Sensitivity studies on cloud droplet number enhancement from the co-condensing NH3, HNO3, and organic vapours in Hyytiälä, Finland

Semi-volatile compounds (organics, nitrate, chloride, ammonium) are ubiquitous in atmospheric aerosols and usually contribute over 50% to the fine aerosol mass worldwide. Their gas precursors (organics, HNO₃, HCl, NH₃) can co-condense with water vapour as an extra source of aerosol particle growth when ambient relative humidity (RH) increases, therefore facilitating hygroscopic growth under sub-saturated conditions and activation to cloud condensation nuclei (CCN). Yet, the attribution of co-condensing semi-volatile compounds to the CCN activation is poorly understood.

Topping et al. (2013) developed a cloud parcel model to simulate the co-condensation effect of organics and sensitivities to key influencing factors (e.g. aerosol concentration, updraft velocity) for the first time. Building on Topping’s study, we further developed a cloud parcel model that simulates co-condensation for both organic and inorganic compounds. We used in-situ observations of gas and aerosols from SMEAR II Hyytiälä Forestry Field Station in Finland as input and quantified co-condensation for inorganics, organics, and their combination. Evaporation losses of particulate semi-volatile compounds in the sampling and non-ideality of organics are also considered.

In Hyytiälä, the inclusion of co-condensing semi-volatile compounds to CCN activation is sensitive to the updraft velocity (0.003 – 5 m s^-1) and assumed volatility distribution and non-ideality of organics. The volatility distribution of organics is highly uncertain but important because it relates the amount of organic gas precursors with measured mass concentration in the condensed phase. Topping et al. (2013) simulated co-condensation of organic compounds with volatility bins up to C* = 10^-3 μg m^-3, saturation mass concentration of organics in condensed phase. To understand the role of more volatile bin C* = 10^-4 μg m^-3 which is usually considered too volatile for co-condensation, we modified volatility basis set of Topping et al (2013) by adding an extra bin C* = 10^-4 μg m^-3. We found that the bin C* = 10^-4 μg m^-3 can play a large role in CCN activation when temperature decreases, resulting in a 30% higher cloud droplet number concentration (CDNC), consistent with Heikkinen et al. (2024). For the combined co-condensation of organics and inorganics increase CDNC by up to 52% with bin C* = 10^-4 μg m^-3 compared to 40% without the bin. The semi-volatile compounds evaporated by ~10% due sampling losses, dryer tubes, and outdoor-indoor temperature changes before detection by the instrument, which should be considered in the total organic mass estimate. Non-ideality of the system is important for considering the co-condensation effect realistically. Assuming ideality, co-condensation is overestimated by 100% in CDNC. The combined enhancement in CDNC of inorganic and organic species goes beyond the sum of individual effects and should be further constrained and properly estimated in models.

Reference:

Heikkinen et al. (2024), Cloud response to co-condensation of water and organic vapors over the boreal forest, Atmos. Chem. Phys., 24(8), 5117-5147.

Topping, D., P. Connolly, and G. McFiggans (2013), Cloud droplet number enhanced by co-condensation of organic vapours, Nature Geoscience, 6, 443.