Spectral Radiative Effects of Extreme Wildfire Aerosols at the Izaña High-Altitude Observatory

High-altitude observatories play a key role in monitoring background atmospheric composition and radiation, yet they are increasingly affected by extreme wildfire plumes. In August 2023, an intense wildfire on Tenerife (Canary Islands, Spain) reached the immediate surroundings of the Izaña Observatory, creating an exceptional near-source observational configuration at a high-altitude mountain site. This event provides a rare opportunity to investigate the spectral radiative effects of freshly emitted biomass-burning aerosols under extreme loading conditions in a clean-background environment. During the most intense phase of the episode, aerosol optical depth reached exceptionally high values, while the Ångström Exponent remained consistently elevated, indicating a strong dominance of fine-mode smoke particles. Spectral measurements of global, direct-normal and diffuse solar irradiance across the ultraviolet to near-infrared range reveal a strong attenuation of the direct solar component and a pronounced enhancement of diffuse radiation, particularly in the visible spectrum. Relative to clean-sky conditions, daily global irradiance experienced a substantial reduction, while direct-normal irradiance was strongly suppressed, in some cases approaching complete extinction. Surface aerosol radiative forcing and radiative forcing efficiency were quantified using radiative transfer simulations assuming pristine atmospheric conditions as reference. The resulting shortwave radiative forcing indicates intense surface cooling, largely driven by aerosol scattering processes. Maximum forcing and efficiency occurred in the visible spectral range, consistent with the optical properties of freshly emitted smoke aerosols. Despite a reduction in aerosol loading during the later stage of the event, radiative forcing efficiency remained comparable or slightly enhanced, reflecting changes in aerosol optical properties and solar geometry. Concurrent increases in particle concentrations, black carbon and trace gases confirm the direct impact of the wildfire plume on atmospheric composition at the observatory. These results demonstrate how extreme wildfire events can temporarily disrupt radiative and compositional conditions at high-altitude background sites and highlight the importance of accurately representing fine-mode smoke aerosols in radiative transfer and climate models.