Diffusion and Equilibration in Viscous Atmospheric Aerosol

Aerosols are a key component of the atmosphere. Defined as either solid particles or liquid droplets dispersed in the gas phase, aerosols can scatter and absorb sunlight and terrestrial infrared radiation influencing the radiation budget and
having a direct effect on climate. They also act as nuclei on which water can condense, leading to the formation of cloud droplets, indirectly influencing the climate. As well as having many natural sources, they can form in polluted environments from the condensation of semi-volatile organic compounds forming secondary organic aerosol (SOA). The composition of SOA is rich in oxidised organic compounds and can contain organic molecules of high molecular weight. When the atmosphere is dry or cold, SOA particles can be highly viscous; indeed, it has been shown that SOA can exist as glassy particles. As such, droplets formed from water or formed from highly viscous SOA can differ in their viscosity by more than 15 orders of magnitude.

Aerosol droplets that are largely water (eg. cloud droplets) have low viscosity, flow readily, and deform and spread when deposited. When exposed to changes in relative humidity and temperature, they can respond quickly to the change in the environment, losing or gaining water and also any semi-volatile or volatile organic compounds. They are, in essence, at equilibrium in composition with the surrounding gas phase. For particles approaching the glass transition, the particles do not deform and have the mechanical properties of a solid. They can only respond slowly to changes in the environment, losing or gaining water, semivolatile and volatile organic components only very slowly. Indeed, it can be estimated that such particles could in principle take many days to equilibrate and suggesting that SOA can exist in a kinetically arrested/hindered state in the atmosphere. Predicting the properties and impacts of aerosol in the atmosphere relies on knowing if the aerosol mass is in thermodynamic equilibrium or if it is kinetically limited, with significant consequences for understanding even the mass of aerosol in the atmosphere and the ability of the aerosol to form liquid cloud droplets or ice crystals.

In this project, we will use a combination of single particle measurements, models and simulations to characterise the viscosity of ambient particles and the diffusion kinetics of water and organic components within viscous aerosol.
Measurements will be made using individual particles captured in aerosol optical tweezers or in an electrodynamic balance. Light scattering measurements that allow the accurate determination of droplet size and refractive index will be used to examine the response of the particle to changes in environmental conditions. From the time-dependence of these changes, the diffusion of molecules within the particle can be determined. The viscosity can be measured directly by coalescing two particles and determining the timescale for the shape of the composite particle to relax to a sphere. Measurements of particles of simple and complex composition will be used to refine models of aerosol viscosity and molecular diffusion constants.

In a final stage, the refined models will be used to assess the properties of viscous aerosol in the atmosphere. Initially, the role of viscous aerosol will be evaluated in a detailed model of the processes occurring in aerosol chamber measurements designed to simulate atmospheric aerosol. This will allow an assessment of the accuracy with which non-equilibrium kinetically limited aerosol processes can be captured and how sensitive the chamber measurements are to non-equilibrium effects. Finally, the sensitivity of atmospheric aerosol to non-equilibrium effects will be investigated using a wider scale regional model.

In summary, we will seek to better define when aerosol can be considered to be at equilibrium and when kinetically limited in the atmosphere.

The specific impact of improving our understanding of the role of kinetic factors in governing the properties of atmospheric aerosol will largely benefit academic researchers working in atmospheric science, with an immediate tier being researchers requiring knowledge of the microphysical processes occurring in aerosol, as described under Academic Beneficiaries. Web-based tools will be extended to allow users to calculate the viscosity and diffusion constants of molecules in viscous aerosol and these will find immediate use by, for example, the project partners who will use them in microphysical kinetic models (Shiraiwa, MPI-Mainz,Germany) and to interpret analytical measurements (eg. particle bounce, Virtanen). The development of a kinetic framework within the Manchester Chamber Model (MANIC) will also lead to improvements in the interpretation of historical aerosol chamber measurements and future campaigns. Wider impact could result from the adoption of the model framework by a wider range of chamber instruments, benefiting from the involvement of two of the PIs in projects such as EUROCHAMP3.

The primary non-academic end-users of the proposed programme output in the UK would be the Met Office via existing links with the UKCA Climate-Chemistry-Community-Aerosol model, a joint NCAS-Met Office programme funded by NCAS,GMR and DEFRA. The impacts of aerosol on climate are still credited with the largest uncertainty in climate forcing and a large part of the radiatively active boundary layer sub-micron aerosol burden is organic. Policy decisions with respect to quantification and mitigation of the climate impacts of aerosol require policy-related model simulations with at least a rudimentary but physically-based representation of organic aerosol. Prior to the proposed work, such model descriptions that include kinetic factors in regulating aerosol composition are unavailable and our study of the properties dictating gas/particle partitioning of organic compounds will be inform such a climate-focused goal. Other international non-academic agencies conducting IPCC simulations would be best placed to use the same reduced complexity secondary organic aerosol formalisms as supplied to the Met Office.

Many of the microphysical processes that will be studied have a significance than extends to academic beneficiaries beyond atmospheric science. The formation and properties of amorphous aerosol are important for large scale industrial processes such as spray drying in which microparticles are fabricated, often in amorphous states, through driving systems far from thermodynamic equilibrium and using the transport kinetics of volatile components to mediate physical transformations. Further, understanding water transport kinetics is important for better quantifying processes occurring during the delivery of drugs to the lungs and in the impact of environmental pollution on human morbidity and mortality.

Both viscosities and diffusion constants are fundamental quantities that are challenging to measure in supersaturated states and in the exotic compositional regimes in which aerosol exist. The measurements will provide a
broad catalogue of values for these properties and the refined models will provide a robust and versatile treatment over a wide range in conditions. Thus, they will find relevance in a broader range of chemical disciplines in which rheological and mass transport quantities are required.

Finally, air quality and climate change are important topics in the new 21st Century Science curriculum at GCSE and A level in the UK, and are of wider interest to the public. The impact from this project will be directly incorporated in the ongoing public engagement contributions made by the researchers leading this project.