Evaluating the response of the Aerodyne aerosol mass spectrometer to monoterpene- and isoprene-derived organic nitrate standards

An accurate quantification of organic nitrate (RONO2) is important owing to its contribution to organic aerosol (OA) and its impacts on NOx cycling and ozone formation. Here, we calibrate and evaluate the response of the Aerodyne aerosol mass spectrometer (AMS) to synthetic monoterpene- and isoprene-derived RONO2 standards. The dominant fraction of nitrogen is detected as NO2+ and NO+ ions in the AMS with the NOx ratio (NO2+/NO+) being substantially lower than that of ammonium nitrate (NH4NO3). The NOx ratio also varies among the RONO2 standards, possibly driven by the proximity of other functional groups to the nitrate group and/or the position to which the nitrate group is attached (aliphatic vs. cyclic). The best match of the nitrogen-containing moiety mass concentrations of particulate RONO2 (pRONO2) between the AMS and a condensation particle counter (CPC) is observed when the CPC-derived concentrations are calculated using a molecular weight of 46 g mol−1 (NO2) rather than commonly assumed 62 g mol−1 (NO3). This result is consistent with the thermal decomposition on the AMS vaporizer predominantly breaking the O–N bond rather than the R–O bond of R–O–NO2 species. The estimation of pRONO2 mass concentration by the AMS depends on the identity of the measured nitrogen-containing moiety of pRONO2, which for the AMS has been historically assumed as NO3 (62 g mol−1) instead of NO2 (46 g mol−1) as found in this study. Therefore, actual pRONO2 mass concentrations as well as their contribution to OA could be higher than previously reported in literature by 35%, further highlighting the prevalence of pRONO2 in ambient environments.