Laboratory evaluation of organic aerosol relative ionization efficiencies

Title: Laboratory evaluation of organic aerosol relative ionization efficiencies in the aerodyne aerosol mass spectrometer and aerosol chemical speciation monitor
Hosted by: University of California - Riverside Student Chapter

Organic aerosol (OA) mass concentrations measured by the Aerodyne aerosol mass spectrometer (AMS) and aerosol chemical speciation monitor (ACSM) depend on the particle relative ionization efficiency (RIE_OA). To understand the importance of RIE_OA, first, a quick discussion about the principles in how AMS and ACSM measure particles will be presented. Then, I will present a series of laboratory experiments that were conducted to investigate how RIE_OA differs for different classes of OA and between the AMS and ACSM. Though we find that for high oxidation states (e.g., SOA-like) an RIE_OA similar to prior studies (~1.60), we also found that at lower oxidation states, the absolute magnitude and variability for the RIE_OA increased but varied across the instruments. This difference allowed us to probe and better understand how OA is sampled with the AMS and ACSM. Specifically, we found that the differences are related to m/z-depend ion transmission/detection efficiencies. Using a new metric, the fractional contribution of ions at or less than m/z 50, we find that when this value is high (e.g., SOA-like), we find an average RIE_OA ~1.20, similar to the value of 1.4 within uncertainty. As this fractional contribution decreases (OA becomes more primary-like), the RIE_OA increases linearly. Though this study provides new insight into how OA is sampled by the AMS and ACSM, I will show why applying what we learn to real-world measurements is more difficult.

About the speaker: Dr. Benjamin A. Nault is currently an assistant research scientist in the Department of Environmental Health and Engineering at Johns Hopkins University and a principal research scientist in the Center for Aerosol and Cloud Chemistry at Aerodyne Research, Inc. Benjamin is from Indiana, where he pursued his bachelor's in chemistry at Purdue University and was introduced to atmospheric chemistry via his undergraduate research at Purdue and with NASA. Then, he pursued his Ph.D. in Earth and Planetary Science at UC Berkeley, where his dissertation focused on data he collected when he chased thunderstorms with the NASA DC-8 research aircraft in order to understand lightning nitrogen oxide chemistry. After his Ph.D., Benjamin went to CU Boulder and started working with particulate matter and the AMS with Prof. Jose Jimenez, where he was on 6 different NASA DC-8 research projects to investigate what controlled the aerosol chemistry from urban sources to the most remote locations in the middle of the Pacific and Atlantic Ocean. Now, at Aerodyne and Johns Hopkins, Benjamin is working on many projects, including better understanding how the AMS and ACSM works, air quality for fence line community, marine aerosol chemistry, urban aerosol chemistry, and aerosol thermodynamics such as aerosol acidity and aerosol liquid water.