🔆Environmental Chemistry I Unit 2 – Atmospheric Composition and Structure

Key Components of Earth's Atmosphere

• Nitrogen (N2) the most abundant gas in the atmosphere at 78% by volume, essential for life as a component of amino acids and proteins
• Oxygen (O2) the second most abundant gas at 21%, vital for respiration in many organisms and plays a crucial role in combustion and oxidation reactions
• Argon (Ar) a noble gas that makes up ~1% of the atmosphere, does not participate in chemical reactions due to its stable electron configuration
• Carbon dioxide (CO2) a trace gas at ~0.04%, plays a significant role in the greenhouse effect and is essential for photosynthesis in plants
  • Atmospheric CO2 levels have increased from ~280 ppm in pre-industrial times to over 400 ppm today due to human activities (fossil fuel combustion, deforestation)
• Water vapor (H2O) varies in concentration from 0.1-4% depending on location and weather conditions, contributes to the greenhouse effect and plays a crucial role in the water cycle
  • Acts as a solvent for chemical reactions and facilitates the formation of clouds and precipitation
• Trace gases include methane (CH4), nitrous oxide (N2O), and ozone (O3), which have significant impacts on climate and atmospheric chemistry despite their low concentrations

Layers of the Atmosphere

• Troposphere the lowest layer, extends from the Earth's surface to ~8-16 km depending on latitude, contains ~75% of the atmosphere's mass and nearly all of its water vapor
  • Characterized by a decrease in temperature with increasing altitude at a rate of ~6.5°C/km (lapse rate)
  • Most weather phenomena (clouds, precipitation) occur in this layer
• Stratosphere extends from the tropopause to ~50 km, contains the ozone layer which absorbs harmful UV radiation from the sun
  • Temperature increases with altitude in this layer due to absorption of UV radiation by ozone
• Mesosphere extends from the stratopause to ~85 km, characterized by a decrease in temperature with altitude
  • The coldest temperatures in the atmosphere occur at the top of this layer (mesopause) at around -90°C
  • Meteors typically burn up in this layer due to friction with the atmosphere
• Thermosphere extends from the mesopause to ~500-1000 km, characterized by a significant increase in temperature with altitude due to absorption of high-energy radiation (UV, X-rays)
  • Temperatures can reach up to 2000°C, but the air is extremely thin, so it would not feel hot to a human
  • The aurora (Northern and Southern Lights) occur in this layer due to collisions between charged particles and atmospheric gases
• Exosphere the outermost layer, extends from the thermopause to ~10,000 km, where the atmosphere gradually fades into space
  • Composed mainly of hydrogen and helium atoms, which can escape Earth's gravity due to their low mass and high velocities

Atmospheric Pressure and Density

• Atmospheric pressure the force exerted by the weight of the atmosphere on a unit area, decreases with increasing altitude as the amount of air above decreases
  • At sea level, the average atmospheric pressure is ~101.3 kPa (1 atm or 760 mmHg)
  • Pressure decreases exponentially with altitude, with half of the atmosphere's mass located below ~5.5 km
• Density the mass of air per unit volume, also decreases with increasing altitude due to the compressibility of gases under the force of gravity
  • At sea level, the average density of air is ~1.225 kg/m³
  • Density is affected by temperature, with warmer air being less dense than colder air at the same pressure
• The relationship between pressure, density, and temperature is described by the ideal gas law: $PV = nRT$, where $P$ is pressure, $V$ is volume, $n$ is the number of moles, $R$ is the universal gas constant, and $T$ is temperature
• Pressure and density variations with altitude have significant implications for aviation, as aircraft performance depends on the density of the air
  • At higher altitudes, the lower density of air requires aircraft to fly at higher speeds to generate sufficient lift
• Atmospheric pressure differences drive wind patterns, as air flows from high-pressure areas to low-pressure areas
  • The Coriolis effect, caused by Earth's rotation, deflects wind to the right in the Northern Hemisphere and to the left in the Southern Hemisphere

Temperature Variations in the Atmosphere

• Temperature in the atmosphere varies with altitude, latitude, and season, primarily due to differences in solar radiation absorption and Earth's surface characteristics
• The troposphere experiences a decrease in temperature with altitude (lapse rate) due to adiabatic cooling as air expands and rises
  • The average lapse rate is ~6.5°C/km, but can vary depending on moisture content and other factors
• The stratosphere has an inverted temperature profile, with temperature increasing with altitude due to absorption of UV radiation by the ozone layer
  • This temperature inversion creates a stable layer that limits vertical mixing between the troposphere and stratosphere
• Mesosphere and thermosphere have temperature profiles that are influenced by the absorption of high-energy radiation and the low density of the air
• Latitudinal temperature variations are primarily driven by differences in the angle of incoming solar radiation and the duration of daylight hours
  • Equatorial regions receive more direct sunlight and have smaller seasonal variations in temperature compared to polar regions
• Seasonal temperature variations are caused by Earth's tilt and its orbit around the sun, which affects the amount of solar radiation received at different latitudes throughout the year
  • The Northern Hemisphere experiences summer during the months of June, July, and August, when it is tilted towards the sun, and winter during December, January, and February, when it is tilted away from the sun
• Urban heat islands are local temperature anomalies where cities experience higher temperatures than surrounding rural areas due to the absorption and re-emission of heat by buildings and pavement
  • This effect can exacerbate heat waves and increase energy consumption for cooling

Major Chemical Reactions in the Atmosphere

• Photochemical reactions initiated by the absorption of solar radiation, play a crucial role in the formation and destruction of atmospheric compounds
  • Photodissociation the breakdown of molecules into smaller fragments upon absorbing high-energy photons (UV, visible light)
    • Example: the photodissociation of ozone ($O_3$) into oxygen molecules ($O_2$) and atomic oxygen ($O$) by UV radiation
• Oxidation reactions involve the transfer of electrons from one molecule to another, often facilitated by the presence of hydroxyl radicals ($OH$) or ozone ($O_3$)
  • Oxidation of volatile organic compounds (VOCs) in the presence of nitrogen oxides ($NO_x$) and sunlight leads to the formation of tropospheric ozone, a major component of photochemical smog
• Acid-base reactions involve the transfer of protons ($H^+$) between molecules, influencing the acidity of atmospheric water droplets and precipitation
  • The dissolution of carbon dioxide ($CO_2$) in water forms carbonic acid ($H_2CO_3$), contributing to the natural acidity of rainwater ($pH \approx 5.6$)
  • Sulfur dioxide ($SO_2$) and nitrogen oxides ($NO_x$) from anthropogenic sources can further increase the acidity of precipitation, leading to acid rain
• Condensation and evaporation processes govern the formation and dissipation of clouds and fog, which play a significant role in the Earth's radiative balance and water cycle
  • Cloud droplets form when water vapor condenses onto atmospheric aerosol particles (cloud condensation nuclei) under suitable temperature and humidity conditions
• Adsorption and desorption reactions involve the attachment and release of molecules from the surface of atmospheric particles, influencing their chemical composition and reactivity
  • The adsorption of water vapor onto hygroscopic aerosol particles (sea salt, sulfates) can lead to the formation of haze and reduced visibility
• Photolysis reactions involve the breaking of chemical bonds by the absorption of photons, often initiating complex reaction chains in the atmosphere
  • The photolysis of nitrogen dioxide ($NO_2$) by sunlight produces nitric oxide ($NO$) and atomic oxygen ($O$), which can then react with other molecules to form ozone and other secondary pollutants

Atmospheric Pollutants and Their Sources

• Primary pollutants emitted directly from sources, such as carbon monoxide (CO), sulfur dioxide (SO2), nitrogen oxides (NOx), and particulate matter (PM)
  • CO is primarily produced by incomplete combustion of fossil fuels and biomass, with transportation and industrial processes being major sources
  • SO2 is mainly emitted from the burning of sulfur-containing fuels (coal, oil) and industrial processes (metal smelting)
  • NOx (NO and NO2) are formed during high-temperature combustion processes, with transportation and power generation being significant contributors
  • PM can be directly emitted from sources such as dust, soot, and sea salt, or formed through chemical reactions in the atmosphere
• Secondary pollutants formed in the atmosphere through chemical reactions involving primary pollutants and other atmospheric constituents
  • Tropospheric ozone (O3) formed by the reaction of NOx and volatile organic compounds (VOCs) in the presence of sunlight, a major component of photochemical smog
  • Secondary particulate matter formed through the oxidation and condensation of gaseous precursors (SO2, NOx, VOCs) or the reaction of acidic gases with ammonia to form fine particles (sulfates, nitrates)
• Natural sources of atmospheric pollutants include volcanic eruptions (SO2, PM), wildfires (CO, PM), and biogenic emissions from vegetation (VOCs)
  • These sources can contribute significantly to regional and global atmospheric composition, but their emissions are often sporadic and difficult to control
• Anthropogenic sources, related to human activities, are the primary contributors to air pollution in urban and industrial areas
  • Fossil fuel combustion for energy production, transportation, and industrial processes releases a wide range of pollutants (CO, SO2, NOx, PM)
  • Agricultural activities, such as livestock farming and fertilizer application, emit ammonia (NH3) and methane (CH4)
  • Waste management practices, including landfills and wastewater treatment, can release methane and other volatile organic compounds
• Indoor air pollution can be caused by a variety of sources, including cooking and heating appliances, building materials, and consumer products
  • Household combustion of solid fuels (wood, coal) for cooking and heating is a major source of indoor air pollution in developing countries, contributing to respiratory health problems

Climate Impact of Atmospheric Composition

• Greenhouse gases (GHGs) absorb and re-emit infrared radiation, trapping heat in the atmosphere and contributing to global warming
  • Carbon dioxide (CO2) the most significant anthropogenic GHG, primarily due to fossil fuel combustion and deforestation
  • Methane (CH4) a potent GHG with a global warming potential ~28 times that of CO2 over a 100-year timeframe, emitted from natural (wetlands) and anthropogenic (agriculture, landfills) sources
  • Nitrous oxide (N2O) a long-lived GHG with a global warming potential ~265 times that of CO2, primarily from agricultural activities (fertilizer use) and industrial processes
  • Water vapor (H2O) the most abundant GHG, but its atmospheric concentration is not directly affected by human activities
• Aerosols can have both cooling and warming effects on the climate, depending on their composition and optical properties
  • Sulfate aerosols from volcanic eruptions and anthropogenic SO2 emissions scatter incoming solar radiation, leading to a cooling effect
  • Black carbon (soot) aerosols from incomplete combustion absorb solar radiation and can contribute to warming when deposited on snow and ice surfaces
• Ozone (O3) acts as a GHG in the troposphere, contributing to warming, but also absorbs harmful UV radiation in the stratosphere, protecting life on Earth
  • Depletion of stratospheric ozone by chlorofluorocarbons (CFCs) and other ozone-depleting substances (ODS) has led to the formation of the Antarctic ozone hole
• Atmospheric composition influences the Earth's radiative balance, which determines the amount of energy absorbed and emitted by the planet
  • Changes in GHG concentrations and aerosol loading can alter this balance, leading to climate change and associated impacts (sea-level rise, extreme weather events, ecosystem disruption)
• Climate feedbacks can amplify or dampen the initial climate response to changes in atmospheric composition
  • The water vapor feedback is a positive feedback, where warming leads to increased atmospheric water vapor, which further enhances the greenhouse effect
  • The ice-albedo feedback is another positive feedback, where melting of snow and ice due to warming reduces the Earth's albedo (reflectivity), leading to increased absorption of solar radiation and further warming

Measurement Techniques and Instruments

• In-situ measurements involve direct sampling of the atmosphere using instruments located on the ground, on aircraft, or on balloons
  • Gas analyzers measure the concentration of specific gases (CO2, CO, O3) using techniques such as infrared absorption, chemiluminescence, or electrochemical cells
  • Particle counters and sizers determine the number concentration and size distribution of aerosol particles using optical or electrical methods
  • Meteorological sensors measure temperature, pressure, humidity, wind speed, and direction to characterize atmospheric conditions
• Remote sensing techniques use electromagnetic radiation to infer atmospheric composition and properties from a distance
  • Ground-based remote sensing instruments, such as lidar (light detection and ranging) and radar, use active sensing techniques to measure vertical profiles of aerosols, clouds, and wind
  • Passive remote sensing instruments, such as sun photometers and Fourier-transform infrared (FTIR) spectrometers, measure the attenuation or emission of radiation by atmospheric constituents
  • Satellite-based instruments provide global coverage and long-term monitoring of atmospheric composition and climate variables
    • Examples include the Moderate Resolution Imaging Spectroradiometer (MODIS) for aerosol optical depth and the Atmospheric Infrared Sounder (AIRS) for temperature and water vapor profiles
• Air sampling techniques involve collecting air samples for later analysis in a laboratory
  • Whole air sampling uses canisters or bags to collect air samples for subsequent analysis of trace gases and volatile organic compounds
  • Filter-based sampling collects particulate matter on filters for gravimetric, chemical, or microscopic analysis
  • Sorbent tubes are used to concentrate and collect specific gases or volatile compounds for later thermal desorption and analysis
• Data analysis and interpretation are crucial for understanding atmospheric composition and its impacts
  • Quality control procedures ensure the accuracy and reliability of measurements, including calibration, intercomparison, and data validation
  • Statistical methods, such as regression analysis and time series analysis, help identify trends, correlations, and variability in atmospheric data
  • Atmospheric models, ranging from simple box models to complex 3D chemical transport models, are used to simulate and predict the behavior of atmospheric constituents and their interactions with climate and ecosystems
• Integration of multiple measurement platforms and techniques provides a comprehensive understanding of atmospheric composition and its spatial and temporal variability
  • Ground-based networks, such as the Global Atmosphere Watch (GAW) programme, coordinate measurements of key atmospheric variables from a global network of stations
  • Field campaigns, involving intensive observations from multiple platforms (ground-based, aircraft, satellite), target specific research questions and regions of interest
  • Data assimilation techniques combine observations with model simulations to provide optimal estimates of atmospheric composition and constrain model uncertainties