Fig. 5.

Key aerosol and cloud microphysics issues involved in atmospheric RFG techniques. For SAI, MSB and CCT, the aerosol and cloud microphysics involved are poorly understood and challenging to simulate – one of the main hindrances to confidently predicting the potential climate effects. a The size distribution of the injected or produced aerosol particles influences their effectiveness. As illustrated for SAI, initial studies indicate that there is an optimal particle size (estimated at r ≈ 0.25 μm⁹⁸); much smaller particles do not effectively reflect sunlight, while much larger particles sediment out too quickly to produce a significant time-integrated radiative forcing. Simulations indicate that MSB has a smaller optimal particle size (r ≈ 0.13 μm), and oversized particles could even lead to a warming instead of cooling^122,157. CCT is instead mainly affected by the injected particle concentration, with an optimum around 20/l, while excessive concentrations (greater than 100/l) could lead to warming¹³². The size distributions and particle concentrations in turn depend on the particle growth characteristics^98,158,159, for which coagulation is a particularly important and uncertain factor. Chemical composition also influences the aerosol optical properties, but considerable research is needed to better understand this. Similarly, few studies have investigated the dependence on the ambient meteorological conditions, including turbulence, the susceptibility of clouds to formation of precipitation^128,160 (for MSB), and the spectrum of vertical velocities, which affects the activation of cloud condensation nuclei and ice nuclei (for MSB and CCT). b An additional complexity for MSB and CCT is introduced by the aerosol-cloud interactions. The impacts of aerosol particles on cloud optical properties are very difficult to simulate in both cloud-resolving and global climate models, and have been repeatedly highlighted by the IPCC² as one of the most significant uncertainties involved in climate change predictions. The chemical composition of the aerosol particles influences their effectiveness as cloud condensation nuclei (MSB) and ice nuclei (CCT). Finally, aerosol particles can affect cloud lifetimes, especially for MSB, since reducing the size of the cloud droplets can increase the lifetime of the clouds before they form precipitation, but can in turn reduce the lifetime by making them more susceptible to evaporation