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.
When fine particles are trapped in cold still air, they cause the brown haze that typically appears over cities in winter. How these pollutants are measured Ozone is measured by shining ultraviolet light through an air sample and determining the amount of light absorbed by the ozone molecules. The more light is absorbed, the higher the concentration of ozone. While nitrogen dioxide cannot be measured directly, levels of this pollutant can be determined by measuring nitric oxide. The measurement technique is called chemiluminescence.