Ozone Formation Potential Calculator

Ozone formation potential ranks a volatile organic compound by how much ground-level ozone it can generate, not by how much of it is emitted. The calculation multiplies emitted mass by a reactivity coefficient, usually the Maximum Incremental Reactivity, expressed in grams of ozone formed per gram of VOC added to a base case urban air mass. Reactivity spans three orders of magnitude: methane sits near 0.014, ethane near 0.28, toluene near 4.0, xylene isomers between about 7.4 and 10.6, formaldehyde near 9.5, propene near 11.7, and 1,3-butadiene near 12.6. Ozone is produced catalytically, with organic radicals converting nitric oxide to nitrogen dioxide, which then photolyzes to release the oxygen atom that forms ozone, so a compound that reacts quickly with the hydroxyl radical and propagates several radical steps yields far more ozone per gram than a slow-reacting alkane. The key caveat is that MIR is defined at the NOx level that maximizes incremental ozone, making it appropriate for the VOC-limited conditions of urban cores while overstating the benefit of VOC control in NOx-limited rural and downwind areas. Values are also specific to the chemical mechanism used to derive them, and simple multiplication ignores interactions among co-emitted species.

The recognition that hydrocarbons differ in smog-forming power dates to the discovery of the mechanism itself. Arie Haagen-Smit, a plant biochemist at the California Institute of Technology, showed in the early 1950s that Los Angeles smog was ozone generated photochemically from hydrocarbons and nitrogen oxides rather than soot, and later laboratory work established that olefins reacted far faster than paraffins. An early regulatory application, Los Angeles County Rule 66 adopted in 1966, split solvents crudely into photochemically reactive and exempt categories. The quantitative scale in use today was developed by William Carter at the University of California, Riverside, who built the SAPRC chemical mechanism and published incremental reactivity scales through the late 1980s, with the definitive Maximum Incremental Reactivity treatment appearing in the Journal of the Air and Waste Management Association in 1994. The California Air Resources Board adopted MIR values in regulation first for the reactivity adjustment factors applied to clean fuel vehicles around 1990, then for consumer products and the aerosol coatings rule adopted in 2000, which limits products by ozone-forming potential rather than total VOC mass. CARB continues to publish and periodically update the official MIR table.