Archive for year: 2024

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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