[pam-users] Nault et al., AS&T, 2025; Ajith et al., AS&T, 2025; Yan et al., J Phys Chem A, 2025

[pam-users]

Nault, B. A., Canagaratna, M., Croteau, P., Fortner, E., Lambe, A. T., Stark, H.,  Sueper, D., Werden, B. S., Williams, A., Williams, L. R., Worsnop, D. R., Jayne, J., DeCarlo, P. F., Cubison, M., Papadopoulos, G., and Urs, R. Characterization of a new higher-resolution time-of-flight aerosol chemical speciation monitor: Application for measurements of atmospheric aerosols. Aerosol Science and Technology, 59(6), 719–742. https://doi.org/10.1080/02786826.2025.2481221, 2025.

Abstract. Long-term measurements of the composition and mass concentration of particulate matter (PM) are important for source apportionment, epidemiological studies, and trends in atmospheric chemistry. The Aerosol Chemical Speciation Monitor (ACSM) has been widely used for in situ, real time measurements of PM. However, ACSMs provide unit mass resolution data, meaning isobaric ions (same unit mass, different exact m/z) cannot be separated, which can impact detection limits and separation and identification of different organic ions (e.g., C2H3O+ vs C3H7+). Here, we present a new Time-of-Flight ACSM with eXtended resolution (TOF-ACSM-X). With a mass resolving power of ∼2000 m/Δm, the TOF-ACSM-X enables higher-resolution, multi-peak fitting of individual ions compared to the two other existing ACSM models, namely the Time-of-Flight ACSM (TOF-ACSM) and quadrupole ACSM (Q-ACSM). This improved resolution leads to a factor of 25 improvement in ammonium detection limits, from ∼0.200 to ∼0.008 μg m−3 (TOF-ACSM versus TOF-ACSM-X, respectively), for 10-minute integration times, allows for elemental analysis (O/C and H/C) of organic aerosol, and enables improved mass spectral separation of the CH2O+ and NO+ signals at m/z = 30 for improved quantification of organic and inorganic particle nitrate. Comparisons of the TOF-ACSM-X with ambient measurements from two separate instruments show that the TOF-ACSM-X agrees quantitatively and that the TOF-ACSM-X provides unconstrained positive matrix factorization results for the organic aerosol that would not be possible with the unit mass resolution TOF-ACSM. Finally, we are now recommending a more direct and unifying calculation of nitrate mass concentration for both AMS and ACSM ionization efficiency calibrations.

Ajith, T. C., Windwer, E., Fang, Z., Li, C., Modini, R. L., Onasch, T. B., Freedman, A., and Rudich, Y. Evaluation of the 365 nm CAPS PM SSA monitor and its use in both laboratory and field measurements. Aerosol Science and Technology, 1–19. https://doi.org/10.1080/02786826.2025.2488478, 2025.

Abstract. The newly designed 365 nm Cavity Attenuated Phase-Shift Single Scattering Albedo Monitor (CAPS PM SSA monitor; Aerodyne Research, Inc) is evaluated. The instrument measures the extinction and scattering coefficients of suspended particles. It has a precision of 0.66 Mm−1 and 0.45 Mm−1 (1σ) in the extinction and scattering channels, respectively with a time resolution of 1 s. The truncation effect in the scattering channel was evaluated using monodisperse polystyrene nanospheres of varying sizes, yielding correction factors of 0.90 ± 0.002 for 200 nm and 0.80 ± 0.002 for 300 nm particles. Mie theory was used for retrieving the complex refractive index (RI) from the SSA monitor and aerosol size distribution measurements. Fresh and aged indole and naphthalene secondary organic aerosols (SOA) generated using a potential aerosol mass (PAM) oxidation flow reactor (OFR) under varying oxidative conditions with and without the presence of NOx were also measured. For indole, the imaginary part of the RI decreased with oxidative aging involving OH radicals, whereas it increased for naphthalene. The retrieved RI values were consistent with previously reported data. Measurements conducted during a field measurement campaign revealed a mean SSA of 0.76 ± 0.05. Several particle burst events were observed during the campaign, characterized by a significant decrease in SSA (<0.65) and an increase in absorption coefficients (from <2 Mm−1 to >10 Mm−1). These events coincided with enhanced organics-to-sulfate ratios (>2.5), indicating the contribution of BrC aerosols. This study demonstrates the application of the newly designed SSA monitor at 365 nm for measuring the optical properties of laboratory and field BrC aerosols.

Jin Yan, N. Cazimir Armstrong, Katherine R. Kolozsvari, Cara M. Waters, Yao Xiao, Alison M. Fankhauser, Madeline E. Cooke, Molly Frauenheim, Nicolas A. Buchenau, Rebecca L. Parham, Zhenfa Zhang, Barbara J. Turpin, Andrew T. Lambe, Avram Gold, Andrew P. Ault, and Jason D. Surratt.  Effect of Initial Seed Aerosol Acidity on the Kinetics and Products of Heterogeneous Hydroxyl Radical Oxidation of Isoprene Epoxydiol-Derived Secondary Organic Aerosol.  The Journal of Physical Chemistry A,  https://pubs.acs.org/doi/abs/10.1021/acs.jpca.4c08082, 2025.

Abstract. At fixed aerosol acidity, we recently demonstrated that dimers in isoprene epoxydiol-derived secondary organic aerosol (IEPOX-SOA) can heterogeneously react with hydroxyl radical (·OH) at faster rates than monomers. Aerosol acidity influences this aging process by enhancing the formation of oligomers in freshly generated IEPOX-SOA. Therefore, we systematically examined the role of aerosol acidity on kinetics and products resulting from heterogeneous ·OH oxidation of freshly generated IEPOX-SOA. IEPOX reacted with inorganic sulfate aerosol of varying initial pH (0.5, 1.5, and 2.5) in a steady-state smog chamber to yield a constant source of freshly generated IEPOX-SOA, which was aged in an oxidation flow reactor for 0–22 equiv days of atmospheric ·OH exposure. Molecular-level chemical analyses revealed that the most acidic sulfate aerosol (pH 0.5) formed the largest oligomeric mass fraction, causing the slowest IEPOX-SOA mass decay with aging. Reactive uptake coefficients of ·OH (γOH) were 0.24 ± 0.06, 0.40 ± 0.05, and 0.49 ± 0.20 for IEPOX-SOA generated at pH 0.5, 1.5, and 2.5, respectively. IEPOX-SOA became more liquid-like for pH 1.5 and 2.5, while exhibiting an irregular pattern for pH 0.5 with aging. Using kinetic and physicochemical data derived for a single aerosol pH in atmospheric models could inaccurately predict the fate of the IEPOX-SOA.

PAM Wiki - Publications Using the PAM Oxidation Flow Reactor<https://sites.google.com/site/pamwiki/publications-using-the-pam-oxidation-flow-reactor?authuser=0>

Andrew Lambe
Principal Scientist
Aerodyne Research, Inc.

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