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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 (), the measured RIE OA  is 1.60 ± 0.56 for all instruments. However, for OA surrogates with lowerthe 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/z50is high (&gt;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/z50is 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/z50may 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.

aerosol science and technology

Laboratory evaluation of organic aerosol relative ionization efficiencies in the aerodyne aerosol mass spectrometer and aerosol chemical speciation monitor