By the early 1980s, daily air quality reports were being released, based specifically on concentrations of ozone and fine particles. The SPI measured ozone and fine particles from 6am to 3pm. This reflected an emphasis on visible pollution and focused on daytime visibility as a measure of air quality. Monitoring was therefore concentrated in these areas. The RPI is based on measured concentrations of ozone and nitrogen dioxide and a measure of visibility, used to indicate the presence of fine particles in the air.
Ozone is a gas which plays different roles at different levels in the atmosphere.
However, in the lower atmosphere ozone can have an adverse impact on human health, vegetation and materials. ROCs and NOx are emitted by motor vehicles, industrial processes, domestic activities and the natural environment. Nitric oxide and nitrogen dioxide , two oxides of nitrogen which occur naturally in the atmosphere, are also formed in combustion processes, and are emitted by cars, factories and home fuel burning. Nitrogen dioxide also contributes to the brown haze in winter and is a key factor in the formation of ozone in summer. Fine particles come from motor vehicles, wood fires and industry.