The representativeness of the monitoring station at L1. Valby, Denmark, has been tested. Furthermore, the relation of gaseous NOy and O-3 is investigated. From the linear orthogonal regression analyses of Ox (sum of O-3+NO2) and other photochemical products measured at L1. Valby Denmark and from literature results it is estimated that the non-photochemical background mixing ratio of O-3 in the Northern Hemisphere is 24+/-6 ppbv. The correlation of HCOOH and CH3COOH with Ox indicates that these acids are of photochemical origin. A high correlation of HNO3 with Ox is also found. The anti-correlation of O-3 with MSA observed in this study (methane sulphonic acid) indicates that O-3 at L1. Valby, Denmark, is produced by emissions from land sources. Finally, evidence is found that NO3 reactions is only a supplementary source of unidentified nitrogen species in the troposphere. Copyright (C) 1996 Elsevier Science Ltd.