Ozone and oxides of nitrogen in the rural upper-midwestern U.S.A

Ozone, nitric oxide, nitrogen dioxide and meteorological parameters were continuously monitored over a period of 4 y at three rural sites in the upper-midwestern U.S.A. A large data set was obtained which allowed accurate descriptions of mean pollutant concentrations, variations among years and sites, mean seasonal and diurnal fluctuations, frequency distributions and relationships among pollutants and between pollutants and meteorological parameters. Evidence was found which illustrated the importance of photochemistry, nocturnal scavenging of O 3 by NO, and possibly intrusion of stratospheric O 3. Examination of individual events showed that transport of ozone and precursors occurred over a distance of 275 km NW from the Minneapolis-St. Paul metropolitan area (metro area). A gradient in mean nitrogen dioxide concentrations was observed which was possibly due to the metro area influence. Mean concentrations of ozone averaged about 33 ppbv, and were similar at all sites, independent of distance from the metro area. However, a greater frequency of extremely high and extremely low concentrations were measured at the site closest to the metro area. Nitric oxide concentrations averaged 3–4 ppbv and were similar at all sites. Nitrogen dioxide concentrations varied with distance from the metro area, averaging nearly 6 ppbv at approximately 50 km, and less than 2 ppbv at a distance of about 500 km.