Comparison of mass spectrometric approaches for real-time characterization of organic aerosol

Atmospheric organic aerosol (OA) contains a mixture of molecules from various sources, both primary and secondary, and with different levels of atmospheric aging and propensity for particle-phase reactions. This highlights the need for detailed characterization of OA composition to better understand its sources, atmospheric transformation, and resulting physicochemical properties. This characterization is also preferably conducted in real-time, to capture both sudden changes of airmasses as well as fast reactions within the particles. We performed an intercomparison at our field station in Hyytiälä in the Finnish boreal forest where we deployed four different mass spectrometers able to measure aerosol composition in real-time. The instruments included an Aerosol Chemical Speciation Monitor (ACSM) as the reference instrument as well as three chemical ionization mass spectrometers (CIMS): a Vaporization Inlet for Aerosols coupled with a nitrate CIMS (VIA-NO₃-CIMS), an Extractive Electrospray Ionization TOF (EESI-TOF), and a Filter Inlet for Gases and AEROsols coupled with an iodide CIMS (FIGAERO-I-CIMS). We also performed a follow-up chamber study to complement some missing comparisons due to instrumental problems during the field campaign.

CIMS has become a key tool for probing gas-phase composition, and studies have demonstrated how different reagent ions are able to detect distinct molecule types. In addition, using different methods for transferring aerosol-phase molecules into the gas-phase for detection by CIMS will most likely also result in differences in detected molecules. This study aimed to evaluate differences in instrument sensitivity for different types of OA and assess the fraction of OA that could be measured with these state-of-the-art methods deployed together.

The campaign in Hyytiälä (Sept 2-25, 2024) provided several interesting results. The foremost finding was a very high correlation between that the organics measured by the ACSM and the VIA–NO₃-CIMS (R² = 0.90) and FIGAERO-I-CIMS (R² = 0.88). Consequently, also the VIA and the FIGAERO correlated extremely well, which was unexpected given that iodide and nitrate CIMS instruments tend to show very few common signals in typical gas-phase measurements. Sulfate measured by the VIA–NO₃-CIMS agreed almost perfectly with ACSM measurements (R² = 0.97), further validating that the instrument was working well throughout the campaign. Due to technical issues, however, the EESI-TOF did not provide enough data during the Hyytiälä campaign, and therefore we instead compared the VIA and EESI instruments with an AMS during a later chamber campaign using different types of aerosol precursors. This data is currently being analyzed in more detail, but at least for monoterpene-derived OA, also the EESI shows good correlation with the VIA, though with a higher sensitivity for less-oxygenated molecules while the VIA had higher sensitivity for the most oxygenated compounds. I will present more in-depth comparison results at the conference, including key differences between the methods, but our comparison indicates that all three aerosol CIMS instruments are able to detect a large fraction of the OA, at least in regions dominated by biogenic secondary OA.