated gasoline mixtures include at least 2.7% oxygen by mass in fuels sold in carbon monoxide non-attainment areas. A non-attainment area is one that is not currently meeting EPA standards for the particular pollutant in question. The requirement for oxygenated fuels in CO non-attainment areas is known as the Wintertime Oxy-Fuel program. Carbon monoxide tends to accumulate when the air is not well mixed, and this occurs more often in the winter, when temperature inversions can trap a stagnant layer of air near the surface. These weather conditions are also more common in the western U.S., particularly in cities bordered by mountains, which tend to help trap the inversions. In ozone non-attainment areas, the EPA requires at least 2.0% oxygen by mass. This program is known as the Summer Reformulated Gasoline (RFG) program.

3) Based on what you know already about smog formation from your readings and the background, why do you think the program for ozone non-attainment areas is required in the summer? (½ point)

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NAMES: GEOL 0840: Introduction to Environmental Science Exercise 10: Air Pollution Spring 2018 Mandi Lyon & Tyler Rohan

4) Figure 3 below shows the daily maximum 8 hour ozone concentration measured at air quality monitoring stations in Allegheny County during the year 2011. The EPA standard for daily maxium 8-hour ozone concentration is 0.08 ppm. (½ point each)

! a. Using figure 3, do you think Pittsburgh was an ozone non-attainment area in

2011? Be sure to explain your reasoning.

b. During what time of year is Pittsburgh most likely to experience ozone concentrations above the EPA standard?

5) For each of the oxygenated fuel additives in Table 1 below, use a periodic table to calculate the mass percentage of oxygen in the pure additive. Methanol is shown as an example. Try the methanol calculation first, and check your answers with the table. (1 point)

6) Compute the mass percentage of that additive needed to meet the EPA standard of 2.7% oxygen by mass (for carbon monoxide non-attainment areas). Methanol is shown as an example. Try the methanol calculation first, and check your answers with the table.

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NAMES: GEOL 0840: Introduction to Environmental Science Exercise 10: Air Pollution Spring 2018 Mandi Lyon & Tyler Rohan

(1 point)

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NAMES: GEOL 0840: Introduction to Environmental Science Exercise 10: Air Pollution Spring 2018 Mandi Lyon & Tyler Rohan

Table 1: Oxygenated Fuel Additives and Amounts Needed to Meet EPA Standard of 2.7% O by Mass for CO Non-attainment Areas

In some states and urban areas where ground-level air pollution is a problem, automobiles are tested for emissions of CO, exhaust VOCs and NOx. In order to pass the test, cars must be properly tuned to provide the engine with the correct mixture of fuel and air to optimize combustion.

7) Why do air quality officials in these areas believe that altering the fuel-air mixture in an engine will affect air pollution? (1 point)

Combustion of gasoline is a chemical reaction and all chemical reactions must conserve mass, as expressed by the balanced chemical equation. Thus, to combust a given amount of gasoline, a specific amount of oxygen will be required to react it completely. Fuel is fed in from the gas tank, and the oxygen comes from air, which is about 21% oxygen. You might suspect that the ideal fuel-air mixture is this stoichiometric mixture, a mixture that is prepared according to the “recipe” given by the balanced chemical equation. The engine should add just the right amount of oxygen to react all the gasoline and leave no leftover reactants. However, the amounts of air and fuel mixed in a real engine varies, depending on the driving conditions and the tuning of the engine. These variations affect both the engine’s performance and the chemical content of tailpipe emissions. A rich mixture is rich in fuel, with more fuel than the stoichiometrically balanced fuel-to-air ratio, while a lean mixture has less fuel than the stoichiometric ratio calls for. In most modern automobile engines, electronic sensors act together with the fuel injection system to vary the air to fuel (A/F) ratio during operation to match driving conditions.

Additive Chemical formula Mass % oxygen in pure additive Mass % additive in fuel mixture meeting

an EPA standard

methanol CH3OH 50% 5.4%

ethanol CH3CH2OH

MTBE (CH3)3COCH3

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NAMES: GEOL 0840: Introduction to Environmental Science Exercise 10: Air Pollution Spring 2018 Mandi Lyon & Tyler Rohan

Figure 4 below shows the typical emissions of pollutants from gasoline engine as a function of intake air to fuel ratio (A/F) from an experiment. The ratio values are based upon mass. Note the different scale for hydrocarbons (HCs), which is 10 times the value for nitric oxide (NO).

! From Figure 1.2, Ch. 1 “Air Quality” in Interagency Assessment of Oxygenated Fuels, National Science and

Technology Council, Committee on Environment and Natural Resources, Office of Science and Technology Policy, June 1997.

8) Use the information in Figure 4 to explain why there is a trade-off when using the air-fuel ratio to address CO and NOx? (1 point)

9) How does the air-fuel ratio affect the two main air pollutants that serve as pre-cursers to ozone formation? (½ point)

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NAMES: GEOL 0840: Introduction to Environmental Science Exercise 10: Air Pollution Spring 2018 Mandi Lyon & Tyler Rohan

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