Most ground-level ozone is formed indirectly by the action of sunlight on volatile organic compounds in the presence of nitrogen dioxide, and as such is a secondary pollutant. There are no direct man-made emissions of ozone to the atmosphere. About 10 to 15% of ground-level ozone is transported from the stratosphere. As ozone concentrations are particularly dependent on sunlight, episodes are always likely to develop following sustained periods of warmth and calm weather. Once formed, ozone is scavenged by nitric oxide (NO), usually present in urban areas, as a result of traffic fumes, but less so in the countryside. Consequently, ozone usually occurs in higher concentrations during summer than winter, and in rural rather than urban areas. Background levels of ozone across Europe are usually less than 15 ppb but can be as high as 60 ppb. During photochemical smog episodes, levels can rise to over 100 ppb.