Air Quality and Air Pollution Control

Air Quality and Air Pollution Control

Introduction

Like ‘pure’ water, H2O, ‘pure’ air is hard to find. Pure air is a mixture of gases, more than 98% of which is nitrogen (78.0%) and oxygen (20.1%).

[P] Composition of clean air (Vesilind and Morgan, p. 301)

Given the composition of ‘pure’ air, we can then define as pollutants those materials (gases and particles) that, when added to ‘pure’ air at sufficiently high concentrations, will cause adverse effects. Note, however, that there are both natural and anthropogenic sources for many pollutants and thus sources must be specified when classifying contaminants.

Major Air Pollutants: Sources and Effects

Particulates

Particulates are classified based on their size and are expressed on a weight per unit volume of air basis, e.g. µg∙m-3.

[P] Particulate size (Vesilind and Morgan, Fig. 11.6)

Particulate matter in air is measured using a high volume (hi-vol) sampler which draws air through a filter. The filter is then weighed before and after sampling to determine the total suspended particulates (TSP). Recognizing that collection of a few large particles can skew measurements of TSP and that smaller particles are inhaled more deeply into the lungs, particulates are now measured as particulate matter less than 10 microns (PM10).

Most particulates in the atmosphere are of natural origin, e.g. pollen, fungal pores, salt spray, smoke and dust. Major anthropogenic sources of particulates include fossil fuel burning and industrial emissions. The particles may cause health problems in and of themselves (respiratory disease) and also carry other pollutants deeper into the respiratory system.

Oxides of Sulfur

Two oxides of sulfur, sulfur dioxide (SO2) and sulfur trioxide (SO3), are of interest and are typically considered together and referred to as SOx. Oxides of sulfur are a serious respiratory irritant causing chronic bronchitis, especially if carried deep into the lungs by associating with fine particulates. Stationary sources, i.e. the combustion of sulfur-containing fossil fuels (in amino acids) and metal refining (sulfide ores) are the major anthropogenic inputs.

Oxides of Nitrogen

Nitric oxide (NO) is formed by the combustion of nitrogen-containing compounds (including fossil fuels) and by thermal fixation of atmospheric nitrogen. NO is then oxidized to nitrogen dioxide (NO2) in the atmosphere.

Nitrogen dioxide (NO2) is a reddish brown gas in concentrated form and gives a yellow-brown tint at lower concentrations. Exposure to elevated concentrations leads to a cough and irritation of the respiratory system. Continued exposure leads to accumulation of fluid in the lungs.

In nature, nitrous oxide (N2O) is produced by soil bacteria and oxidized to nitric oxide (NO) by atomic oxygen in the atmosphere:

which is then oxidized to nitrogen dioxide by ozone:

High temperature combustion is an anthropogenic source of nitrogen dioxide. Nitrogen and oxygen, typically uncreative in nature, form nitric oxide at temperatures in excess of 1600 K:

with subsequent oxidation by ozone to nitrogen dioxide in the atmosphere:

Sources of NOx are equally split between mobile and stationary sources.

Carbon Monoxide

Carbon monoxide (CO) is a lethal gas. It acts by reacting with hemoglobin the blood to form carboxyhemoglobin. Hemoglobin has a greater affinity for CO than for oxygen, thus the presence of carbon monoxide effectively deprives the body of its oxygen supply. The effects of carbon monoxide on human health provide an excellent illustration of a time-dose response relationship.

[P] Effects of carbon monoxide on health (Vesilind and Morgan, Figure 11.10)

Carbon monoxide is produced through the incomplete combustion (oxidation) of carbonaceous material. It is produced naturally as an intermediate step in the oxidation of methane. Anthropogenic sources include motor vehicles, fossil fuel burning and industrial processes. Motor vehicles account for most of the anthropogenic emissions.

Lead

In contrast to other major air pollutants, lead is a cumulative poison. Retention of lead in the body is higher for that entering through the air than through food and water. Lead poisoning manifests itself through anemia (deficiency of red blood cells) and ultimately brain damage.

Atmospheric lead occurs as a particulate, with volcanic activity and airborne soil being the primary natural sources. Anthropogenic contributors include smelters and refining processes and the incineration of lead-containing wastes. Approximately 80% of the lead which used to be added to gasoline was discharged to the atmosphere.

Photochemical Oxidants

The substances described above are examples of primary pollutants, i.e. those emitted directly to the atmosphere. Secondary pollutants result entirely from atmospheric reactions and are not directly attributable to either natural or anthropogenic emissions.

Photochemical oxidants are important secondary pollutants and include chemicals such as peroxyacetyl nitrate (PAN) and peroxybenzoyl nitrates (PBzN). The major photochemical oxidant, however, is ozone (O3), and it is this chemical which is commonly used as an indicator of the total amount of oxidant present. Photochemical oxidants can cause eye irritation and can aggravate respiratory problems.

Ozone is formed when nitrogen dioxide is exposed to sunlight and dissociates to form nitric oxide and atomic oxygen:

the atomic oxygen then reacts with molecular oxygen to form ozone:

The ozone can then again react with any NO present to form more nitrogen dioxide:

thus continuing the cycle. If hydrocarbons are present in the atmosphere, e.g. through incomplete fuel combustion, a variety of other photochemical oxidants (e.g. PAN) can be formed as well. Over the day, the formation of photochemical oxidants tends to follow the emissions of nitric oxide and hydrocarbons.

[P] Formation of photochemical smog (Vesilind and Morgan, Figure 11.12)

This is the classic photochemical ‘smog’ for which Los Angeles is famous.

Meteorology and Air Pollution Episodes

In air quality work, the word episode is equivalent to disaster. There have been a variety of episodes which have drawn attention to the need for air quality regulation. One of these, The Donora (Pennsylvania) Episode is described in Chapter 1 of the text. In this episode, 17 people died and more than 7,000 were made ill. The episode occurred as a result of the discharge of sulfur oxides and particulates from steel plants at a time when an atmospheric inversion trapped the emissions in the valley where Donora is located. A type of inversion is responsible for air pollution episodes in Los Angeles.

Wind moves air masses both horizontally and vertically, dispersing pollutants and reducing concentrations with distance from the source. The amount of dispersion which occurs is directly related to the stability of the air or how much vertical air movement is taking place.

As a parcel of air rises in the atmosphere, it experiences lower pressure and thus expands and cools. A rising parcel of air, e.g. from a stack, cools at 1°C per 100m: the adiabatic lapse rate. The actual or prevailing lapse rate may differ from the adiabatic lapse rate. When the air cools more rapidly than the adiabatic lapse rate, it is called a superadiabatic lapse rate and when it cools less rapidly, it is a subadiabatic lapse rate. An inversion is a special case of the subadiabatic lapse rate where the air is colder at the bottom and warms with increased elevation.

[P] Prevailing and adiabatic lapse rates

[Excel] Lapse rates and pollutant fate

This demonstrates how inversions can lead to air pollution episodes.

Regulation of Major Air Pollutants

The need to control air pollutants has been known for a long time: the first air pollution control legislation was passed in Los Angeles in 1905. The Clean Air Act of 1963, focusing on the seven major air pollutants introduced previously, was the first federal effort to regulate air quality. Amendments to the act, passed in 1990, added over 180 air toxics to the air quality standards.

Under the Clean Air Act, the U.S. EPA has set National Ambient Air Quality Standards (NAAQSs) for each of the seven major air pollutants. There are two types of standards: primary standards relate to human health and secondary standards address other problems such as corrosion, animal health and visibility.

[P] NAAQS (Davis and Cornwell, p. 465)

U.S. EPA sets emission standards for these pollutants to seek attainment of the NAAQS. Areas in the U.S. where standards are exceeded on average more than twice per year for any pollutant are termed nonattainment areas and air pollution control programs must be implemented to bring the region back into compliance.

[P] Non-attainment areas

Industries must show how new sources will reduce emissions and may engage in ‘trading’ to meet emission standards. Emission standards for automobiles are also set to support clean air initiatives.

Other Air Pollution Issues

Acid Rain

Oxides of sulfur (SO2 from smelters and sulfur-containing fossil fuels) and nitrogen (NO from high temperature combustion) emitted to the atmosphere react to form sulfuric and nitric acids:

which return to the earth as acid rain. A full discussion of the acid rain phenomenon is beyond the scope of this course. Suffice it to say that a combination of source location, prevailing winds and sensitive geology/geography has led to the death of hundreds of lakes in the northeast.

[P] Acid rain

The Clean Air Act Amendments of 1990 required significant cuts in the emissions of nitrogen and sulfur oxides.

Ozone Depletion

The ozone which is a pollutant at the land surface should not be confused with stratospheric ozone, 7-10 miles above the earth. This ‘upper’ ozone acts as a shield for ultraviolet radiation and its alteration can increase cancer risks and alter ecosystem function.

Depletion of the stratospheric ozone layer has occurred due to the production of chlorofluorocarbons (CFCs), once widely used in aerosols and refrigeration systems. CFCs drift into the atmosphere where they are destroyed by short wave radiation, releasing chlorine. The chlorine reacts with and destroys the ozone. The chlorine atom acts as a non-consumptive catalyst in this reaction and a single chlorine atom can make the ‘loop’ thousands of times before it becomes involved in another reaction and changes form.

The Montreal Accord of 1987 sought a reduction in the use of CFCs by 50% by 1999 and an accelerated complete phase out after that. Recent measurements indicate that concentrations of atmospheric ozone have increased in recent years.

[P] Ozone hole

Global Warming

The earth receives radiation from the sun, reflecting some into space and absorbing the rest; later radiating absorbed energy back into space. As long as this energy balance is not changed, the temperature of the earth remains constant. The earth’s atmosphere acts as a gatekeeper for this radiation, allowing short wave, light energy to penetrate and keeping some of the long wave, heat energy from escaping – the greenhouse effect.

The earth’s atmosphere is made up of a variety of gases, each absorbing heat energy at specific wavelengths. Carbon dioxide, water vapor, methane and nitrous oxide (the greenhouse gases) are very effective energy absorbers of heat radiation.

Increases in the concentration of greenhouse gases in the atmosphere over the past 50 years are well documented. It is more difficult to clearly document changes in temperature. To combat global warming, 141 nations have signed a treaty, the Kyoto Accord, agreeing to cut emissions of greenhouse gases by 5.6% by 2012. The U.S., the largest emitter of greenhouse gases, refuses to sign.

[P] National Geographic

Air Quality Control

The best way to control air pollution is to eliminate the source and efforts are underway to do this, reducing the ecological footprint of our society. Most often, however, air pollutants are removed by some form of treatment analogous to that applied for water.

The sizes of air pollutants range over many orders of magnitude and so it is not reasonable to expect a single device to be effective and cost efficient for all pollutants.

[P] Particulate size (Vesilind and Morgan, Fig. 11.6)

Thus, air pollution control devices are typically divided into those used for controlling particulate pollutants and those for controlling gaseous pollutants.

Control of Particulates

Settling chambers: simply wide places in the exhaust flue, often with a baffle to slow air flow, where larger (>100 µm) particles can settle. Only larger particles can be removed by such a device.

Cyclones: a conical cylinder where dirty air is blown in, off-center, creating a tornado-like effect. Acting like a centrifuge, large particles are moved to the wall where they are slowed by friction and eventually settle, exiting on the bottom. The clean air in the middle moves out the top.

[P] Cyclone (Vesilind and Morgan, Fig. 12.2)

Bag filters: these operate like a vacuum cleaner, with fabric bags that collect the dust. Nearly all particulates can be removed by this technology which is widely used in industrial applications.

[P] Bag filter (Vesilind and Morgan, Fig. 12.3)

Scrubbers: spray towers or scrubbers are effective in removing large particulates, but can be designed to remove smaller particulates as well. A drawback to scrubbers is that the waste is now in liquid form and some treatment of the water is necessary.

[P] Scrubbers (Vesilind and Morgan, Fig. 12.4)

Electrostatic precipitators: operate by transferring a negative electric charge to the particulates which then migrate to a positively charged electrode. Able to remove even submicron particles, precipitators are commonly used in power plants where electricity is readily available.

[P] Electrostatic precipitator (Vesilind and Morgan, Fig. 12.5)

Control of Gaseous Pollutants

Gases are removed from the air stream by:

• direct removal (wet scrubbing, carbon adsorption)
• changing the form of the pollutant (incineration of organics to CO2)
• changing the process producing the pollutant (low sulfur fuels)

Coal-fired power plants illustrate these options for reducing sulfur oxide emissions: changing to low sulfur fuels, desulfurization of coal (washing for inorganic sulfur, gasification for organic sulfur), and flue gas desulfurization (addition of limestone):

In review, note that the effectiveness of the various air pollution control devices depends on particle size.

[P] Removal and particle size (Vesilind and Morgan, Fig. 12.9)

Control of Moving Sources

Moving sources, largely automobiles, require control of two types of emissions:

• hydrocarbons from the fuel tank, carburetor and crankcase
• NOx, HC and CO from the exhaust

Automobiles are now produced with gas tank caps that control HC emissions, carburetors have activated carbon canisters which store vapors for later re-burning and PCV valves (positive crankcase ventilation) eliminate venting to the atmosphere with recycling and combustion of blowby gases.

Exhaust accounts for ~60% of the HC and all of the NOx and CO emissions and is more difficult to control. Three types of emission control strategies are employed:

• engine tuning for efficient fuel combustion (air/fuel ratio)
• engine re-design (e.g. fuel inject to insure proper amount of gas)
• catalytic converter (burns HC and CO to CO2)

[P] Emissions and air:fuel ratio (Vesilind and Morgan, Fig. 12.16)