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

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.

First, as highlighted in the Academic Beneficiaries section, the beneficiaries comprise a much wider group than that of the investigators' immediate professional circle, reaching directly out to large-scale modellers carrying out direct impact-related research for climate integrations, for example. Whilst these activities may be considered to be within the academic community, they are frequently the end-use global and regional climate and weather tools developed and used by the met services of many countries. The form of the parameterisations developed in task 3 lend themselves directly to incorporation in these models. This will be directly demonstrated in task 4 by the implementation in a regional model. The WRF-Chem model is a US development, but is widely adopted worldwide and carries appropriate gas, aerosol and cloud descriptions to represent the process. In Europe, the COSMO-ART and ECHAM models are capable recipients and hence European Met services using these models will benefit, being able to predict the changes in cloud properties because of the process investigated in the proposal. The parameterisations will be developed in conjunction with Prof. Nenes (see Letter of Support) and so will be directly usable within the NASA GEOS-5, GEOS-Chem and NCAR CAM-5 models.

The Met Office is the obvious UK beneficiary, with UKCA not too far from being sufficiently well-developed to incorporate the parameterisation and with simulations feeding the IPCC AR5/CMIP5 process. The effect will be manifested at the scale of "weather" as well as "climate". However, none of the Numerical Weather Prediction (NWP) models are sufficiently well-developed to operationally predict the impacts. Uncoupled models of weather and chemistry cannot capture the effects, but demonstration of substantial effects in WRF-Chem within WP4 may stimulate a need for adoption of such coupled frameworks.

The Met Office, through project partner, Paul Field will provide access to the new UKCA-4A microphysics scheme, which is an aerosol-cloud scheme that consistently couples
aerosol particles and cloud microphysics. It is being used in research configurations of the Met Office Unified Model; the Large Eddy Model (LEM) and the Kinematic Driver Model (KiD). As part of the project we will implement the semi-volatile co-condensation code and test it for case studies in within the LEM (SAMBBA). We will also implement the ice nucleation scheme and test it for case studies from the up and coming INUPIAQ data. All code updates will be provided to the UKCA-4A community. An assessment will be made as to whether the updates lead to an improvement in simulated cloud properties and hence whether the parameterisations should be developed for inclusion in the Met Office NWP simulations.

Agency beneficiaries such as DECC would need to respond in the longer term, since the models are currently too poorly developed. Local authorities will also benefit from this research, once reliable predictions of air quality as well as weather are necessary, probably over the coming decade. The process under investigation is one of the most likely to lead to strong coupling between the atmospheric composition and weather.