🌦️Atmospheric Science Unit 13 – Atmospheric Chemistry & Air Pollution

Key Concepts and Definitions

• Atmospheric chemistry studies chemical processes occurring in Earth's atmosphere including reactions, transport, and removal of gases and particles
• Air pollution refers to the presence of substances in the atmosphere that adversely affect human health and the environment
• Primary pollutants are emitted directly from sources (carbon monoxide, sulfur dioxide, nitrogen oxides)
• Secondary pollutants form in the atmosphere through chemical reactions (ozone, secondary particulate matter)
• Criteria air pollutants are six common pollutants regulated by the U.S. EPA (particulate matter, ozone, carbon monoxide, sulfur dioxide, nitrogen dioxide, lead)
• Volatile organic compounds (VOCs) are organic chemicals that easily evaporate at room temperature and contribute to ozone formation
• Particulate matter (PM) consists of solid particles and liquid droplets suspended in the air classified by size (PM10, PM2.5)
• Photochemical smog is a type of air pollution formed by chemical reactions between nitrogen oxides and VOCs in the presence of sunlight

Atmospheric Composition and Structure

• Earth's atmosphere is a thin layer of gases held by gravity consisting of 78% nitrogen, 21% oxygen, and trace gases
• The atmosphere is divided into layers based on temperature variations with altitude (troposphere, stratosphere, mesosphere, thermosphere)
• The troposphere is the lowest layer extending from the surface to ~10-15 km containing 75-80% of the atmosphere's mass
  • Most weather phenomena and air pollution occur in the troposphere
• The stratosphere extends from the tropopause to ~50 km and contains the ozone layer which absorbs harmful UV radiation
• The tropopause is the boundary between the troposphere and stratosphere marked by a temperature inversion
• Atmospheric pressure decreases exponentially with altitude as the weight of the overlying air decreases
• Temperature in the troposphere generally decreases with altitude at a rate of ~6.5°C/km (environmental lapse rate)
• The atmospheric boundary layer is the lowest part of the troposphere directly influenced by the Earth's surface extending to ~1-2 km

Chemical Processes in the Atmosphere

• Photochemical reactions are initiated by sunlight and play a key role in the formation of air pollutants like ozone and secondary aerosols
• Nitrogen oxide (NO) reacts with ozone (O3) to form nitrogen dioxide (NO2) which can photodissociate to reform NO and atomic oxygen (O)
  • This NO-NO2-O3 cycle is a key component of tropospheric ozone formation
• Volatile organic compounds (VOCs) oxidize in the atmosphere to form peroxy radicals (RO2) which convert NO to NO2 and lead to ozone production
• Sulfur dioxide (SO2) oxidizes in the atmosphere to form sulfuric acid (H2SO4) a major component of acid rain and secondary aerosols
  • Oxidation can occur in the gas phase with hydroxyl radicals (OH) or in the aqueous phase with hydrogen peroxide (H2O2) or ozone
• Ammonia (NH3) neutralizes acids in the atmosphere to form ammonium salts like ammonium sulfate ((NH4)2SO4) and ammonium nitrate (NH4NO3)
• Atmospheric oxidants like hydroxyl radicals (OH), ozone (O3), and nitrate radicals (NO3) initiate the oxidation and removal of many trace gases
• Deposition is the process by which gases and particles are removed from the atmosphere through dry deposition (direct contact) or wet deposition (precipitation)

Major Air Pollutants and Their Sources

• Carbon monoxide (CO) is a colorless, odorless gas emitted from incomplete combustion of fossil fuels (vehicle exhaust, industrial processes)
• Nitrogen oxides (NOx) refer to NO and NO2 formed during high-temperature combustion (power plants, vehicles) and contribute to ozone and acid rain
• Sulfur dioxide (SO2) is emitted from burning sulfur-containing fuels (coal, oil) and industrial processes (metal smelting) and forms acid rain and particulates
• Particulate matter (PM) has both natural sources (dust, sea salt, wildfires) and anthropogenic sources (fossil fuel combustion, industrial processes)
  • Fine particulates (PM2.5) pose the greatest health risk due to their ability to penetrate deep into the lungs
• Ozone (O3) is a secondary pollutant formed by photochemical reactions between NOx and VOCs and is a main component of smog
  • Ground-level ozone is harmful to human health and vegetation, while stratospheric ozone protects against UV radiation
• Volatile organic compounds (VOCs) are emitted from a variety of sources (solvents, paints, vehicles) and contribute to ozone and particulate formation
• Hazardous air pollutants (HAPs) or air toxics are pollutants known or suspected to cause cancer or other serious health effects (benzene, mercury, dioxins)
• Greenhouse gases like carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) absorb infrared radiation and contribute to climate change

Atmospheric Transport and Dispersion

• Wind is the primary mechanism for horizontal transport and dispersion of air pollutants from their sources to receptors
• Atmospheric stability determines the vertical mixing and dispersion of pollutants with stable conditions leading to poor dispersion and high concentrations
  • Stability is determined by the vertical temperature gradient and wind shear
• Temperature inversions occur when temperature increases with altitude trapping pollutants near the surface and leading to high concentrations
  • Inversions can be caused by radiative cooling at night or subsidence of air aloft
• Mixing height is the vertical extent of the atmosphere over which pollutants can be mixed and dispersed, typically reaching a maximum in the afternoon
• Long-range transport can carry pollutants hundreds to thousands of kilometers from their sources crossing regional and national boundaries
  • Examples include Asian dust transport to North America and Saharan dust transport to the Caribbean
• Deposition removes pollutants from the atmosphere through dry deposition (gravitational settling, impaction) or wet deposition (precipitation scavenging)
• Atmospheric lifetime is the average time a pollutant remains in the atmosphere before being removed by chemical reactions or deposition
• Dispersion models use mathematical equations to simulate the transport, dispersion, and removal of air pollutants based on emissions, meteorology, and chemistry

Air Quality Measurement and Monitoring

• Air quality monitoring networks measure concentrations of criteria pollutants and other substances to assess compliance with standards and trends
  • Networks include state and local agencies, national programs (NAAQS), and research-grade measurements
• Sampling methods collect air samples over a period of time for later analysis in a laboratory (filters, canisters, adsorbent tubes)
• Continuous monitors provide real-time measurements of pollutant concentrations using various techniques (chemiluminescence, spectroscopy, mass measurement)
• Passive samplers collect pollutants by diffusion onto a sorbent material over an extended period (weeks to months) without using pumps
• Remote sensing techniques use satellite or ground-based instruments to measure pollutants over a wide spatial area (TROPOMI, MOPITT, OMI)
  • Can provide global coverage but have lower spatial and temporal resolution than ground-based measurements
• Air quality indices (AQI) communicate the level of health concern based on pollutant concentrations using a color-coded scale (good to hazardous)
• Source apportionment uses statistical methods to identify and quantify the contributions of different emission sources to ambient pollutant concentrations
• Data quality assurance and control (QA/QC) ensures the accuracy, precision, and reliability of air quality measurements through calibration, audits, and data validation

Health and Environmental Impacts

• Exposure to air pollution can cause a range of adverse health effects from minor irritation to chronic diseases and premature death
  • Effects depend on the pollutant, concentration, duration, and individual susceptibility
• Short-term exposure to ozone and particulates can aggravate respiratory diseases (asthma), cause reduced lung function, and increase hospital admissions
• Long-term exposure to particulates is associated with increased risk of cardiovascular disease, respiratory disease, and lung cancer
• Vulnerable populations such as children, the elderly, and those with pre-existing conditions are at higher risk for air pollution-related health effects
• Ecosystems can be damaged by air pollution through direct effects on plants (ozone damage) or indirect effects on soil and water (acid rain)
  • Ozone reduces crop yields and forest growth, while acid rain acidifies soils and streams and damages vegetation
• Deposition of nitrogen and sulfur compounds can lead to eutrophication of water bodies stimulating algal blooms and fish kills
• Air pollution impairs visibility by scattering and absorbing light, forming haze that reduces visual range and clarity
  • Regional haze affects scenic vistas in national parks and wilderness areas
• Climate change is influenced by air pollutants that absorb or scatter radiation (greenhouse gases, aerosols) leading to warming or cooling effects
  • Black carbon (soot) absorbs solar radiation and contributes to warming, while sulfate aerosols scatter radiation and have a cooling effect

Air Pollution Control Strategies

• Emission standards set limits on the amount of pollutants that can be emitted from specific sources (vehicles, power plants, industries)
  • Standards are set by national and state agencies based on health and environmental criteria
• Emission control technologies reduce pollutant emissions from sources through physical or chemical processes (filters, scrubbers, catalytic converters)
  • Best Available Control Technology (BACT) is required for new or modified major sources in attainment areas
• Fuel standards regulate the composition and properties of fuels to reduce emissions (low-sulfur diesel, reformulated gasoline)
• Transportation control measures aim to reduce vehicle emissions through improved efficiency, alternative fuels, or reduced travel demand
  • Measures include vehicle inspection and maintenance programs, congestion pricing, and public transit improvements
• Market-based approaches use economic incentives to reduce emissions through tradable permits or emission taxes
  • Cap and trade programs set an overall emission limit and allow sources to trade allowances, providing flexibility and cost-effectiveness
• Urban planning strategies reduce air pollution by promoting compact development, mixed land use, and alternative transportation modes
• International agreements address transboundary air pollution through cooperation and commitments to reduce emissions
  • Examples include the Convention on Long-Range Transboundary Air Pollution (CLRTAP) and the Gothenburg Protocol
• Public education and outreach raise awareness about air pollution sources, health effects, and actions individuals can take to reduce emissions

Current Research and Challenges

• Interactions between air pollution and climate change are complex and not fully understood, requiring further research on feedbacks and co-benefits of mitigation
• Characterizing the health effects of air pollution mixtures and identifying the most harmful components is an ongoing challenge
  • Research is needed on the toxicity and epidemiology of emerging pollutants and the combined effects of multiple pollutants
• Improving the spatial and temporal resolution of air quality monitoring and modeling is necessary for exposure assessment and environmental justice applications
  • Low-cost sensors and mobile monitoring platforms offer opportunities for enhanced data collection and community engagement
• Developing and implementing effective control strategies for multiple pollutants and sources requires coordination across sectors and jurisdictions
  • Integrated assessment models can help evaluate the costs and benefits of different policy scenarios
• Reducing emissions from transportation sources, particularly in developing countries with rapidly growing vehicle fleets, is a major challenge
  • Cleaner fuels, advanced vehicle technologies, and sustainable transportation infrastructure are needed
• Mitigating air pollution impacts on vulnerable populations and ecosystems requires targeted interventions and adaptation strategies
• Communicating air quality information to the public in a clear and actionable manner is essential for protecting public health during pollution episodes
• Capacity building and technology transfer are needed to address air pollution in developing countries with limited resources and expertise