![]() Such a lengthening has been used to propose significant kinetic limitations on growth and evaporation (Fig. The dramatic decrease in diffusion constant that can be expected to accompany an increase in viscosity can lead to a lengthening in the diffusional mixing time, τ mix, within a particle of radius a, which can be estimated from 16: d In combination, differences in the mechanisms and rates of microphysical processing in viscous aerosol particles when compared to low-viscosity solution droplets can have important consequences for the impacts of aerosols on climate, visibility, air quality and health In viscous particles, heterogeneous chemistry may occur only very slowly and be confined to the particle surface. c Heterogeneous chemistry can occur rapidly in low-viscosity particles throughout the particle bulk. ![]() b Unlike liquid droplets, glassy particles can act as heterogeneous nuclei for ice nucleation. a Although low-viscosity aerosol particles equilibrate in composition rapidly, highly viscous particles may require time to achieve an equilibrium composition through the gas-particle partitioning of water, semi-volatile organic compounds (SVOCs) and other pollutants. Impacts of ambient particle viscosity and phase on climate and health. Einstein first established the slowing of diffusional motion in a medium of increasing viscosity, with this inverse relationship apparent in the Stokes–Einstein equation: By contrast, highly viscous organic particles can be slow to respond to changes in gas-phase composition, particularly under dry conditions or at low temperatures 5, 7, 13. Liquid organic particles with low viscosity are responsive to changes in gas-phase composition and take up or lose water in response to variations in ambient relative humidity (RH) and temperature 13, 14. As an example, the rates of growth and evaporation of organic particles are dependent on particle viscosity, with direct implications for climate, visibility and air quality 10– 12. The viscosity of atmospheric OA is central to rationalising and predicting their atmospheric impacts (Fig. Thus, much of our review necessarily considers the consensus that is emerging from laboratory work on the phase state of ambient particles, supported by the limited number of field measurements that are now emerging. Many of the studies on which our understanding of atmospheric aerosol particle viscosity is based have been undertaken in the laboratory using surrogates of ambient particles this is a consequence of the challenges of making aerosol particle viscosity measurements directly. Our focus here is to explore the compositional and environmental factors that govern particle viscosity, the consequences of particle viscosity for aerosol microphysics and the global impacts that can then result. In stark contrast, recent measurements of organic particle properties often imply the existence of highly viscous semi-solid and even amorphous solid particles 4– 6. Until recently, researchers assumed that atmospheric organic particles are liquid in phase and, hence, have low viscosity. The composition of SOA is especially uncertain, with only approximately 10% of the mass of secondary OA identified at the molecular level 2. Ambient organic aerosols (OA) can be emitted directly (referred to as primary OA) or can be formed by a complex series of reactions (referred to as secondary OA, SOA) 3. The organic component can represent 50% or more of the mass of the fine aerosol particle fraction (particles smaller than 1 μm in diameter) in the atmosphere 2. Atmospheric particles, which range in size from <10 nm to ∼10 μm, consist of both inorganic and organic material 1, 2. ![]()
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