The effects of organic material on warm and cold cloud formation: from the laboratory to regional and global impacts

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Clouds have a profound influence on weather and climate. Formation of cloud droplets by condensation of water vapour on particles has been studied for many decades. For inert involatile particles, this process and its impacts are relatively well understood. However, a substantial proportion of fine particle material can evaporate under some atmospheric conditions. Our recent Nature Geoscience Letter suggests that the role of this fraction on cloud droplet formation is large enough to be globally significant, is not normally considered in cloud parcel models and is completely untreated in large-scale models. This results from the co-condensation of partly volatile material along with the water vapour during droplet activation. Indirect evidence supports this effect, but direct measurements are unavailable. There has also been considerable interest in the potential role of amorphous "glassy" particles as seeds for ice crystals in cold and mixed-phase clouds. The Nature publication and subsequent work by project partner Virtanen identified that secondary organic aerosol from both biogenic and anthropogenic precursors could exist in an amorphous state dependent on relative humidity and temperature. The impact of glassy particles as ice nuclei is potentially very significant, but direct evidence is currently confused and realistic supporting measurements are sparse. It is proposed to quantify the impacts of organic components on warm and cold cloud formation by both processes through simulation chamber measurements, to use the measurements to evaluate a recently developed model treatment, to parameterise the model and use the parameterisation to quantify the regional impacts on cloud physical and radiative properties.We have conducted proof of concept laboratory work showing that we are able to study both processes. We have coupled the Manchester Aerosol Chamber (MAC), where we can make particles from the atmospheric chemistry of both natural plant emissions and man-made emissions, to the Manchester Ice Cloud Chamber (MICC), where we can form a cloud under reasonable atmospheric conditions. We have further measured the changes in the effectiveness of the particles to act as seeds for liquid cloud droplets, cloud condensation nuclei (CCN), along with the volatility, composition and phase behaviour. We propose to build on this proof-of-concept to systematically quantify the effects in a range of atmospherically-representative systems and quantify their impacts.The proposed work will be carried out in 4 parts. The first two are laboratory-based with numerical model interpretation and the second two solely use numerical modelling: i) quantification of the effect of organic vapours in two instruments that are used in the field and laboratory, one measuring particle water uptake below 100% RH and the other the ability to form a cloud droplet just above 100% RH. Particles will be exposed to controlled concentration of semi-volatile vapour and introduced into the instruments. Detailed flow modelling of the second instrument will be carried out, in collaboration with the author as project partner.ii) involves the coupling of the MAC and MICC chambers as in the proof-of-concept, but covering particles formed in a wide range of natural, manmade and mixed systems. We will measure all relevant parameters to quantify the formation of warm and cold clouds under a reasonable range of atmospheric conditions.iii) informed by the experiments, the effects of organic compounds on warm and cold clouds will be included in a numerical model and this will be used to develop physically-based parameterisations for use in large-scale models.iv) the parameterised process description will be used in large-scale models informed by our project partner Nenes to estimate the impact on cloud properties and radiation, hence quantifying the couplings between organic compounds and weather and climate under representative conditions.