Abstract. 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, H3O+/H3O+(H2O)n or NO+/NO2+. The proposed approach has been proved to be appropriate for the on-line detection of individual alkyl nitrates and functionalized nitrates. It has been shown that hydroxyl- and keto-nitrates have a high affinity towards NO+, leading to the formation of an adduct that allows to easily identify the organic nitrate (R) with 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 keto-nitrates. Alkyl nitrates exhibit a moderate affinity towards NO+ ionization leading to detection units of few hundreds of ppt and the highest sensitivity in H3O+ mode was obtained for the water adducts signals. This method exhibits however lower capabilities for the detection of PANs with detection limits in the ppb range.

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