☁️Atmospheric Physics Unit 6 – Atmospheric Chemistry & Air Pollution

Key Concepts and Terminology

• Atmospheric chemistry studies chemical reactions and processes occurring in Earth's atmosphere
• Air pollution refers to the presence of harmful substances in the atmosphere that can adversely affect human health and the environment
• Primary pollutants are emitted directly from sources (sulfur dioxide, nitrogen oxides)
• Secondary pollutants form through chemical reactions in the atmosphere (ozone, secondary particulate matter)
• Criteria air pollutants are six common pollutants regulated by the U.S. Environmental Protection Agency (EPA) under the Clean Air Act (carbon monoxide, lead, nitrogen dioxide, ozone, particulate matter, sulfur dioxide)
• Photochemical smog is a type of air pollution formed by reactions between nitrogen oxides and volatile organic compounds in the presence of sunlight
• Greenhouse gases absorb and emit infrared radiation, contributing to the greenhouse effect and global warming (carbon dioxide, methane, water vapor)

Atmospheric Composition and Structure

• Earth's atmosphere is a mixture of gases, with nitrogen (78%) and oxygen (21%) being the most abundant
• The atmosphere is divided into layers based on temperature variations: troposphere, stratosphere, mesosphere, thermosphere, and exosphere
• The troposphere is the lowest layer, extending from the Earth's surface to an average height of 12 km, where most weather phenomena occur
• The stratosphere extends from the top of the troposphere to about 50 km, containing the ozone layer that absorbs harmful ultraviolet radiation
  • The ozone layer is crucial for protecting life on Earth from damaging UV rays
• The mesosphere extends from the top of the stratosphere to about 85 km, characterized by decreasing temperature with increasing altitude
• The thermosphere extends from the top of the mesosphere to about 600 km, featuring high temperatures due to absorption of solar radiation by oxygen and nitrogen molecules
• The exosphere is the outermost layer, extending from the top of the thermosphere to about 10,000 km, where atoms and molecules can escape into space

Chemical Reactions in the Atmosphere

• Photochemical reactions are initiated by the absorption of solar radiation, leading to the formation of reactive species (hydroxyl radicals, ozone)
• Oxidation reactions involve the transfer of electrons from one species to another, often facilitated by oxidants such as hydroxyl radicals and ozone
  • Oxidation reactions play a crucial role in the removal of pollutants from the atmosphere
• Reduction reactions involve the gain of electrons by a species, such as the conversion of nitrogen dioxide to nitric oxide
• Acid-base reactions occur between acidic and basic compounds, influencing the acidity of precipitation (acid rain)
• Photolysis is the breakdown of chemical compounds by light, such as the photolysis of nitrogen dioxide to form ozone in the troposphere
• Homogeneous reactions occur within a single phase (gas-phase reactions), while heterogeneous reactions involve multiple phases (gas-liquid, gas-solid)
• Catalytic cycles involve the regeneration of reactive species, allowing them to participate in multiple reactions without being consumed (chlorine-catalyzed ozone depletion)

Sources of Air Pollution

• Anthropogenic sources are human activities that emit pollutants (fossil fuel combustion, industrial processes, transportation)
  • Fossil fuel combustion in power plants, vehicles, and industries releases carbon dioxide, sulfur dioxide, and nitrogen oxides
• Biogenic sources are natural processes that emit pollutants (volcanic eruptions, wildfires, vegetation)
  • Volcanic eruptions release sulfur dioxide, ash, and other particulates into the atmosphere
• Stationary sources are fixed-location sources (power plants, factories, refineries)
• Mobile sources are moving sources (cars, trucks, airplanes, ships)
• Point sources are single, identifiable sources (smokestacks, exhaust pipes)
• Area sources are diffuse sources spread over a large area (agricultural fields, construction sites)
• Indoor sources contribute to indoor air pollution (building materials, cleaning products, cooking, smoking)

Types of Air Pollutants

• Particulate matter (PM) consists of solid or liquid particles suspended in the air, classified by size (PM10, PM2.5)
  • PM can penetrate deep into the lungs, causing respiratory issues and cardiovascular disease
• Sulfur dioxide (SO2) is a colorless, reactive gas emitted from fossil fuel combustion and industrial processes, contributing to acid rain and respiratory problems
• Nitrogen oxides (NOx) are highly reactive gases formed during high-temperature combustion, contributing to ozone formation and acid rain
  • Nitrogen dioxide (NO2) is a reddish-brown, toxic gas that can irritate the lungs and lower resistance to respiratory infections
• Carbon monoxide (CO) is a colorless, odorless, toxic gas produced by incomplete combustion of carbon-containing fuels, reducing the blood's oxygen-carrying capacity
• Volatile organic compounds (VOCs) are organic chemicals that easily evaporate at room temperature, contributing to ozone formation and indoor air pollution (benzene, formaldehyde)
• Ozone (O3) is a highly reactive gas formed by photochemical reactions between NOx and VOCs, causing respiratory issues and damaging vegetation
• Lead (Pb) is a toxic heavy metal that can accumulate in the body, causing neurological and developmental problems, primarily emitted from leaded gasoline (now phased out in most countries)

Air Quality Measurement and Monitoring

• Air quality monitoring networks measure pollutant concentrations using various instruments (gas analyzers, particulate matter samplers)
• The Air Quality Index (AQI) is a scale used to communicate the level of air pollution to the public, ranging from 0 (good) to 500 (hazardous)
  • The AQI is calculated based on the concentrations of criteria air pollutants
• Ambient air quality standards set maximum allowable concentrations of pollutants in the outdoor air to protect public health and welfare
  • National Ambient Air Quality Standards (NAAQS) are set by the U.S. EPA for criteria air pollutants
• Emissions inventories estimate the amount of pollutants released from various sources in a given area and time period
• Dispersion modeling predicts the concentration and spatial distribution of pollutants using mathematical models and emission data
• Remote sensing techniques (satellites, lidar) provide spatial coverage and data on pollutants and atmospheric composition
• Source apportionment identifies the contribution of different sources to the overall pollution levels using statistical methods and chemical fingerprinting

Environmental and Health Impacts

• Respiratory effects include irritation, inflammation, and exacerbation of pre-existing conditions (asthma, chronic obstructive pulmonary disease)
• Cardiovascular effects include increased risk of heart attack, stroke, and heart failure due to exposure to particulate matter and other pollutants
• Neurological effects include cognitive decline, increased risk of neurodegenerative diseases (Alzheimer's, Parkinson's), and developmental issues in children exposed to lead and other neurotoxins
• Reproductive and developmental effects include increased risk of birth defects, low birth weight, and preterm birth due to exposure to air pollutants during pregnancy
• Acid rain occurs when sulfur dioxide and nitrogen oxides react with water and oxygen in the atmosphere, damaging ecosystems, buildings, and monuments
• Eutrophication is the excessive growth of algae in water bodies due to increased nitrogen input from air pollution, leading to oxygen depletion and fish kills
• Climate change is exacerbated by greenhouse gases emitted from human activities, leading to rising temperatures, sea levels, and extreme weather events
  • Black carbon (soot) also contributes to climate change by absorbing solar radiation and reducing the reflectivity of snow and ice surfaces

Pollution Control and Mitigation Strategies

• Emission standards set limits on the amount of pollutants that can be released from specific sources (vehicles, industrial facilities)
• Clean Air Act Amendments of 1990 strengthened the U.S. air quality regulations, introducing market-based approaches (cap-and-trade) and technology-forcing standards
• Cleaner fuels and technologies reduce emissions from transportation and industrial sources (low-sulfur diesel, electric vehicles, renewable energy)
• Best available control technology (BACT) is required for new or modified major sources in attainment areas to minimize emissions
• Lowest achievable emission rate (LAER) is required for new or modified major sources in non-attainment areas to ensure the most stringent emission control
• Pollution prevention focuses on reducing or eliminating pollution at the source through process modifications, material substitution, and improved efficiency
• Urban planning and land use strategies aim to reduce emissions by promoting compact development, mixed-use neighborhoods, and alternative transportation modes
• International agreements and protocols address transboundary air pollution issues and set targets for emission reductions (Convention on Long-Range Transboundary Air Pollution, Montreal Protocol, Paris Agreement)