precursor - a substance which may participate in or influence a reaction in air or water to produce a new substance; for example, in the atmosphere, nitrogen oxides and hydrocarbons with sunlight are precursors of ozone and photochemical smog (ESD)
The recent California Clean Air Act requires a permit program to be adopted to achieve a "no net increase" of nonattainment pollutants or their precursors from all new and modified sources; this meant that changes to its permitting program were necessary to ensure that increases in precursor organic compounds (POCs) or NOx emissions from new or modified sources be accompanied by compensating reductions of the same pollutant, since POC and NOx are precursors of ozone.
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