The chemistry of gases, particles and droplets in planetary atmospheres. Chemical conversion competes with radiation, transport, mixing and removal to surfaces, so local equilibrium is not a general assumption.
In the usual atmospheric convention, hydroxyl radical and HO2 form a rapidly interconverting hydrogen-oxide radical family. Some budgets also explicitly include H; state the counting convention. Radical production, recycling and termination differ from the abundance of the water reservoir.
A reactive iodine oxide radical. Its gas-phase abundance need not represent total reactive iodine when particulate reservoirs and multiphase recycling are important.
Changes in temperature, water vapor and radical chemistry alter methane removal. Warming can increase its hydroxyl radical reaction coefficient and the hydroxyl radical source from oxygen singlet D atom plus water, tending to reduce chemical lifetime at fixed precursor conditions. Changed emissions and competing radical sinks can modify or reverse that conditional response.
A local or box-model balance between hydroxyl-radical production and effective losses. For fixed production and pseudo-first-order total loss frequency , . Additional hydrogen, carbon monoxide or methane consumption can lower hydroxyl radical if production and other terms stay fixed; radical recycling and spatial transport can alter that simple result.
In a fixed-production hydroxyl radical box, adding hydrogen by changes to . Methane lifetime scales as when its hydroxyl radical reaction coefficient is held fixed. The pre-existing hydrogen contribution is already in and must not be counted again.
At fixed relative humidity and fixed excited-oxygen concentration, a water-reaction hydroxyl radical source scales as when concentrations are number densities. Thus under constant latent heat and rate coefficient. Using only instead gives a vapor pressure or fixed-pressure mole-fraction approximation. Neither derivative follows without a humidity condition.
A unit of atmospheric column abundance. One Dobson unit corresponds to approximately molecules per square centimetre. Multiplying a column decrement by an absorption cross section gives its contribution to a monochromatic optical-depth decrement.
The radical participates in catalytic atmospheric ozone destruction. Its measured abundance is only one partition of the total bromine inventory; precursor delivery, photodissociation and reservoir reactions determine the relation between the two.
In the productive mixed-radical pathway, chlorine monoxide and BrO yield chlorine and bromine atoms; subsequent atom reactions with ozone regenerate the oxides. The net cycle consumes two ozone molecules per radical-pair event, giving when the subsequent steps are fast. The branching of a real chlorine monoxide–BrO reaction must be included if not already absorbed into its specified effective rate coefficient.
The reservoir species , formed from ClO and NO2 with third-body stabilization. Heterogeneous reaction with hydrogen chloride can produce molecular chlorine and nitric acid, activating chlorine while removing reactive nitrogen from the gas phase. It belongs to the wider reactive nitrogen family, not to the narrow NO-plus-NO2 definition of NOx.
Transfer of a gas across the atmosphere–ocean interface, driven by a departure from the gas's interfacial equilibrium and controlled by transfer resistances and mixing. This can move an added atmospheric carbon-dioxide perturbation into the ocean without immediately returning all of that carbon to a permanent geological store.
A low-temperature catalytic ozone destruction pathway involving ClO association to ClOOCl, photodissociation of the dimer and regeneration of chlorine atoms. Combined with two Cl + O3 reactions its net effect is . Low temperature suppresses thermal dimer dissociation, while sunlight is needed for the photolytic step. Polar stratospheric clouds can activate reservoir chlorine before the sunlit cycle becomes rapid.
For dimer photodissociation frequency and competing thermal dissociation frequency , the successful branch fraction is . With steady dimer abundance and rapid subsequent recycling, ozone destruction is . The factor two counts ozone molecules consumed per productive dimer event.
The peroxy-linked chlorine oxide , often written in a chlorine monoxide dimer cycle. Thermal return to two chlorine monoxide radicals competes with a photolytic pathway that ultimately yields two chlorine atoms. The formula alone should not be taken as specifying every possible dichlorine-dioxide isomer.
Removal of a gas or particle to a surface without precipitation. A simple parameterization uses downward flux , where is the concentration near the surface and the deposition velocity. For a well-mixed layer of depth , the loss is first order with coefficient .
The ratio of a specified downward dry deposition flux to its reference near-surface concentration. It has units of speed but need not be an actual particle settling speed: transfer resistance and surface uptake can both control gas dry deposition.
An organic compound readily present in the gas phase under the conditions of interest. In polluted-air chemistry, its oxidation can generate peroxy radicals which convert nitrogen monoxide to nitrogen dioxide without consuming ozone. Volatility alone does not determine reactivity or ozone yield.
A compound composed of carbon, chlorine and fluorine, with no hydrogen. Many such fully halogenated molecules resist lower-atmospheric attack but release reactive chlorine after stratospheric photodissociation, contributing to catalytic ozone destruction.
A relative measure of stratospheric ozone destruction associated with emissions of a substance, conventionally referenced to CFC-11. It depends on atmospheric survival, transport, halogen delivery and catalytic effectiveness, not only the number of chlorine atoms in the formula. Fluorine-only replacements can have negligible direct ozone-depletion potential while remaining strong greenhouse gases.
For a well-mixed atmospheric burden removed by a single first-order sink, , the atmospheric lifetime is the exponential decay time . At steady emission rate , this model gives . Independent sinks acting on the same burden have additive inverse lifetimes. Local removal times in separate compartments cannot be combined this way without considering exchange and partitioning. A multi-reservoir perturbation, especially atmospheric carbon dioxide, generally needs more than one timescale.
The e-folding relaxation time of an added methane burden after accounting for chemical feedbacks. It need not equal burden divided by the contemporaneous methane loss rate: extra methane can consume hydroxyl radical and reduce its own removal. A fixed-OH exponential estimate omits that coupled adjustment.
Environmental Research Letters 4 (2009), 045108. Coupled aerosol-size and chemistry–climate simulations show why constant forcing per unit injection is an approximation.
Chapter 7 of Climate change 2021: The Physical Science Basis, pp. 923–1054. §§7.3.2–7.3.3 contrast greenhouse-gas and aerosol effective radiative forcing.
Chapter 6 of Climate change 2021: The Physical Science Basis. Relevant sections are §6.3.1 for methane chemical and perturbation lifetime and §6.4.5 for climate–chemistry feedbacks.
Atmospheric chemistry and Physics 23 (2023), 13451–13467. Coupled chemical modelling distinguishes hydrogen leakage effects from accompanying changes in other precursor emissions.
PNAS 117 (2020), 1860–1866. Aircraft IO and particulate-iodine measurements constrain nonzero stratospheric iodine injection and multiphase recycling.
Atmospheric Chemistry and Physics 7, 1961–1971. DOI 10.5194/acp-7-1961-2007. The hygroscopicity parameter and dry diameter determine approximate activation properties.
Journal of the Atmospheric Sciences 34, 1149–1152. The theoretical droplet-number and shortwave-reflectance response is distinguished from absorption and subsequent cloud adjustments.
Journal of Geophysical Research 95, 1837–1851. Regional photochemical-model comparison of ozone precursor sensitivity in rural and urban environments.
Journal of Physical Chemistry 91, 433–436. The NASA abstract describes flow-system dimer production at 220–240 K and infrared/ultraviolet identification. Institutional record: ntrs.nasa.gov/citations/19870061046
Earth System Dynamics 7, 697–715. DOI 10.5194/esd-7-697-2016. A window-grey radiation model separates the blocking effect from the indirect solar-heating effect.
Chapter 7 of Climate Change 2021: The Physical Science Basis, pp. 923–1054. DOI 10.1017/9781009157896.009. Cited sections: emissions metrics and aerosol–cloud interactions; this institutional assessment is not an answer set for the Cambridge paper.
Annual Review of Earth and Planetary Sciences 37, 117–134. The physical ocean and solid-Earth portions distinguish fast exchange, slow ocean equilibration and a long carbonate/weathering tail. Biological mechanisms are outside the present expansion.
Cited physical-chemistry passages: Table 1, p. 119, and physical ocean/solid-Earth conclusions, p. 131.
Growth by collision and attachment of existing particles. Coagulation reduces particle number while conserving particulate material in a closed system without other sources or sinks. Larger particles can have a different scattering-per-mass efficiency and faster sedimentation, so increasing precursor injection need not increase albedo proportionally.
Atmospheric particles containing sulfate, commonly in sulfuric-acid/water or neutralized-salt mixtures. Particle size, water uptake, acidity and accompanying material determine scattering, absorption, cloud activation and heterogeneous chemistry; sulfate mass alone does not specify radiative forcing.
A particle's tendency to take up water vapor, which depends on composition, solubility and ambient humidity. In a common parameterization, a dimensionless summarizes soluble-water uptake for use with dry diameter in Köhler theory. A single composition parameter does not remove the importance of particle size or parcel supersaturation.
A change in cloud microphysics or cloud properties caused by aerosol, including droplet activation, precipitation changes and adjustments of water content or coverage. The Twomey effect isolates a fixed-water-content optical response; it does not by itself establish the sign or magnitude of every cloud adjustment.
Cloud changes caused by aerosol absorption of radiation and the resulting heating. Heating can evaporate clouds and reduce shortwave reflection, but the effect can reverse when heating changes stability and cloud maintenance differently. The forcing sign depends on the relative positions of cloud and absorbing aerosol and on dynamical adjustment.
Increasing cloud droplet number at fixed liquid-water content and cloud thickness reduces mean droplet size and increases total extinction area, raising shortwave optical depth and cloud albedo. This idealized response produces negative shortwave radiative forcing. Changes in cloud amount, water content and dynamics are separate adjustments.[ref]
The Influence of Pollution on the Shortwave Albedo of Clouds by S. Twomey. Fixed-water-content droplet-number and albedo response
For at fixed liquid-water content and thickness, and , the derivative is . For a fixed fractional number increase, the initial response is greatest at . The exact finite-change expression is with . Uncertain activation of additional particles and changes in water content limit direct application to real clouds.
An aerosol particle able to activate into a growing cloud droplet at the supersaturation attained by a cloud parcel. Activity depends on particle dry size, hygroscopicity and interfacial properties. The number active at a specified supersaturation differs from total aerosol number.
The equilibrium water-vapor saturation ratio above a solution droplet combines a curvature effect, which raises vapor pressure, with dissolved-solute reduction of water activity. Its maximum defines a critical supersaturation for activation. In a dilute hygroscopicity approximation using droplet diameter and dry diameter , , where . Then . These approximate forms assume the stated diameter convention and dilute solution; composition and surface tension must be consistent.[ref]
A single parameter representation of hygroscopic growth and cloud condensation nucleus activity by M. D. Petters and S. M. Kreidenweis. Equations describing hygroscopic growth and critical activation supersaturation
The maximum equilibrium supersaturation along a solution-droplet growth curve. Above this threshold, an aerosol particle can grow into an activated cloud droplet under the conditions of the parcel. In Köhler theory, larger dry diameter or greater hygroscopicity generally lowers the threshold; aerosol number alone does not specify it.
In the atmospheric convention used here, denotes nitric oxide plus nitrogen dioxide. The broader reactive-nitrogen family includes additional oxidized reservoirs; the chosen nitrogen-atom counting convention must be specified.
The atmospheric nitrogen oxide . Daylight photodissociation forms nitrogen monoxide and an oxygen atom; subsequent third-body recombination with oxygen forms ozone. Whether this makes net ozone depends on the pathway that regenerated nitrogen dioxide.
The atmospheric nitrogen oxide . It consumes ozone when converted to nitrogen dioxide by , but its oxidation by peroxy radicals instead enables net photochemical ozone production.
A chemical-family bookkeeping variable in a simple stratospheric model. Interconversion of atomic oxygen and ozone preserves this count, while removes two units. The definition must be extended consistently when including additional reservoirs or excited atomic states.
A basic oxygen–ozone mechanism combining oxygen photodissociation, three-body ozone formation, ozone photodissociation and . The last reaction removes two units of odd oxygen; a catalytic cycle can add another loss pathway.
Suppose a reduced steady balance is , where , and hold and all specified background concentrations fixed. With , and , implicit differentiation gives . The quadratic loss term includes temperature-dependent O/O3 partitioning as well as the direct reaction coefficient. Holding the loss ratio fixed over a finite change is an additional approximation; solving the new quadratic provides a check.
A number or mole fraction relative to a specified atmospheric reference population. For ideal gas partial pressures with a consistent wet/dry convention, . A quoted ppm or ppb must state whether it denotes this number fraction, rather than a mass fraction.
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Atmospheric chemistry is the study of the chemical composition of the Earth's atmosphere and the chemical reactions that occur within it. It encompasses various processes that affect the atmosphere's composition, including the interactions between gases, aerosols, and other particles. This field examines both natural and anthropogenic (human-made) sources of atmospheric constituents and how they influence air quality, climate, and atmospheric processes.