A commercial PTR-TOF-MS has been optimized in order to allow the measurement of individual organic nitrates in the atmosphere. This has been accomplished by shifting the distribution between different ionizing analytes, H 3 O + / H 3 O + (H 2 O) n or NO + / NO + 2. The proposed approach has been proven to be appropriate for the online detection of individual alkyl nitrates and functionalized nitrates. It has been shown that hydroxyl and ketonitrates have a high affinity towards NO + , leading to the formation of an adduct that allows the easy identification of the organic nitrate (R) from the R-NO + ion signal. The recorded sensitivities for both ionization modes correspond to detection limits of tens of ppt min −1 in the case of hydroxy-and ketonitrates. Alkyl nitrates exhibit a moderate affinity towards NO + ionization leading to detection units of few hundreds of ppt and the highest sensitivity in H 3 O + mode was obtained for the water adducts signals. However, this method exhibits much lower capabilities for the detection of peroxyacetyl nitrates with detection limits in the ppb range.