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Photoexcitation is the first step in a photochemical process where the reactant is elevated to a state of higher energy, an excited state.The first law of photochemistry, known as the Grotthuss–Draper law (for chemists Theodor Grotthuss and John W. Draper), states that light must be absorbed by a chemical substance in order for a photochemical reaction to take place.
Photolysis occurs in the atmosphere as part of a series of reactions by which primary pollutants such as hydrocarbons and nitrogen oxides react to form secondary pollutants such as peroxyacyl nitrates. See Photochemical smog. The two most important photodissociation reactions in the troposphere are firstly:
Photochemical smog, often referred to as "summer smog", is the chemical reaction of sunlight, nitrogen oxides and volatile organic compounds in the atmosphere, which leaves airborne particles and ground-level ozone. [21] Photochemical smog depends on primary pollutants as well as the formation of secondary pollutants.
Atmospheric chemistry is a branch of atmospheric science that studies the chemistry of the Earth's atmosphere and that of other planets. This multidisciplinary approach of research draws on environmental chemistry, physics, meteorology, computer modeling, oceanography, geology and volcanology, climatology and other disciplines to understand both natural and human-induced changes in atmospheric ...
Photochemical and chemical reactions involving it fuel many of the chemical activities that occur in the atmosphere during the day and night. It is a pollutant and a component of smog that is produced in large quantities as a result of human activities (mostly the combustion of fossil fuels). [ 90 ]
In the atmosphere, the organic compounds are degraded by hydroxyl radicals, which are produced from water and ozone. [3] Photochemical reactions are initiated by the absorption of a photon, typically in the wavelength range 290–700 nm (at the surface of the Earth).
PAN is produced in the atmosphere via photochemical oxidation of hydrocarbons to peroxyacetic acid radicals, which react reversibly with nitrogen dioxide (NO 2) to form PAN. [4]: 2680 Night-time reaction of acetaldehyde with nitrogen trioxide is another possible source. [4] Since there are no direct emissions, it is a secondary pollutant.
Photogeochemical reactions are described by the same principles used to describe photochemical reactions in general, and may be classified similarly: Photosynthesis: in the most general sense, photosynthesis refers to any light-activated reaction for which the change in free energy (ΔG o ) is positive for the reaction itself (without ...