| 15 Jul 2026
Multi-year characterization of low volatility vapors in a boreal forest
Abstract. Atmospheric low volatility vapors play an essential role in aerosol particle formation, growth, and cloud condensation nuclei production, thereby influencing climate. While intensive measurements have been carried out in different locations, little is known about their seasonal variability due to a lack of long-term measurements. To address this gap, we present nearly 4 years of continuous observations of low volatility vapors in a boreal forest measured using a NO3- atmospheric pressure interface time-of flight (MION-Api-TOF) mass spectrometer. Our results reveal the seasonal variation in the concentration, molecular composition, formation mechanism and volatility distribution of highly oxygenated organic molecules (HOMs). We show that while temperature-dependent terpene emissions regulate the overall seasonal abundance of HOMs, distinct formation pathways govern their diurnal profiles: monoterpene-derived monomers peak during the day due to the availability of precursors and oxidants, whereas their dimers peak at night driven by low concentrations of terminating species (NO and HO2) and extended RO2 radical lifetimes. We observed differing seasonal changes across different HOMs species and, although C10 HOMs remains the dominant HOM species during the whole year, sesquiterpene-derived C15 HOMs shows the steepest increase during summer. Using binned Nonnegative Matrix Factorization (bin-NMF), we found that HOM composition is strongly modulated by seasonality. While monoterpene-derived HOMs containing nitrogen atoms (“CHON”) dominate during colder months, summertime chemistry is characterized by a shift toward “CHO” species and a substantial contribution from heavy terpenes (sesquiterpenes and diterpenes), which can account for 40 %–80 % of the signal in the high-mass range (m/z 450–700, including NO3-). We also analysed the seasonal volatility distribution of HOMs and identified the primary contributors to each volatility class. Crucially, challenging the long-held assumption that monoterpene derived C17-20 HOM dimers are the most important biogenic precursors of new particle formation in the boreal forest, we found sesquiterpene-derived C13-15 HOMs and C17-20 HOMs have comparable contribution to Ultra-Low Volatility Organic Compounds (ULVOCs) during daytime throughout much of the year. Moreover, in spring – the season with the highest NPF frequency at our measurement site, sesquiterpene-derived C13-15 HOM account for approximately 40 % of ULVOC during daytime, compared to 28 % for C17-20 HOMs. Our findings provide critical new insights into the seasonal dynamics of HOM composition and their broader atmospheric implications.
Competing interests: At least one of the (co-)authors is a member of the editorial board of Atmospheric Chemistry and Physics.
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General comments:
This manuscript presents a valuable dataset of nearly four years of continuous measurements of low-volatility vapors in a boreal forest using NO3-CIMS. By employing high resolution peak fitting, bin-NMF analysis, and volatility estimation, the authors reveal the seasonal variability of concentrations, molecular composition, formation mechanisms, and volatility distributions of highly oxygenated organic molecules (HOMs). They show that temperature-dependent terpene emissions regulate the overall seasonal abundance of HOMs, while distinct formation pathways govern their diurnal profiles. In particular, they highlight an important contribution of sesquiterpenes to HOM concentrations in summer and to ULVOCs across all seasons, which challenges previous assumptions that monoterpenes are the dominant precursors for new particle formation in boreal forests. The long-term dataset presented here is a significant contribution to the field and is valuable for understanding the seasonal dynamics of HOMs and their atmospheric and climate implications. Overall, the study is well executed and the manuscript is nicely written. I recommend its publication after the following comments are addressed.
Specific comments:
L81: The authors noted that “under low-NO regime, NO can enhance the HOM formation… (Nie et al., 2023)”, yet they only qualitatively correlated seasonal and diurnal patterns of NO with HOM concentrations and molecular composition. Could the authors provide a more quantitative analysis of the effects of NO on HOM formation? In particular, was the non-linear effect of NO observed during the long-term measurements in this study?
L158: The authors mentioned a “limited number of calibrations”. It would be helpful to specify how many calibrations were performed and at what intervals throughout the observation period.
L161: The authors estimated a large uncertainty in the quantification of HOMs (±600%), yet many conclusions are presented in terms of absolute concentrations and relative contributions. While the authors appropriately noted these caveats, a more explicit discussion of how the large quantification uncertainty affects the robustness of the key conclusions should be provided. For example, when stating that C15 compounds account for ~40% of daytime ULVOC in spring, how does the quantification uncertainty affect this percentage? In addition, the authors should briefly explain how these uncertainty values (±200% for SA and ±600% for HOMs) are estimated.
L246: This statement should be rephrased, as H-abstraction pathways in OH-initiated oxidation of alpha-pinene can also contribute significantly to the formation of C10H15O6,8,10 radicals (See Shen et al., Sci. Adv., 2022, 8, eabp8702).
L263-265: The C17-20HyOz species with fewer than 10 oxygen atoms cloud arise from dimerization of RO2 species with fewer than 6 oxygen atoms, which often dominate the RO2 pool during monoterpene oxidation. The authors should refine this discussion to account for the diversity of oxidation states among HOM dimers formed via RO2 cross reactions.
L340: It would be helpful to briefly explain why bin-NMF analysis was performed separately for the m/z 260-450 and m/z 450-700 ranges.
L380: The authors assigned the C15/C17 HOM factor to isoprene-monoterpene HOM dimers, but this appears inconsistent with the C17H28O6 fingerprint molecule. The proposed assignment should be reconsidered or at least more cautiously worded.
L382-383: The authors stated that the compounds in Factor 7 are mostly direct closed-shell products from O3-initiated RO2 radicals (e.g., C10H15O8,10). As noted above, H-abstraction channel during OH oxidation can also yield such species. In addition, this factor peaked after 18:00, substantially later than other three types of MT HOM monomers. The authors should provide an explanation for this difference?
L507-509: As shown in Fig. 9, a marked difference in HOM volatility distribution was also observed between daytime and nighttime. What are the main driving factors for such variation?
L518: The contribution of sesquiterpenes to ULVOCs is stated as approximately 50% here but as 40% in the Abstract. Please confirm and reconcile these values for consistency.