Abstract. 

Organic aerosol (OA) mass concentrations measured by the Aerodyne Aerosol Mass Spectrometer (AMS) and Aerosol Chemical Species Monitor (ACSM) depend on particle relative ionization efficiency (RIE_OA). Here, a series of laboratory experiments were conducted to investigate how RIE_OA differs for different classes of OA and between AMS and ACSM instruments. For OA surrogates with high oxidation state (OSc¯), the measured RIE_OA is 1.60 ± 0.56 for all instruments. However, for OA surrogates with lower OSc¯, the absolute magnitude and the variability of the measured RIE_OA increases. The increase in RIE_OA is different between the AMS and ACSMs due to differences in m/z-dependent ion transmission/detection efficiencies in the mass spectrometers. A new metric is introduced to explore RIE_OA—the fractional contribution of ions at less than or equal to m/z 50 (f _Σm/z50). When f _Σm/z50 is high (>64%, similar to ambient OA), the average RIE_OA is 1.23 ± 0.62. This supports the use of a default RIE_OA of 1.4 for typical ambient conditions, where secondary OA dominates the OA budget. When f _Σm/z50 is less than 64%, the RIE_OA increases as higher m/z ions contribute more to the total signal. These observations reflect the fact that the observed RIE_OA is a combination of multiple processes. The f _Σm/z50 may be used as a metric to determine if primary-like OA is contributing significantly to total OA. Overall, the results indicate that changes in RIE_OA are most important for studies measuring primary-like OA, and the magnitude of the changes will depend on the instrument being used to measure the OA.