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

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 (RIEOA). Here, a series of laboratory experiments were conducted to investigate how RIEOA differs for different classes of OA and between AMS and ACSM instruments. For OA surrogates with high oxidation state ((OSc) over bar), the measured RIEOA is 1.60 & PLUSMN; 0.56 for all instruments. However, for OA surrogates with lower (OSc) over bar(,) the absolute magnitude and the variability of the measured RIEOA increases. The increase in RIEOA 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 RIEOA-the fractional contribution of ions at less than or equal to m/z 50 (f(Sm/z50)). When f(Sm/z50) is high (>64%, similar to ambient OA), the average RIEOA is 1.23 & PLUSMN; 0.62. This supports the use of a default RIEOA of 1.4 for typical ambient conditions, where secondary OA dominates the OA budget. When f(Sm/z50) is less than 64%, the RIEOA increases as higher m/z ions contribute more to the total signal. These observations reflect the fact that the observed RIEOA is a combination of multiple processes. The f(Sm/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 RIEOA 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.