6.1 Chemical composition of the atmosphere

Major atmospheric constituents

Earth's atmosphere is a mixture of gases and particles, and the major constituents account for over 99% of its volume. These gases form the baseline against which all trace species, chemical reactions, and energy transfer processes operate.

Nitrogen and oxygen

Nitrogen ($N_2$) makes up about 78% of the atmosphere by volume. Its triple bond ($N \equiv N$) makes it chemically inert under most atmospheric conditions, which is why it acts largely as a passive background gas rather than participating in everyday atmospheric reactions.

Oxygen ($O_2$) accounts for roughly 21%. It's essential for respiration and combustion, and it's produced primarily through photosynthesis by plants and marine phytoplankton. Oxygen also participates directly in atmospheric chemistry, particularly in ozone formation.

Argon and other noble gases

Argon ($Ar$) is the third most abundant atmospheric gas at about 0.93%. It's produced by the radioactive decay of potassium-40 ($^{40}K$) in Earth's crust. Other noble gases (helium, neon, krypton, xenon) are present in much smaller trace amounts.

Because noble gases are chemically inert, they're valuable as tracers for studying atmospheric processes. Their ratios provide information about atmospheric evolution, mixing, and exchange between reservoirs.

Carbon dioxide

Carbon dioxide ($CO_2$) makes up about 0.04% of the atmosphere, but its role far exceeds its concentration. It's a key greenhouse gas that absorbs and re-emits infrared radiation, directly influencing Earth's energy budget.

• Atmospheric $CO_2$ has risen from ~280 ppm in pre-industrial times to over 420 ppm today, primarily due to fossil fuel combustion and deforestation
• $CO_2$ variations correlate closely with global temperature changes over geological timescales, as shown in ice core records
• It's a central component of the global carbon cycle, exchanged continuously between the atmosphere, oceans, biosphere, and lithosphere

Trace gases

Trace gases exist at very low concentrations, yet they drive much of atmospheric chemistry and have outsized effects on climate. Many act as catalysts, absorbers of radiation, or precursors to secondary pollutants.

Water vapor

Water vapor is the most variable gas in the atmosphere, ranging from nearly 0% in cold, dry regions to about 4% in warm, humid tropical air. This variability makes it unique among atmospheric constituents.

• It's the strongest greenhouse gas, responsible for the majority of Earth's natural greenhouse effect
• It's the primary source of latent heat in the atmosphere: when water vapor condenses, it releases energy that drives weather systems
• It forms clouds and precipitation, making it central to the hydrological cycle
• Its concentration depends strongly on temperature (governed by the Clausius-Clapeyron relation), so it varies enormously with location and season

Ozone

Ozone ($O_3$) exists in two distinct atmospheric layers, and context matters:

• Stratospheric ozone ("good ozone") absorbs harmful UV-B and UV-C radiation, protecting life at the surface. It forms through photochemical reactions: UV light splits $O_2$ into oxygen atoms, which then combine with other $O_2$ molecules to form $O_3$
• Tropospheric ozone ("bad ozone") is a pollutant and greenhouse gas. It forms near the surface through reactions involving nitrogen oxides ($NO_x$) and volatile organic compounds (VOCs) in the presence of sunlight

Ozone concentrations are affected by both natural photochemistry and human activities, particularly CFC emissions (stratosphere) and $NO_x$ emissions (troposphere).

Methane and other hydrocarbons

Methane ($CH_4$) is a potent greenhouse gas with a global warming potential about 28 times that of $CO_2$ over a 100-year horizon. Its sources include wetlands, rice paddies, livestock, landfills, and fossil fuel extraction.

Other atmospheric hydrocarbons include ethane, propane, and biogenic compounds like isoprene (emitted by vegetation). These play important roles in tropospheric chemistry:

• They react with $OH$ radicals, influencing the atmosphere's oxidizing capacity
• Some serve as precursors to secondary organic aerosols
• They contribute to the formation of tropospheric ozone through $NO_x$-VOC photochemistry

Vertical structure of composition

The atmosphere's composition isn't uniform with altitude. Different physical and chemical processes dominate at different heights, creating a layered structure that matters for remote sensing, modeling, and understanding energy transfer.

Homosphere vs heterosphere

The homosphere extends from the surface to about 100 km altitude (the turbopause). Within this region, turbulent mixing keeps the major gas mixing ratios roughly constant. $N_2$, $O_2$, and $Ar$ maintain nearly the same proportions whether you're at sea level or 80 km up.

Above 100 km lies the heterosphere, where molecular diffusion dominates over turbulent mixing. Here, gases separate by molecular weight: lighter species like atomic hydrogen ($H$) and helium ($He$) become increasingly abundant at higher altitudes, while heavier molecules concentrate lower down. Photochemical processes also become important, breaking apart molecules like $O_2$ and $N_2$.

Mixing ratios and partial pressures

Two key quantities describe gas abundance in the atmosphere:

• Mixing ratio (or mole fraction): the number of moles of a gas per mole of total air. For long-lived gases in the homosphere, mixing ratios stay constant with altitude even as total pressure drops. This makes mixing ratio a convenient, conserved quantity.
• Partial pressure: the pressure contribution of a single gas in the mixture, calculated using Dalton's Law:

$P_i = x_i \cdot P_{total}$

where $P_i$ is the partial pressure, $x_i$ is the mixing ratio, and $P_{total}$ is the total atmospheric pressure. Unlike mixing ratios, partial pressures decrease with altitude because total pressure decreases.

Atmospheric aerosols

Aerosols are solid or liquid particles suspended in the atmosphere, ranging from nanometers to tens of micrometers in size. They influence climate by scattering and absorbing radiation (direct effect) and by serving as cloud condensation nuclei that modify cloud properties (indirect effect). They also degrade air quality and affect human health.

Types and sources

Aerosol sources fall into two broad categories:

• Natural: volcanic eruptions, mineral dust storms, sea spray, wildfires, biogenic emissions (pollen, spores, bacteria)
• Anthropogenic: industrial emissions, vehicle exhaust, biomass burning, agricultural activities

Aerosols are also classified by how they form:

• Primary aerosols are emitted directly as particles (dust, soot, sea salt)
• Secondary aerosols form in the atmosphere through gas-to-particle conversion. For example, $SO_2$ oxidizes to sulfuric acid, which then nucleates into sulfate aerosol particles.

Size distribution

Aerosol sizes span several orders of magnitude and are typically grouped into three modes:

Size distribution matters because it controls aerosol lifetime, optical properties, and health effects. Smaller particles remain suspended longer and can penetrate deep into the lungs. Accumulation-mode particles are most efficient at scattering visible light and are therefore most important for climate effects.

Chemical composition

Aerosol composition varies widely depending on source and atmospheric processing. Major chemical components include:

• Sulfates and nitrates (from oxidation of $SO_2$ and $NO_x$)
• Ammonium (from $NH_3$ neutralization of acids)
• Organic carbon (from both primary emissions and secondary formation)
• Elemental carbon / black carbon (from incomplete combustion)
• Mineral dust (from wind erosion of soils)

Composition determines an aerosol's hygroscopicity (how readily it takes up water) and its optical properties (whether it primarily scatters or absorbs light). Analytical techniques like mass spectrometry, ion chromatography, and X-ray fluorescence are used to characterize aerosol chemistry.

Atmospheric chemistry

Atmospheric chemistry governs how gases and particles interact, transform, and are ultimately removed from the atmosphere. These processes control air quality, the lifetime of greenhouse gases, and the formation of secondary pollutants.

Gas-phase reactions

Most atmospheric chemistry occurs in the gas phase. Two main reaction types dominate:

• Bimolecular reactions: $A + B \rightarrow \text{products}$
• Termolecular reactions: $A + B + M \rightarrow \text{products} + M$, where $M$ is a third body (usually $N_2$ or $O_2$) that carries away excess energy

Rate constants typically depend on temperature and, for termolecular reactions, on pressure. The hydroxyl radical ($OH$) is the single most important reactive species in the troposphere, initiating the oxidation of most trace gases. For example:

$CH_4 + OH \rightarrow CH_3 + H_2O$

Chain reactions involving free radicals are central to both ozone formation and destruction cycles.

Photochemistry

Photochemistry refers to reactions initiated by the absorption of a photon. Solar radiation drives many of the atmosphere's most important processes by breaking chemical bonds and producing reactive species (photolysis).

A key example is ozone photolysis in the stratosphere:

$O_3 + h\nu \rightarrow O_2 + O(^1D)$

The excited oxygen atom $O(^1D)$ can then react with water vapor to produce $OH$ radicals, linking stratospheric photochemistry to tropospheric oxidation. In urban areas, photochemistry drives the formation of photochemical smog through a complex cycle involving $NO_x$, VOCs, and sunlight.

Oxidation processes

The atmosphere functions as a giant oxidation reactor. The $OH$ radical is the primary tropospheric oxidant, and it's responsible for removing most pollutants and trace gases.

Oxidation generally converts gases into more water-soluble compounds, which can then be removed by precipitation (wet deposition). A classic example is the oxidation of $SO_2$ to sulfuric acid:

1. $SO_2 + OH \rightarrow HOSO_2$
2. $HOSO_2 + O_2 \rightarrow HO_2 + SO_3$
3. $SO_3 + H_2O \rightarrow H_2SO_4$

The resulting sulfuric acid contributes to acid rain and forms sulfate aerosols. This pathway also illustrates how gas-phase chemistry connects to aerosol formation.

Biogeochemical cycles

Biogeochemical cycles describe how elements move between the atmosphere, biosphere, hydrosphere, geosphere, and lithosphere. Atmospheric composition is both a driver and a product of these cycles.

Carbon cycle

The carbon cycle tracks carbon as it moves between the atmosphere, land, oceans, and Earth's interior.

• Atmospheric reservoir: ~800 Gt of carbon (as $CO_2$)
• Terrestrial biosphere: ~2000 Gt
• Oceans: ~38,000 Gt (the largest active reservoir)
• Fossil fuels: ~10,000 Gt

Natural fluxes include photosynthesis (which removes $CO_2$), respiration and decomposition (which release it), and ocean-atmosphere gas exchange. Human activities have disrupted the balance: fossil fuel combustion and deforestation add roughly 10 Gt of carbon per year to the atmosphere, driving $CO_2$ from ~280 ppm pre-industrial to over 420 ppm today.

Nitrogen cycle

Nitrogen cycles through the atmosphere primarily as $N_2$, which is by far the largest nitrogen reservoir. The key transformations are:

• Nitrogen fixation: converts $N_2$ into biologically available forms ($NH_3$, $NO_3^-$), done by certain bacteria and, industrially, by the Haber-Bosch process
• Nitrification: microbial oxidation of $NH_3$ to $NO_3^-$
• Denitrification: microbial reduction of $NO_3^-$ back to $N_2$, returning nitrogen to the atmosphere

Human activities have roughly doubled the rate of reactive nitrogen entering the biosphere, mainly through synthetic fertilizer production and fossil fuel combustion. Atmospheric nitrogen compounds ($NO_x$, $NH_3$, $N_2O$) affect air quality, contribute to acid deposition, and $N_2O$ is a potent greenhouse gas.

Sulfur cycle

The sulfur cycle involves movement of sulfur compounds through the Earth system.

• Natural sources: volcanic emissions ($SO_2$, $H_2S$) and marine phytoplankton that produce dimethyl sulfide (DMS), which oxidizes to form sulfate aerosols
• Anthropogenic sources: fossil fuel combustion and metal smelting, which emit $SO_2$

Sulfate aerosols have a net cooling effect on climate through both direct scattering of sunlight and indirect effects on cloud properties. However, $SO_2$ emissions also cause acid deposition (acid rain), which damages ecosystems, soils, and infrastructure.

Anthropogenic influences

Human activities have measurably altered atmospheric composition over the past two centuries. These changes affect air quality, climate, and ecosystems on local to global scales.

Air pollution

Air pollution results from the emission of harmful substances into the atmosphere. The major pollutants are:

• Particulate matter ($PM_{2.5}$, $PM_{10}$): from combustion, dust, and secondary formation
• Tropospheric ozone ($O_3$): formed photochemically from $NO_x$ and VOC precursors
• Nitrogen oxides ($NO_x$): from vehicle engines and power plants
• Sulfur dioxide ($SO_2$): from coal combustion and industrial processes
• Carbon monoxide ($CO$): from incomplete combustion

These pollutants harm human health, damage ecosystems, and reduce visibility. In urban areas, $NO_x$ and VOCs react in sunlight to produce photochemical smog. Mitigation strategies include emission controls (catalytic converters, scrubbers), cleaner fuels, and urban planning to reduce traffic congestion.

Greenhouse gas emissions

The primary anthropogenic greenhouse gases are $CO_2$, $CH_4$, $N_2O$, and halocarbons (HFCs, PFCs, $SF_6$). Their atmospheric concentrations have increased substantially since the Industrial Revolution, driven by fossil fuel combustion, agriculture, and industrial processes.

The enhanced greenhouse effect from these elevated concentrations leads to global warming and associated climate impacts: sea-level rise, more frequent extreme weather events, and ecosystem disruption. International agreements like the Paris Agreement aim to limit global temperature increase to well below 2°C above pre-industrial levels.

Ozone depletion

Stratospheric ozone depletion is caused primarily by chlorofluorocarbons (CFCs) and halons. These long-lived compounds are transported to the stratosphere, where UV radiation releases chlorine and bromine atoms that catalytically destroy ozone. A single chlorine atom can destroy thousands of ozone molecules.

The Antarctic ozone hole forms each austral spring (September–October) due to the unique polar vortex conditions that concentrate ozone-depleting chemistry on polar stratospheric cloud surfaces. The Montreal Protocol (1987) successfully phased out production of most ozone-depleting substances, and the ozone layer is expected to recover by the mid-to-late 21st century. This remains one of the most successful examples of international environmental cooperation.

Measurement techniques

Measuring atmospheric composition accurately is essential for understanding chemical processes, validating models, and tracking changes over time. Techniques range from direct sampling to satellite-based remote sensing.

In-situ sampling

In-situ methods involve collecting and analyzing air directly at the measurement location. They provide high accuracy and precision but are limited to specific sites.

• Gas chromatography and mass spectrometry identify and quantify trace gases
• Optical spectroscopy measures absorption or fluorescence of specific species
• Flask sampling collects air in containers for later laboratory analysis (used in global monitoring networks like NOAA's)
• Continuous analyzers provide high time-resolution data at fixed stations
• Aircraft and balloon-borne instruments enable vertical profiling of composition through the atmosphere

Remote sensing methods

Remote sensing measures atmospheric properties from a distance using electromagnetic radiation. These methods trade some precision for much broader spatial coverage.

• Passive remote sensing relies on natural radiation sources (reflected sunlight, Earth's thermal emission)
• Active remote sensing (lidar, radar) emits its own radiation and measures the returned signal
• Spectroscopic techniques like FTIR (Fourier Transform Infrared) and DOAS (Differential Optical Absorption Spectroscopy) measure absorption features of specific gases
• Ground-based, airborne, and satellite platforms all use remote sensing approaches

Satellite observations

Satellites provide global coverage and continuous long-term monitoring that no ground network can match.

• Key missions include NASA's Aura, ESA's Sentinel-5P (TROPOMI instrument), and JAXA's GOSAT
• These instruments measure reflected solar radiation and emitted thermal radiation to retrieve concentrations of $O_3$, $CO_2$, $CH_4$, $NO_2$, aerosol optical depth, and more
• Data assimilation combines satellite observations with numerical models to produce more complete and accurate atmospheric analyses
• Challenges include cloud interference, limited vertical resolution, and uncertainties in retrieval algorithms

Atmospheric composition models

Numerical models simulate how atmospheric composition evolves over time by integrating emissions, chemistry, transport, and removal processes. They range from simple box models to complex three-dimensional global simulations.

Chemical transport models

Chemical transport models (CTMs) simulate the transport and chemical transformation of trace species. They work by solving continuity equations for each species, accounting for:

1. Emissions from natural and anthropogenic sources
2. Chemical production and loss through gas-phase, photochemical, and heterogeneous reactions
3. Transport by winds (advection) and turbulent mixing
4. Removal by dry deposition and wet deposition (washout by precipitation)

CTMs are driven by meteorological fields from weather models or reanalyses. Widely used examples include GEOS-Chem, WRF-Chem, and CMAQ. They can run in forward mode (predicting concentrations from known emissions) or inverse mode (using observed concentrations to constrain emission estimates).

Global climate models

Global climate models (also called Earth System Models) simulate the full coupled system: atmosphere, ocean, land surface, and cryosphere. Modern versions include atmospheric chemistry modules of varying complexity.

• They couple composition changes with radiation, dynamics, and biogeochemical cycles, capturing important feedbacks (e.g., warming increases water vapor, which increases warming)
• Used for long-term climate projections on decadal to centennial timescales
• Examples: CESM, GFDL-ESM, HadGEM
• Major challenges include computational cost and representing processes that occur at scales smaller than the model grid (sub-grid parameterization)
• Ensemble simulations (running the model many times with slightly different conditions) help quantify uncertainty in projections

Temporal variations

Atmospheric composition changes on timescales from hours to millennia. Recognizing these patterns helps you interpret measurements correctly and distinguish natural variability from long-term trends.

Diurnal changes

Daily cycles are driven by the rise and fall of solar radiation and by human activity patterns.

• Photochemistry peaks during daytime: $O_3$ and $OH$ concentrations increase as sunlight drives photolysis reactions
• Nighttime chemistry is dominated by the $NO_3$ radical and $N_2O_5$, since $NO_3$ is rapidly destroyed by sunlight during the day
• Urban pollutants like $NO_x$ and $CO$ show morning and evening rush-hour peaks
• The planetary boundary layer grows during the day (mixing pollutants through a deeper layer) and collapses at night (trapping pollutants near the surface), causing large swings in surface concentrations
• Biogenic emissions like isoprene are strongly temperature- and light-dependent, peaking in the afternoon

Seasonal fluctuations

Seasonal cycles reflect changes in solar radiation, temperature, and biological activity.

• $CO_2$ shows a clear annual cycle: concentrations drop during Northern Hemisphere summer as vegetation absorbs $CO_2$ through photosynthesis, then rise in winter as respiration and decomposition dominate. This cycle is stronger in the Northern Hemisphere because it has more land area and vegetation.
• Ozone varies seasonally with UV radiation intensity and precursor emission patterns
• Dust and biomass burning aerosols follow strong seasonal patterns tied to dry seasons and agricultural practices
• Water vapor peaks in summer due to higher temperatures and evaporation rates

Recognizing seasonal cycles is important because you need to remove them to detect underlying long-term trends.

Long-term trends

Long-term trends reflect sustained changes in emissions, land use, and climate feedbacks over years to decades.

• Greenhouse gases ($CO_2$, $CH_4$, $N_2O$) show persistent upward trends driven by human activities
• Ozone-depleting substances show declining trends following the Montreal Protocol
• Aerosol concentrations vary regionally: decreasing in North America and Europe due to emission controls, but increasing in parts of Asia and Africa due to industrialization
• Long-term monitoring datasets (e.g., Mauna Loa $CO_2$ record, starting 1958) are invaluable for detecting and attributing these changes
• Satellite observations complement ground-based records by providing a global perspective on decadal trends

Spatial variations

Atmospheric composition varies across space at every scale, from meters (near a smokestack) to hemispheric gradients. These patterns reflect the distribution of sources and sinks, atmospheric transport, and removal processes.

Latitude dependence

Many trace gases and aerosols show strong latitudinal gradients.

• $CO_2$ and $CH_4$ concentrations are higher in the Northern Hemisphere because most anthropogenic sources are located there
• Ozone column abundance peaks at mid-latitudes and is lowest over the tropics (where strong upwelling brings ozone-poor air from the troposphere into the stratosphere)
• Water vapor generally decreases from equator to poles, following the temperature gradient
• These gradients are shaped by large-scale circulation patterns: Hadley cells redistribute air in the tropics, while the jet streams and planetary waves mix air at mid-latitudes

Urban vs rural differences

Urban and rural areas have distinctly different atmospheric compositions.

• Cities have elevated $NO_x$, $CO$, and particulate matter from vehicle emissions, industry, and heating
• Tropospheric ozone is often lower in city centers (where fresh $NO$ emissions titrate $O_3$) but higher in suburban and rural areas downwind, where $NO_x$-VOC photochemistry has had time to produce $O_3$
• The urban heat island effect raises temperatures in cities, which can enhance photochemical reaction rates and alter local meteorology
• Rural areas may have higher biogenic VOC emissions from forests and vegetation

Understanding these urban-rural gradients is critical for air quality management and for assessing health impacts of pollution exposure.

Marine vs continental air

Air masses originating over oceans versus continents have very different chemical signatures.

• Marine air is generally cleaner, with lower particulate matter and $NO_x$ concentrations, but higher sea salt aerosol content
• Continental air carries more mineral dust, anthropogenic pollutants, and typically higher ozone levels
• Marine phytoplankton emit dimethyl sulfide (DMS), which oxidizes to form sulfate aerosols that influence cloud formation over the oceans
• The chemical regime in the marine boundary layer differs from continental regions: lower $NO_x$ means different photochemical pathways dominate

Identifying whether an air mass is marine or continental in origin is a standard first step when interpreting atmospheric measurements at coastal or island stations.