Cloud droplet number enhancement from co-condensing NH₃, HNO₃, and organic vapours: boreal case study
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Type
Journal Article
Abstract
Semi-volatile compounds such as organics, nitrate, chloride, and ammonium are ubiquitous in atmospheric aerosols. Their gaseous precursors (organics, HNO3, HCl, NH3) co-condense with water vapour when
ambient relative humidity (RH) increases, thus enhancing hygroscopic growth under sub-saturated conditions
and facilitating activation as cloud condensation nuclei (CCN) to cloud droplets. In this study, we investigate the
co-condensation effect on CCN activation for inorganics, organics, and their combination in a boreal forest site
in autumn with our cloud parcel model that includes non-ideality of organic-inorganic mixtures. The volatility
distribution of organics is highly uncertain but critically important to estimate the co-condensation effect. We
compare two distinct volatility basis sets (VBS) established from experimental and modelling data at 25 °C, which we amended with a volatility bin of saturation concentration C∗ = 10⁴µg m−3, which proved to be highly
relevant for CCN activation. The combined co-condensation of organics and inorganics increases CDNC by up to 44 % in simulations initialized with RH of 80 %, depending on VBS distribution and updraft velocity during the air parcel uplifts. Non-ideality of the system is relevant for considering the co-condensation effect realistically. For the ideal case, the maximum CDNC enhancement due to the combined co-condensation effect is 53 % while it is 44 % for the non-ideal case. The combined enhancement in CDNC of inorganic and organic species exceeds the sum of individual effects and should be further constrained in different environments in cloud parcel models as a basis for regional and global simulations.
ambient relative humidity (RH) increases, thus enhancing hygroscopic growth under sub-saturated conditions
and facilitating activation as cloud condensation nuclei (CCN) to cloud droplets. In this study, we investigate the
co-condensation effect on CCN activation for inorganics, organics, and their combination in a boreal forest site
in autumn with our cloud parcel model that includes non-ideality of organic-inorganic mixtures. The volatility
distribution of organics is highly uncertain but critically important to estimate the co-condensation effect. We
compare two distinct volatility basis sets (VBS) established from experimental and modelling data at 25 °C, which we amended with a volatility bin of saturation concentration C∗ = 10⁴µg m−3, which proved to be highly
relevant for CCN activation. The combined co-condensation of organics and inorganics increases CDNC by up to 44 % in simulations initialized with RH of 80 %, depending on VBS distribution and updraft velocity during the air parcel uplifts. Non-ideality of the system is relevant for considering the co-condensation effect realistically. For the ideal case, the maximum CDNC enhancement due to the combined co-condensation effect is 53 % while it is 44 % for the non-ideal case. The combined enhancement in CDNC of inorganic and organic species exceeds the sum of individual effects and should be further constrained in different environments in cloud parcel models as a basis for regional and global simulations.
Date Issued
2026-02-03
Date Acceptance
2026-01-12
Citation
Atmospheric Chemistry and Physics (ACP), 2026, 26 (3), pp.1735-1749
ISSN
1680-7316
Publisher
Copernicus Publications
Start Page
1735
End Page
1749
Journal / Book Title
Atmospheric Chemistry and Physics (ACP)
Volume
26
Issue
3
Copyright Statement
© Author(s) 2026. This work is distributed under the Creative Commons Attribution 4.0 License.
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Publication Status
Published
Date Publish Online
2026-02-03