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

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.