Ozone O3 is formed by processes involving nitrogen oxides NOx and volatile organic compounds VOCs, which are catalyzed by UV radiation from sunshine. Each VOC may generate varying levels of ozone. One kilogram me of ethane, for example, creates half the quantity of ozone as one kilogram me of formaldehyde. Ozone levels are typically maximum on warm, bright, windless days. Ozone concentrations peak in the morning and/or afternoon and fall in the evening during rush hour. Motor vehicles approximately 40% of emissions; industrial activities, notably the chemical and petroleum sectors; and any usage of paints, varnishes, and solvents another 40% for these sources combined are among the sources of VOCs. Other notable producers of VOCs are service stations, pesticide application, dry cleaning, fuel combustion, and open burning
