A well-known secondary photochemical pollutant is ozone (O3). Its formation results from the sunlight-initiated oxidation (reaction with oxygen) of volatile organic compounds (VOCs) such as benzene in the presence of nitrogen oxides (NOx), mostly nitric oxide (NO) and nitrogen dioxide (NO2). Once formed, ozone is scavenged by NO, and in the absence of other competing reactions, a "photostationary state" is formed where concentrations of NO, NO2 and O3 are all inter-related. In rural areas away from major sources of NO, such as urban road transport, ozone scavenging by NO is lower, and consequently ozone concentrations in the atmosphere are higher.