Air quality standards and measurement

Warmer temperatures and air pollution are interrelated: Rising temperatures result in increases in ozone production and energy utilization. Increased energy demands result in more power plant utilization, which leads to greater emissions.

Background

Climate change affects atmospheric composition and dynamics. In addition to changing global weather patterns, climate change may have a negative impact on air quality. In particular, the formation of ozone and is influenced by weather conditions such as temperature and precipitation. Air pollution concentrations are also influenced by management strategies that control emissions. The need to reduce influences on the climate, through mitigation of greenhouse gas (GHG) emissions, and to adapt to future climate change is a significant environmental problem. Adapting to is necessary to protect air quality.

Climate Change and Air Quality

Air quality is affected by human activities such as driving automobiles or other vehicles, burning coal or other fossil fuels, and manufacturing chemicals. In most countries, transportation is one of the major contributors to air pollution, and it generates many pollutants. Transportation causes wear and tear on cars and on roads, which produces road dust. Heating and cooling residential and commercial buildings requires a great deal of energy, most of which is supplied by burning fossil fuels, constituting another major source of air pollutants and GHGs. Residential heating, particularly from burning wood, also contributes particulate matter and many toxic compounds to the atmosphere.

There are a number of connections and synergistic effects between climate change and air quality. Climate change, including changes to temperature, precipitation, cloud cover, and relative humidity, may affect the atmospheric concentrations of many important chemical species and change the rate at which ozone and particulate matter are formed in the atmosphere, with warmer temperatures increasing ozone and particulate-matter formation. High temperatures also cause the evaporation of toxic substances such as mercury, polycyclic aromatic hydrocarbons (PAHs), and polychlorinated biphenyls (PCBs), from sediments.

Warmer temperatures lengthenpollen and mold seasons and encourage spore production. Climate change may increase the frequency of stagnant air masses, which may cause pollutant buildup in certain regions. Changes in temperature can affect methane emissions. Widespread climate changes may also alter human activity patterns such as agriculture, burning, and energy consumption, including the demand for heating or cooling. This, in turn, would affect the emissions of pollutant gases and particles resulting from these activities. Changes in land use and in fire and drought patterns may affect smoke and mineral dust aerosols in the atmosphere.

Air Quality Monitoring Techniques

Air quality is measured through direct monitoring or through emissions inventories. Air quality monitoring is the main method for determining concentrations of particular pollutants in the atmosphere at specific points in time. Air quality monitoring is used to determine the Air Quality Index (AQI), a composite indicator of outdoor air quality that is communicated to the public through the media and generally includes recommendations for protection against pollutant-associated health effects.

Emissions inventories estimate the amount of pollutants emitted into the atmosphere from major mobile, stationary, and natural sources over a specific period of time—for example, a day or a year—and form the basis for efforts such as trend analysis, air quality monitoring, regulatory impact assessments, and human exposure modeling. The Environmental Protection Agency (EPA) maintains the National Emission Inventory database, which contains information about sources that emit criteria air pollutants and air pollutants.

In many countries, air quality monitoring networks provide measurements of pollutant species. Many air quality monitoring programs include both global and regional networks that measure background atmospheric composition at selected remote sites. Other programs include regulatory monitoring networks that analyze day-to-day variations in air quality at numerous sites, primarily in urban areas. Most air quality monitoring sites are focused on heavily populated areas and are designed to determine whether a specific area is in compliance with air quality standards. Urban and regional air quality monitoring networks rely on ground-based sites that sample within the boundary layer.

Remote-sensing or satellite instruments provide global-scale observations of specific pollutants in the atmosphere. A variety of instruments attached to balloons or aircraft are used for atmospheric measurements over a wide range of altitudes. Particulate matter measurements can be conducted in real time, but routine aerosol mass measurements generally rely on particle accumulation over extended sampling times followed by laboratory analyses. Urban areas in the United States are monitored through the EPA’s air quality programs, but large data gaps exist in a number of rural areas, and insufficient data are available for large regions of the Earth.

Air Quality Management Methods

Contemporary air quality management plans focus on controlling state and local emissions, although the EPA has also initiated regional air quality management strategies. In general, there are two types of air quality standards. The first type, theNational Ambient Air Quality Standards (NAAQS’s), are set by the EPA and include target levels for specific pollutants that apply to outdoor air throughout the country. The EPA has set NAAQS’s for six principal pollutants, called “criteria pollutants.” They are carbon monoxide, lead, nitrogen dioxide, particulate matter (PM₁₀ and PM_2.5), ozone, and sulfur dioxide. The second class of standards constitutes the Air Quality Index (AQI), which uses a scale to communicate the relative risk of outdoor activity to the public.

The Clean Air Acts (1963-1990) require the EPA to set NAAQS’s for pollutants from a variety of sources considered harmful to public health and the environment and have significantly strengthened air pollution regulation. The set target levels for air pollutants and provide reporting and enforcement mechanisms. Two types of national air quality standards were established through these acts: Primary standards set limits to protect public health, including the health of sensitive populations such as asthmatics, children, and the elderly. Secondary standards set limits to protect public welfare, including protecting against decreased visibility and damage to animals, crops, vegetation, and buildings.

The Clean Air Rules, established in 2004, are a suite of rules focused on improving US air quality. Three of the rules, the Clean Air Interstate Rule (CAIR), the Clean Air Mercury Rule, and the Clean Air Nonroad Diesel Rule, address the transportation of pollution across state borders. CAIR is a management strategy that covers twenty-eight eastern states and the District of Columbia and focuses on the problem of power plant pollution drifting from one state to another. This rule uses a cap-and-trade system to reduce sulfur dioxide and nitrogen oxides.

The Clean Air Mercury Rule (CAMR) represents the first federally mandated requirement that coal-fired electric utilities reduce their emissions of mercury. Coal-fired power plants are the largest remaining domestic source of anthropogenic mercury emissions, and with the CAMR, the United States is the first nation in the world to control emissions from this major source of mercury pollution. Together, the CAMR and the CAIR create a multipollutant strategy to reduce U.S. emissions. The effects of these laws have been very positive, with substantial reductions in emissions of carbon monoxide, nitrogen oxide, sulfur dioxide, particulate matter, and lead. The Clean Air Nonroad Diesel Rule changes the way diesel engines function to remove emissions and the way diesel fuel is refined to remove sulfur. These rules provide national tools to achieve significant improvement in air quality.

On February 7, 2024, the EPA strengthened the National Ambient Air Quality Standards for Particulate Matter. These standards were amended to 9.0 micrograms per cubic meter. Experts believe that strengthening these standards would improve the public's respiratory health. Additionally, that same year, the EPA revised the Air Quality Index to improve public communication.

Global Air Quality Management

Air pollution emissions can affect air quality beyond national borders. Westerly winds transport ozone from the eastern United States into Canada and the North Atlantic. Air masses reaching the United States carry pollution originating from many other parts of the world, including Asian industrial pollutants and African dust aerosols. Saharan dust has affected atmospheric particulate-matter concentrations in several inland areas of the American southeast. Long-range transport of gases and of aerosols from burning biomass has also been detected.

Many areas of the world are moving toward mitigating the GHG emissions that contribute to global climate change. As urbanization and industrialization have increased, urban air quality has become a significant public health concern throughout many regions, particularly in developing countries. Particulate air pollution is a chronic problem in much of Asia as a result of coal combustion in factories and power plants and the use of coal and wood for cooking and heating homes.

Automobiles continue to be an increasingly important contributor to air pollution in much of the world, with more than 600 million vehicles in use, a number that is growing exponentially. Motor vehicles are the predominant source of air pollution in many Latin American cities, where automobile use has been restricted in order to manage severe air pollution occurrences. In the United States, air quality has shown steady improvement, partly because of air quality regulatory programs and new, cleaner technologies that have improved both motor vehicles and stationary pollution sources.

The includes provisions that limit GHG emissions from industrialized countries throughout the world. is the predominant GHG emitted by most countries, and controlling CO₂ emissions is an essential component of air quality monitoring strategies. Non-CO₂ GHG emissions also have considerable global warming potentials and are important targets for emission reductions. Although ozone and particulate matter also contribute to global warming, control of these species is not currently included in the Kyoto Protocol. Ozone and particulate matter could be effective targets for emission control efforts, however, since many countries already have regulations focused on controlling these species and since reducing ozone and particulate-matter emissions can improve local air quality, health, and agricultural productivity.

Context

Climate change will alter the extent and nature of air pollution and the general composition of the atmosphere and will be influenced by both natural and anthropogenic factors. While climate change may exacerbate the frequency of smog episodes and related health effects, air pollution is already a serious health concern around the world. Changes in air pollution emissions occurring over the next several decades could affect global health, as emissions may reach areas far beyond their local sources. Efforts under way and contemplated to address climate change may have the related benefit of reducing air pollution in general. However, climate change may also reduce the effectiveness of existing programs: Many of the national control programs currently in place may prove be less effective than was originally expected. As climate change progresses, more stringent air quality standards and management will likely be necessary. Multipollutant approaches that protect human health and climate will require a global perspective to meet air quality objectives.

Key Concepts

• Air Quality Index (AQI): a numerical index for reporting air pollution levels to the public
• Clean Air Acts: a set of federal laws that form the basis for the United States’ air pollution control effort
• criteria air pollutant: air pollutants for which acceptable levels of exposure can be determined and ambient air quality standards have been set
• National Ambient Air Quality Standards (NAAQS): standards established by the EPA that limit acceptable outdoor air pollution levels throughout the United States
• particulate matter: any nongaseous material in the atmosphere

Bibliography

Alley, E. Roberts, Lem B. Stevens, and William L. Cleland. Air Quality Control Handbook. New York: McGraw-Hill, 1998.

Godish, Thad. Air Quality. 4th ed. Boca Raton, Fla.: CRC Press, 2003.

Griffin, Roger D. Principles of Air Quality Management. Boca Raton, Fla.: CRC Press, 1994.

Harrop, Owen. Air Quality Assessment and Management. London: Taylor & Francis, 2002.

"National Ambient Air Quality Standards (NAAQS) for PM." EPA, 9 Dec. 2024, www.epa.gov/pm-pollution/national-ambient-air-quality-standards-naaqs-pm#. Accessed 13 Dec. 2024.

National Research Council. Committee on Air Quality Management in the United States. Air Quality Management in the United States. Washington, D.C.: National Academies Press, 2004.