A method to determine the total content of particulate organic nitrates (PON) has been developed and ambient air measurements of PON, NO, NO2, HNO3, peroxyacetyl nitrate (PAN), peroxypropionyl nitrate (PPN), gas NOy and particulate inorganic nitrate have been performed in the spring and early summer al an agricultural site in Denmark and compared with measurements of ozone, H2O2, SO2, formic acid, acetic acid and methane sulphonic acid. The gas NOy detector determines the sum NO + NO2 + HNO2 + HNO3 + PAN + PPN + gas phase organic nitrates + 2 x N2O5 + NO3. The content of residual gas NOy ( = gas NOy - NO - NO2 - HNO3 - PAN - PPN) was determined and a group of unidentified NO, compounds was found. The phenomenon was observed at a site with relatively high NOx/NOy ratios compared to previous observations in U.S.A. and Canada. The residual gas NOy made up 7 +/- 6% of total NOy (total NOy = gas NOy + particulate inorganic nitrate). Residual gas NOy was much higher than the particulate fraction of organic nitrates (PON). PON was only 0.25 +/- 0.11% of concentrations of photochemical oxidants in connection with high-pressure systems suggesting atmospheric processes being the major source. Clean marine air can be discarded as a source for PON and residual gas NOy.