🍂Environmental Chemistry II Unit 3 – Tropospheric Pollution & Photochemical Smog

Key Concepts and Definitions

• Troposphere the lowest layer of Earth's atmosphere, extending from the surface to an average height of 10-12 km
• Photochemical smog a type of air pollution formed by the reaction of nitrogen oxides (NOx) and volatile organic compounds (VOCs) in the presence of sunlight
• Ozone (O3) a key component of photochemical smog, formed by the reaction of NOx and VOCs in the presence of sunlight
  • Ground-level ozone is a major air pollutant and can cause respiratory issues
• Nitrogen oxides (NOx) a group of highly reactive gases, primarily nitric oxide (NO) and nitrogen dioxide (NO2), that contribute to the formation of photochemical smog
• Volatile organic compounds (VOCs) organic chemicals that easily evaporate at room temperature and participate in photochemical reactions
  • Examples include benzene, formaldehyde, and methane
• Hydroxyl radical (OH•) a highly reactive molecule that plays a crucial role in the formation of photochemical smog by initiating the oxidation of VOCs
• Peroxy radicals (RO2•) formed during the oxidation of VOCs and contribute to the formation of ozone and other secondary pollutants

Composition of the Troposphere

• Nitrogen (N2) the most abundant gas in the troposphere, making up about 78% of the air by volume
• Oxygen (O2) the second most abundant gas, comprising approximately 21% of the troposphere
• Water vapor (H2O) varies in concentration depending on location and weather conditions, typically ranging from 0.1% to 4% by volume
• Carbon dioxide (CO2) a minor but important component, with a concentration of about 0.04% (400 ppm) and rising due to human activities
• Argon (Ar) a noble gas that makes up about 0.93% of the troposphere
• Trace gases include methane (CH4), nitrous oxide (N2O), and ozone (O3), which play important roles in atmospheric chemistry despite their low concentrations
• Aerosols tiny solid or liquid particles suspended in the air, such as dust, soot, and sea salt, that can affect air quality and climate

Sources of Tropospheric Pollutants

• Fossil fuel combustion the burning of coal, oil, and natural gas in power plants, vehicles, and industrial processes releases NOx, SOx, CO, and particulate matter
• Biomass burning wildfires, agricultural burning, and deforestation contribute to emissions of NOx, VOCs, and particulate matter
• Industrial processes manufacturing, chemical production, and solvent use can release a variety of VOCs and other pollutants
  • Examples include oil refineries, paint factories, and plastic manufacturing plants
• Transportation emissions from cars, trucks, ships, and aircraft include NOx, VOCs, CO, and particulate matter
• Biogenic sources natural emissions from vegetation, such as isoprene and terpenes, which are VOCs that can contribute to ozone formation
• Agricultural activities livestock farming, fertilizer application, and pesticide use can release ammonia (NH3), NOx, and VOCs
• Waste treatment and disposal landfills and wastewater treatment plants can emit methane and other VOCs

Chemistry of Photochemical Smog Formation

• Photolysis of NO2 nitrogen dioxide absorbs sunlight and breaks down into nitric oxide (NO) and atomic oxygen (O), which quickly reacts with molecular oxygen (O2) to form ozone (O3)
  • $NO2 + hν → NO + O$
  • $O + O2 + M → O3 + M$
• VOC oxidation initiated by reaction with hydroxyl radicals (OH•), leading to the formation of peroxy radicals (RO2•) and other reactive intermediates
  • $RH + OH• → R• + H2O$
  • $R• + O2 → RO2•$
• NOx cycling peroxy radicals react with NO to form NO2, which can then undergo photolysis to regenerate NO and produce ozone
  • $RO2• + NO → RO• + NO2$
  • $NO2 + hν → NO + O$
  • $O + O2 + M → O3 + M$
• Ozone accumulation occurs when the rate of ozone formation exceeds the rate of ozone destruction, leading to high concentrations of ground-level ozone
• Secondary pollutant formation reactions between ozone, NOx, and VOCs can produce other harmful pollutants, such as peroxyacetyl nitrate (PAN) and formaldehyde (HCHO)

Environmental and Health Impacts

• Respiratory health effects exposure to ozone and other components of photochemical smog can cause coughing, throat irritation, and lung inflammation
  • Aggravates pre-existing conditions such as asthma and chronic obstructive pulmonary disease (COPD)
• Cardiovascular effects air pollution can increase the risk of heart attacks, strokes, and other cardiovascular problems
• Damage to vegetation ozone can damage plant tissues, reduce photosynthesis, and decrease crop yields
  • Sensitive species include soybeans, wheat, and tomatoes
• Reduced visibility photochemical smog can cause haze and reduce visibility, affecting transportation and outdoor activities
• Climate change some pollutants, such as ozone and black carbon, are short-lived climate forcers that contribute to global warming
• Ecosystem impacts air pollution can alter nutrient cycles, damage forests, and harm aquatic ecosystems through acid deposition and eutrophication
• Economic costs health care expenses, reduced agricultural productivity, and decreased tourism due to poor air quality can have significant economic impacts

Monitoring and Measurement Techniques

• Ground-based monitoring networks stations equipped with instruments to measure concentrations of ozone, NOx, VOCs, and particulate matter
  • Examples include the US EPA's Air Quality System (AQS) and the European Environment Agency's AirBase
• Satellite observations provide global coverage and can measure tropospheric column concentrations of pollutants such as NO2 and formaldehyde
  • Instruments include OMI (Ozone Monitoring Instrument) and TROPOMI (TROPOspheric Monitoring Instrument)
• Aircraft measurements allow for detailed vertical profiling of pollutants and can help validate satellite observations
• Chemical transport models simulate the emission, transport, and chemical transformation of pollutants in the atmosphere
  • Used to predict air quality and assess the effectiveness of control strategies
• Emission inventories databases that estimate the amount and sources of pollutant emissions, used as inputs for air quality models
• Source apportionment techniques used to identify the relative contributions of different emission sources to observed pollutant concentrations
  • Examples include positive matrix factorization (PMF) and chemical mass balance (CMB)

Control Strategies and Regulations

• Emission standards set limits on the amount of pollutants that can be released from specific sources, such as vehicles and industrial facilities
  • Examples include the US EPA's National Ambient Air Quality Standards (NAAQS) and the EU's Euro emission standards for vehicles
• Fuel quality improvements reducing the sulfur content of diesel fuel or promoting the use of cleaner alternatives like natural gas and electricity
• Industrial process controls installing pollution control devices (scrubbers, electrostatic precipitators) or implementing best management practices to reduce emissions
• Transportation management strategies promoting public transit, carpooling, and active transportation to reduce vehicle emissions
  • Implementing congestion pricing or low-emission zones in cities
• Urban planning and land use policies designing cities to minimize the need for motorized transportation and promote green spaces
• International agreements such as the Convention on Long-Range Transboundary Air Pollution (CLRTAP) and the Gothenburg Protocol, which set emission reduction targets for multiple countries
• Market-based instruments emissions trading schemes, pollution taxes, and incentives for clean technology adoption

Case Studies and Current Research

• Los Angeles, California known for its history of severe photochemical smog, the city has implemented strict emission controls and seen significant improvements in air quality
  • Ongoing challenges include the high volume of vehicle traffic and the city's geography, which can trap pollutants
• Beijing, China rapid industrialization and urbanization have led to severe air pollution, including high levels of ozone and particulate matter
  • Recent efforts to control emissions from coal-fired power plants and promote electric vehicles have shown some success
• Tropospheric ozone trends studies have shown that background ozone levels have increased in many regions due to global emissions of precursors
  • Research is ongoing to understand the relative contributions of natural and anthropogenic sources
• Interaction between air quality and climate change investigating the feedback between air pollutants and climate forcers, such as the impact of methane emissions on ozone formation
• Satellite-based emission estimates using remote sensing data to improve the accuracy and resolution of emission inventories, particularly for regions with limited ground-based monitoring
• Health impact assessments quantifying the public health burden of air pollution and evaluating the benefits of emission reduction strategies
  • Focus on vulnerable populations, such as children, the elderly, and low-income communities
• Development of low-cost sensors increasing the spatial and temporal resolution of air quality monitoring networks through the deployment of affordable, portable sensors
  • Challenges include ensuring data quality and integrating sensor data with traditional monitoring methods