Photochemical ozone production

Net daylight ozone production occurs when peroxy radicals oxidize NO to NO2 without using ozone, followed by NO2 photodissociation and oxygen-atom addition to O2. The competing reaction NO + O3 to NO2 + O2 only recycles odd oxygen. Radical initiation, termination, precursor loading and transport determine the net rate.

A regime in which precursor/radical supply and NOx-dependent radical termination make net ozone production more responsive to additional reactive organic compounds than to additional nitrogen oxides. Reducing nitrogen oxides can initially raise ozone in some such conditions. Diagnose the chemical regime before extrapolating an emission reduction to every location.

A regime in which additional available nitrogen oxides tends to increase net photochemical ozone production when other factors are fixed. Reducing nitrogen oxides generally suppresses that production, but local ozone concentration also depends on transport, deposition and NO titration.

A contour plot of a specified ozone outcome against the loadings of nitrogen oxides and volatile organic compounds. The ridge separating opposite NOx sensitivities distinguishes an NOx-limited ozone-production regime from a VOC-limited ozone-production regime. The outcome may be peak concentration, a time-integrated measure or a model production rate; axes and averaging conditions must be stated. A schematic diagram is not a calibrated emissions-response prediction.

A regime with abundant nitrogen oxides in which volatile-organic oxidation and radical supply limit net tropospheric ozone production. Reducing volatile organic compounds lowers production. A small NOx reduction can initially increase ozone by weakening OH termination and NO titration; sufficiently large reductions can cross into an NOx-limited ozone-production regime. Local concentration and regional emission sensitivities need not coincide.

A regime in which additional nitrogen oxides promote net tropospheric ozone production because NO availability limits peroxy-radical oxidation of NO. Reducing NOx then generally lowers ozone production, subject to transport and chemical history. At much larger NOx burdens, OH removal by nitric-acid formation can instead produce a VOC-limited ozone-production regime.