Purely light-scattering aerosols could induce positive shortwave radiative forcing over high-albedo surfaces

Abstract. Anthropogenic particles enter the stratosphere both by design and by accident: stratospheric aerosol injection (SAI) proposes deliberate particle injection to partially counteract global warming by increasing planetary albedo, while spacecraft reentry ablation already deposits alumina in stratospheric aerosol at rates growing with launch activity. Scattering particle layers in the stratosphere are conventionally expected to cool the surface; however, their radiative behavior over high-albedo surfaces remains under-examined. This study investigates the counter-intuitive positive shortwave radiative forcing induced by purely scattering aerosols over bright surfaces such as snow and ice. By conducting radiative transfer simulations of an SAI-relevant aerosol layer with libRadtran, we compare the rigorous discrete-ordinate method (DISORT) against the standard two-stream approximation commonly utilized in climate models. We identify significant positive forcing at low solar zenith angles over high-albedo regions, a phenomenon systematically underestimated by the two-stream approximation. To elucidate the mechanism, we derive a first-principles "single-layer adding model" that attributes this positive forcing to the optical path length asymmetry between collimated downwelling solar radiation and diffuse upwelling radiation. We introduce the reflectance asymmetry parameter (Φ) to quantify this asymmetry: diffuse upwelling light encounters a longer optical path through the layer and is preferentially backscattered to the surface. Furthermore, we show that standard daily-averaged forcing metrics obscure these strong diurnal forcing peaks, potentially masking critical impacts on snowmelt. These findings suggest that current assessments may underestimate the localized climatic risks of SAI and extend to any optically thin scattering layer over bright surfaces – including the growing burden of reentry-derived alumina.