the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 21 Jul 2026
Overestimated organic condensation reveals an underperformance in estimation of ambient nanoparticle growth
Abstract. Atmospheric new particle formation (NPF) supplies up to half of global cloud condensation nuclei, yet the growth of sub-15 nm nanoparticles—the stage most vulnerable to scavenging—remains poorly constrained, largely because the volatility of oxygenated organic molecules (OOMs) is highly uncertain. Using a purpose-built laminar flow reactor that isolates particle–particle coagulation from OOM condensation, we show that six of seven widely used OOM volatility parameterizations substantially overestimate nanoparticle growth rates, with the largest bias under the high-NOx conditions. A recent parameterization constrained by ambient organic aerosol volatility reproduces our laboratory observations across diverse OOM precursors, seed sizes (3–5 nm), and NOx regimes. By applying this laboratory-validated framework to NPF events at Lake Tai, China, in summer 2023, OOM and H2SO4 condensation explain about 53 % of the observed 3–15 nm growth rate, leaving a residual that persists even at the upper bound of measurement uncertainty. Together, our laboratory experiments and field observations provide strong evidence that particle coagulation and vapour condensation alone cannot account for ambient nanoparticle growth, revealing a clear gap in our understanding of this process. This gap may point to additional processes, not yet accounted for in current frameworks, that could contribute to nanoparticle growth in polluted atmospheres, or it may reflect uncertainties in other parameters.
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Status: open (until 01 Sep 2026)
- RC1: 'Comment on egusphere-2026-3174', Anonymous Referee #1, 05 Aug 2026 reply
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RC2: 'Comment on egusphere-2026-3174', Anonymous Referee #2, 21 Aug 2026
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The study investgates the role of OOM volatility in determining nanoparticle growth rates and demonstrates how uncertainties in volatility parameterizations can affect particle growth predictions. The findings are especially relevant for polluted environments where anthropogenic emissions and NOx chemistry strongly influence OOM composition. The manuscript makes a contribution to understanding the mechanisms controlling NPF and nanoparticle growth. Overall, the manuscript is well written and scientifically sound. I have some minor comments and suggestions below
Comments #1. “OOMs were generated by hydroxyl radical (OH)-initiated oxidation of toluene or ozonolysis of α-pinene in a Potential Aerosol Mass (PAM) oxidation flow reactor (Aerodyne Research Inc.) operated at a residence time of ~52 s.” The OH concentrations employed in PAM reactors are typically much higher than those encountered in the ambient atmosphere. Could the authors discuss how the elevated OH exposure may influence gas-phase oxidation pathways and resulting OOM compositions? To what extent are the OOM yields and molecular compositions measured in this study representative of those formed under atmospheric conditions? A discussion of previous evaluations comparing PAM-generated OOMs with ambient observations would strengthen the atmospheric relevance of the results.
Comment #2. "Section 2.3 describes the measurement and quantification of OOMs." Please discuss the limitations and uncertainties associated with the ionization techniques used. Are there classes of OOMs that may be underdetected or missed due to ionization efficiency, molecular structure, volatility, or elemental composition? What fraction of the total OOM population is expected to be detected by the employed instruments? A more comprehensive discussion of measurement biases would aid interpretation of the results.Comment #3. "Section 2.4 discusses volatility parameterizations and growth simulations." Please elaborate on the uncertainties associated with the applied volatility parameterizations. Specifically, what are the uncertainties in estimated saturation vapor pressures (or C*)?
Comment #4. On Page 5, “We then simulated nanoparticle growth from the measured saturation vapor pressures and axial concentration profiles of citric acid or PEG-7…” Please clarify whether particle curvature effects on equilibrium vapor pressure were considered once citric acid had condensed into the particle phase. Since Kelvin effects can substantially influence condensation onto nanoparticles in the size range investigated, additional information regarding the treatment of size-dependent vapor pressures in the simulations would be helpful.
Comment #5. On Page 7, “Concentrations of individual OOMs were quantified by instrument-specific calibrations... Note that identical molecular formulas across instruments do not necessarily correspond to identical structures...” Could the authors provide an estimate of the proportion of total gas-phase oxidation products or total OOM mass that was detected and quantified by the instrumentation? Although structural isomers cannot be resolved, it would be useful to understand the extent to which the observed OOM population represents the overall oxidation product distribution.Comment #6. On Page 9, “They are less transferable to polluted, high-NOx environments, where anthropogenic precursors and intensified NOx pathways likely generate OOM populations with distinct functionality, volatility, and condensational behavior.” This is an nice conclusion and deserves further discussion. Specifically, how do OOM populations formed under low-NOx and high-NOx conditions differ chemically? Which facotrs are most responsible for the performance of existing volatility frameworks under low-NOx and high-NOx conditions: molecular functionality, volatility distributions, molecular weight, degree of oxidation, nitrate functionality, or condensational behavior?
Comment #7. "In Section 3.3, the authors report the volatility classification of ambient OOMs. One question is whether sulfur-containing OOMs were detected in the ambient measurements. Sulfur-containing organic compounds, including organosulfates and sulfur-containing highly oxidized molecules, may contribute to NPF and nanoparticle growth in polluted environments. If such species were detected, please discuss their abundance and possible role. If not, please clarify whether this reflects their actual absence or limitations of the measurement technique.Citation: https://doi.org/10.5194/egusphere-2026-3174-RC2
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The manuscript by Li et al. investigates the roles of oxygenated organic molecules (OOMs) and sulfuric acid (H₂SO₄) in explaining atmospheric nanoparticle growth. To isolate condensational growth from coagulation, the authors developed a laminar flow reactor that enables the independent quantification of these two processes. The study combines laboratory experiments using both biogenic and anthropogenic precursors with field observations, providing a comprehensive assessment of particle growth under controlled and ambient conditions. The measurements employ three complementary mass spectrometers to maximize the detection of condensable organic vapours. My expertise lies primarily in atmospheric measurements and modelling, and therefore my comments focus on these aspects rather than on the design and characterization of the newly developed reactor.
Overall, the laboratory and field measurements are comprehensive, the methodology is robust, and the manuscript is well organized and easy to follow. The authors compare several published volatility parameterizations and demonstrate that six of the seven overestimate condensational growth. They attribute this primarily to parameterizations developed for biogenic environments that predict lower OOM volatilities, particularly under high-NOₓ conditions. I believe this is an important contribution to the field and recommend publication after addressing the following minor comments.
Minor comments
1. A summary table comparing the seven volatility parameterizations would greatly improve readability. In particular, it would be useful to summarize the required input variables and intended application of each parameterization. Since the Zhang et al. parameterization is the only one that underestimates condensational growth, the authors should also briefly discuss the key differences that distinguish it from the others.
2. The title, abstract, and conclusions attribute the remaining unexplained particle growth to additional processes that are not represented in the current framework. While this is certainly a plausible interpretation, the manuscript should more explicitly acknowledge other sources of uncertainty, including incomplete detection of condensable vapours by current instrumentation, uncertainties in particle size distribution and growth-rate measurements, and uncertainties within the modelling framework itself. I therefore encourage the authors to slightly soften the title and conclusions to better reflect these remaining uncertainties.
3. The discussion would also benefit from placing the results within the context of other recent studies on condensational growth. In addition, it would be interesting to comment on the broader applicability of the conclusions beyond Lake Tai. For example, would the Zhang et al. parameterization also underestimate particle growth under low-NOₓ, biogenic environments such as Hyytiälä, Finland, or is its performance expected to be location dependent?
Technical comments
• Line 135: "limited resolution" is missing a unit.
• Figure 4 legend panel c: "Zhang (2025)" should be updated to "Zhang (2026)."
• Supplementary Figure 13: "Fig." should read "Fig. S13."