the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 31 Dec 2025
Refining the Evolution of Gas-Particle Partitioning in Cooking Emissions Oxidation via FIGAERO-CIMS Analysis
Abstract. This study examines atmospheric oxidation and gas-particle partitioning of cooking-emitted organic aerosols. Using a Potential Aerosol Mass (PAM) flow reactor coupled with a Filter Inlet for Gases and AEROsols and a Chemical Ionization Mass Spectrometer (FIGAERO-CIMS), we monitored chemical composition, volatility distribution, and partitioning behavior under realistic conditions. A key aspect was applying high-resolution mass spectrometry within a two-dimensional volatility basis set (2-D VBS) framework to mechanistically analyze aerosol evolution. Experiments identified a two-stage particle formation: primary emissions rapidly produced fine particles (~10 nm) within two hours of oxidation, followed by secondary aerosol formation (30–50 nm) after 0.5–1 day of atmospheric aging. Oxidation products were primarily semi-volatile and intermediate-volatility organic compounds (S/IVOCs), shifting systematically toward semi-volatile organic compounds (SVOCs) over time, despite stable average molecular weight and oxidation state. Using Positive Matrix Factorization (PMF), we classified compounds by volatility and oxidation degree, identifying molecular markers for each stage. Highly oxidized small organic acids (≤C3) and C7–C10 multi-generation products were significant, showing moderate volatility and high oxidation states. A major finding was non-equilibrium gas-particle partitioning, strongly dependent on molecular class. Small organic acids and fragmentation products neared equilibrium, whereas first-generation oxidation products (C3–8O3–4) and large, non-fragmented compounds (>C14O5) exhibited kinetic limitations due to particle-phase diffusion constraints. This work enhances understanding of cooking aerosol behavior and provides a basis for improving emission inventories and air quality models.
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Status: final response (author comments only)
- RC1: 'Comment on egusphere-2025-6416', Anonymous Referee #1, 23 Jan 2026
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RC2: 'Comment on egusphere-2025-6416', Anonymous Referee #2, 09 Mar 2026
This manuscript investigates the chemical evolution and gas-particle partitioning behavior of cooking-emitted organic aerosols using PAM flow-tube oxidation combined with FIGAERO-CIMS measurements and a 2-D volatility basis set framework. The authors aim to elucidate the compositional evolution and volatility distribution of oxidation products and demonstrate that non-equilibrium partitioning plays a dominant role in cooking-related SOA formation.
The topic is timely and relevant. Cooking emissions have increasingly been recognized as an important urban source of organic aerosols, and their oxidation chemistry and SOA formation mechanisms remain insufficiently understood. The combination of high-resolution FIGAERO-CIMS measurements, PMF-based non-targeted analysis, and volatility characterization provides potentially valuable insights into the evolution of cooking aerosols. The manuscript presents a large dataset and attempts to connect molecular composition with gas-particle partitioning behavior.
Overall, the study has the potential to contribute to the understanding of cooking-derived SOA and its kinetic limitations. However, several aspects of the manuscript require clarification and improvement before it can be considered for publication, as outlined below.
- A central conclusion of the manuscript is that gas-particle partitioning of cooking-derived SOA deviates significantly from equilibrium due to diffusion limitations. However, the evidence presented for this claim is not entirely convincing. Several alternative explanations should be discussed more thoroughly, including uncertainties in FIGAERO-CIMS quantification, potential thermal decomposition artifacts during desorption, uncertainties in vapor pressure estimation, and limitations associated with the assumption of ideal activity coefficients in the equilibrium calculations. In particular, the comparison of partitioning coefficients relies heavily on estimated vapor pressures and assumed particle composition. Moreover, FIGAERO thermograms are known to produce fragmentation or rearrangement products, which may bias the inferred volatility and therefore affect the interpretation of partitioning behavior.
- The experiments rely on PAM reactor oxidation with OH exposure up to ~1.8×10¹⁰ cm⁻³ s, corresponding to several hours to days of atmospheric aging. However, PAM reactors can introduce well-known artifacts: unrealistically high oxidant concentrations, secondary photolysis, altered radical chemistry, and enhanced fragmentation. The manuscript should clarify whether artifacts such as OH suppression and RO₂ chemistry differences were considered, and how the chosen oxidation conditions compare to typical indoor or urban atmospheric environments. In addition, cooking emissions are often influenced by NOx chemistry, which is largely absent here due to the use of pure nitrogen carrier gas. The implications of this experimental design for real-world cooking emissions should be discussed more carefully.
- The authors note that iodide-CIMS detection may underestimate low-oxygenated or nonpolar species, such as long-chain hydrocarbons and fatty acids. This limitation could strongly influence the conclusions. For example, the reported small molecular size of primary aerosols and the inferred dominance of S/IVOCs may partly reflect instrumental detection bias. The manuscript should clearly discuss the detection efficiency of iodide-CIMS for cooking emissions, potential missing compound classes, and the implications for interpreting 2-D VBS distributions.
- The PMF analysis identifies 13 factors representing volatile, semi-volatile, and low-volatility species. However, the physical interpretation of these factors appears somewhat speculative. For example, the assignment of factors to specific oxidation generations, the identification of fatty-acid cleavage products, and the interpretation of thermogram-derived volatility should be supported by stronger evidence. The authors should clarify how the optimal number of factors was determined, assess the robustness of the results with respect to changes in PMF configuration, and discuss whether the identified factors are chemically meaningful beyond statistical separation.
- The partitioning analysis presented in Figure 4 compares experimental Kp values with equilibrium predictions. However, the methodology used to derive Kp from the FIGAERO data is complex, and the assumptions underlying the equilibrium calculations are not clearly described in the main text. Key parameters, such as the particle-phase molecular weight, organic mass concentration, and activity coefficients, should be explicitly stated. At present, much of the derivation is only described in the Supplement, which reduces clarity and makes it difficult for readers to fully assess the analysis.
- Trace amounts of sulfur appear in the reported elemental compositions, but the manuscript does not discuss their origin. Since sulfur-containing compounds in cooking emissions are often associated with ingredients such as garlic or onions, clarification is needed. However, the experiments appear to involve heating corn oil only, without food ingredients. The authors should therefore explain whether the detected sulfur species originate from cooking emissions, background contamination, or uncertainties in molecular formula assignment.
- Several details should be clarified: the exact composition of the cooking emissions (oil only vs food ingredients), repeatability of the experiments, and the number of experimental replicates. Currently, only corn oil heating at 120-130 °C is described. This may not represent typical cooking conditions.
- Figures: The font size in several figures is too small and should be enlarged for readability (e.g., Figure 1(e)). In addition, the subpanel label “(e)” is not clearly marked in the figure and should be added to match the caption.
Citation: https://doi.org/10.5194/egusphere-2025-6416-RC2
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- 1
Shen et al. investigate oxidative aging and gas–particle partitioning of cooking emissions using a Go:PAM flow reactor coupled with FIGAERO-CIMS (and supporting measurements). The manuscript argues that secondary organic aerosol (SOA) formation is driven primarily by nucleation/condensation of gas-phase oxidation products rather than by oxidative aging of primary aerosol. The authors further interpret the evolution of composition and volatility using a 2-D VBS framework and Positive Matrix Factorization (PMF), and they propose that smaller molecules approach equilibrium partitioning while larger molecules show non-equilibrium behavior that may reflect kinetic limitations.
The topic is potentially important. Cooking is increasingly recognized as a major contributor to urban primary emissions and to indoor particulate matter exposure, and detailed chemical characterization of cooking-related oxidation and partitioning can be valuable for both air quality and exposure studies.
However, in its current form the manuscript has substantial issues in analysis and presentation that prevent a rigorous evaluation of the main conclusions. My primary concerns center on the PMF approach/interpretation and the strength of several mechanistic statements. For these reasons, I do not think the manuscript is ready for publication in ACP without major re-analysis and restructuring, and I recommend rejection with encouragement to resubmit after substantial revision.
Major comments
1) Joint PMF on gas- and particle-phase signals requires stronger justification and validation.
The manuscript applies PMF to a combined dataset that includes both gas-phase signals and particle-phase thermal-desorption signals from FIGAERO. While a joint analysis can be feasible in some contexts, it requires clear justification and evidence that the solution reflects chemical/process factors rather than differences in measurement mode, time resolution, and uncertainty structure (real-time inlet vs. integrated filter collection + desorption). As presented, the manuscript does not provide sufficient validation that combining the two phases in a single PMF run yields physically meaningful factors. At minimum, the authors should demonstrate robustness (e.g., compare solutions from gas-only, particle-only, and combined analyses, and show whether the main conclusions are consistent).
2) The PMF results are presented in an overly descriptive manner, making the factor interpretation difficult to follow and limiting scientific insight.
Section 3.3 reports 13 factors and discusses their dominant ions/tracer-like species, but the factors are not assigned intuitive labels (source/process-oriented) and the narrative does not clearly articulate what each factor represents in a way that readers can track across the manuscript. As a result, it is difficult to understand why resolving these factors is important, how they connect to the SOA formation stages, and what new chemistry/mechanistic insight is gained beyond listing factor constituents. The structure of Section 3.3 also makes the discussion hard to follow; additional organization (e.g., subheadings and grouping factors by interpretation) would greatly improve clarity.
3) Mechanistic attribution of non-equilibrium partitioning to particle-phase diffusion / mass-transfer limitations is overstated relative to the evidence shown.
The abstract and conclusions present particle-phase diffusion/mass-transfer limitation as a key explanation for kinetic limitations in gas–particle partitioning of larger molecules. In the main text (Section 3.4), this appears to be one proposed explanation among others rather than a demonstrated mechanism. The manuscript does not provide quantitative constraints (e.g., phase state/viscosity proxies, diffusion coefficients or mixing timescales, uptake/accommodation considerations, sensitivity analyses) that would allow the reader to evaluate whether diffusion or mass-transfer limitations are sufficient to explain the observed deviations. Without such evidence, the current wording reads as a conclusion stronger than what is actually supported, and should be reconsidered.
Additional major comments / requests for clarification