Antonio Fernando Almansa

Extreme wildfires represent a highly variable source of atmospheric aerosols with potentially strong impacts on surface solar radiation. In August 2023, an exceptional wildfire on Tenerife (Canary Islands, Spain) reached the neighbourhoods of the Izaña Observatory (IZO, 2400 m a.s.l.). This near-source configuration enabled a rare observational characterisation of the spectral radiative effects of biomass-burning aerosols. During the most intense phases of the event (17–18 August), aerosol optical depth (AOD) at 500 nm reached extreme values of 3.63 and 2.25, respectively, with Ångström Exponent (AE) above 2, indicating a strong dominance of fine-mode smoke particles. Spectral measurements of global-horizontal, direct-normal and diffuse-horizontal solar irradiance (300–1100 nm) show a pronounced attenuation of direct and global irradiances, particularly in the visible range, together with a strong enhancement of diffuse radiation. Relative to clean-sky conditions, daily global irradiance decreased by 21 %–27 %, while direct-normal irradiance was reduced by 72 %–99 %. Spectral aerosol radiative forcing and radiative forcing efficiency at the surface were quantified using radiative transfer simulations under pristine atmospheric conditions as a reference. The integrated spectral radiative forcing (300–1100 nm) for global irradiance reached −395 and −299 W m−2 on 17 and 18 August, respectively, indicating strong surface cooling dominated by scattering processes. Maximum forcing and efficiency occurred in the visible spectral range, consistent with the optical properties of freshly emitted smoke aerosols. At the same time, increases in the amount of present particles, equivalent black carbon (eBC) and greenhouse gases (CO2, CH4 and CO) confirm the direct influence of the wildfire plume on atmospheric composition at IZO. These observations provide one of the few detailed spectral assessments of surface radiative forcing by extreme biomass-burning aerosols at a high-altitude site and highlight the need to accurately represent fine-mode smoke aerosols in radiative transfer and climate models.

Ultraviolet atmospheric measurements play a key role in characterizing aerosol properties, as absorption and scattering in this spectral range were highly sensitive to particle size and composition. The Izaña Observatory offers an exceptional natural laboratory for such observations due to its stable free‑tropospheric conditions and long‑term reference datasets. Within this framework, the aerosol optical depth (AOD) retrieved from a BTS spectroradiometer was evaluated by comparing its AOD measurements with those from a CIMEL CE318‑T sunphotometer (AERONET) and a other spectrometer.Langley analysis, applied to the BTS spectral measurements, characterized the instrument under stable atmospheric conditions and provided a consistent reference framework. Daily AOD values were then obtained from the processed morning measurements using standard quality‑control procedures. The comparison with the CIMEL and Brewer instruments considered both clean free‑tropospheric days and periods affected by transported mineral dust, allowing to assess the behaviour of the BTS across a wide range of aerosol loadings and atmospheric conditions.The BTS spectrometer reproduced the temporal evolution and spectral behaviour captured by the reference instruments, with consistent agreement at both wavelengths and slightly more stable performance at 340 and 380 nm. Differences generally remained within expected uncertainty ranges, although larger deviations appeared under higher aerosol concentrations, reflecting the increased sensitivity of UV retrievals to atmospheric variability. Overall, the comparison indicated that the BTS provided reliable UV AOD measurements under high-altitude conditions.

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.

The complex interactions of atmospheric aerosols with solar radiation and clouds represent a major source of uncertainty in atmospheric effective radiative forcing. These interactions are driven by aerosol optical and microphysical properties and depend critically on their spatial and vertical distribution. Consequently, obtaining accurate measurements with high spatial and temporal resolution is essential for improving climate models and reducing uncertainty. To this end, remote sensing techniques are employed globally from both space-borne and ground-based platforms. While space-borne instruments provide superior spatial and temporal coverage, ground-based techniques offer more limited spatial extent but higher measurement quality and precision.However, both techniques measure optical quantities that depend on aerosol properties, which necessitates the use of inversion algorithms to retrieve these underlying characteristics. Among ground-based techniques, sun-sky-lunar photometers and lidars are two of the most prominent instruments, and inversion methods have been developed and applied to each separately. For instance, the AERONET inversion algorithm for photometers employs both aerosol optical depth (AOD) and sky radiance measurements at multiple viewing geometries across four wavelengths to retrieve the aerosol volume size distribution and complex refractive index. For lidars, methods as the Klett-Fernald and Raman enable the retrieval of vertically resolved aerosol properties. Nevertheless, each technique has inherent limitations: while sun photometer inversions can retrieve both optical and microphysical properties, they lack vertical resolution due to the column-integrated nature of their measurements. Conversely, lidar-based methods provide excellent vertical resolution but often rely on assumptions or ancillary data.Consequently, the combined use of photometers and lidar systems offers the potential to provide complete and robust characterization of vertically resolved aerosol properties. To this end, numerous inversion algorithms have been developed that combine sun-sky-lunar photometers with low-power lidar systems to retrieve both columnar and vertically resolved aerosol optical and microphysical properties. This approach benefits from enhanced spatial and temporal coverage due to the widespread availability of both instrument types. Among these algorithms, GARRLiC (Now integrated into the GRASP algorithm) stands out as a flexible option capable of being applied to a wide range of photometers and lidar configurations, providing both intensive and extensive aerosol properties for two aerosol modes in the atmospheric column and with vertical resolution when the lidar system includes multiple wavelengths or polarization channels.This study presents a synthetic evaluation of a GRASP-based inversion combining AOD and sky radiance observations (440, 675, 870, and 1020 nm) from a CE318-T sun-sky-lunar photometer with dual-wavelength elastic lidar measurements (532 and 808 nm) from a CE376 micro-pulse lidar to retrieve both columnar and vertically resolved optical and microphysical properties for two aerosol modes (fine and coarse). Our methodology involves generating synthetic observations from selected aerosol properties, adding realistic noise based on reported uncertainties, performing GRASP inversions, and comparing retrieved parameters with input values under diverse aerosol loadings and viewing geometries. This framework provides comprehensive characterization of the algorithm's performance, accuracy, and sensitivity, validating the method for operational application.

Extreme wildfires represent a highly variable source of atmospheric aerosols with potentially strong impacts on surface solar radiation. In August 2023, an exceptional wildfire on Tenerife (Canary Islands, Spain) reached the neighbourhoods of the Izana Observatory (IZO, 2400 m a.s.l.). This near-source configuration enabled a rare observational characterisation of the spectral radiative effects of biomass-burning aerosols. During the most intense phases of the event (17–18 August), aerosol optical depth (AOD) at 500 nm reached extreme values of 3.63 and 2.25, respectively, with Angstrom Exponent (AE) above, indicating a strong dominance of fine-mode smoke particles. Spectral measurements of global-horizontal, direct-normal and diffuse-horizontal solar irradiance (300–1100 nm) show a pronounced attenuation of direct and global irradiances, particularly in the visible range, together with a strong enhancement of diffuse radiation. Relative to clean-sky conditions, daily global irradiance decreased by 21–27 %, while direct-normal irradiance was reduced by 72–99 %. Spectral aerosol radiative forcing and radiative forcing efficiency at the surface were quantified using radiative transfer simulations under pristine atmospheric conditions as a reference. The integrated spectral radiative forcing (300–1100 nm) for global irradiance reached -395 and -299 W m⁻² on 17 and 18 August, respectively, indicating strong surface cooling dominated by scattering processes. Maximum forcing and efficiency occurred in the visible spectral range, consistent with the optical properties of freshly emitted smoke aerosols. At the same time, increases in the amount of present particles, equivalent black carbon (eBC) and greenhouse gases (CO₂, CH₄ and CO) confirm the direct influence of the wildfire plume on atmospheric composition at IZO. These observations provide one of the few detailed spectral assessments of surface radiative forcing by extreme biomass-burning aerosols at a high-altitude site and highlight the need to accurately represent fine-mode smoke aerosols in radiative transfer and climate models.

Extreme wildfires represent a highly variable source of atmospheric aerosols with potentially strong impacts on surface solar radiation. In August 2023, an exceptional wildfire on Tenerife (Canary Islands, Spain) reached the neighbourhoods of the Izana Observatory (IZO, 2400 m a.s.l.). This near-source configuration enabled a rare observational characterisation of the spectral radiative effects of biomass-burning aerosols. During the most intense phases of the event (17–18 August), aerosol optical depth (AOD) at 500 nm reached extreme values of 3.63 and 2.25, respectively, with Angstrom Exponent (AE) above, indicating a strong dominance of fine-mode smoke particles. Spectral measurements of global-horizontal, direct-normal and diffuse-horizontal solar irradiance (300–1100 nm) show a pronounced attenuation of direct and global irradiances, particularly in the visible range, together with a strong enhancement of diffuse radiation. Relative to clean-sky conditions, daily global irradiance decreased by 21–27 %, while direct-normal irradiance was reduced by 72–99 %. Spectral aerosol radiative forcing and radiative forcing efficiency at the surface were quantified using radiative transfer simulations under pristine atmospheric conditions as a reference. The integrated spectral radiative forcing (300–1100 nm) for global irradiance reached -395 and -299 W m⁻² on 17 and 18 August, respectively, indicating strong surface cooling dominated by scattering processes. Maximum forcing and efficiency occurred in the visible spectral range, consistent with the optical properties of freshly emitted smoke aerosols. At the same time, increases in the amount of present particles, equivalent black carbon (eBC) and greenhouse gases (CO₂, CH₄ and CO) confirm the direct influence of the wildfire plume on atmospheric composition at IZO. These observations provide one of the few detailed spectral assessments of surface radiative forcing by extreme biomass-burning aerosols at a high-altitude site and highlight the need to accurately represent fine-mode smoke aerosols in radiative transfer and climate models.

Extreme wildfires represent a highly variable source of atmospheric aerosols with potentially strong impacts on surface solar radiation. In August 2023, an exceptional wildfire on Tenerife (Canary Islands, Spain) reached the neighbourhoods of the Izana Observatory (IZO, 2400 m a.s.l.). This near-source configuration enabled a rare observational characterisation of the spectral radiative effects of biomass-burning aerosols. During the most intense phases of the event (17–18 August), aerosol optical depth (AOD) at 500 nm reached extreme values of 3.63 and 2.25, respectively, with Angstrom Exponent (AE) above, indicating a strong dominance of fine-mode smoke particles. Spectral measurements of global-horizontal, direct-normal and diffuse-horizontal solar irradiance (300–1100 nm) show a pronounced attenuation of direct and global irradiances, particularly in the visible range, together with a strong enhancement of diffuse radiation. Relative to clean-sky conditions, daily global irradiance decreased by 21–27 %, while direct-normal irradiance was reduced by 72–99 %. Spectral aerosol radiative forcing and radiative forcing efficiency at the surface were quantified using radiative transfer simulations under pristine atmospheric conditions as a reference. The integrated spectral radiative forcing (300–1100 nm) for global irradiance reached -395 and -299 W m⁻² on 17 and 18 August, respectively, indicating strong surface cooling dominated by scattering processes. Maximum forcing and efficiency occurred in the visible spectral range, consistent with the optical properties of freshly emitted smoke aerosols. At the same time, increases in the amount of present particles, equivalent black carbon (eBC) and greenhouse gases (CO₂, CH₄ and CO) confirm the direct influence of the wildfire plume on atmospheric composition at IZO. These observations provide one of the few detailed spectral assessments of surface radiative forcing by extreme biomass-burning aerosols at a high-altitude site and highlight the need to accurately represent fine-mode smoke aerosols in radiative transfer and climate models.

We present a characterization of the optical properties of different aerosol types based on data collected using a compact dual-wavelength depolarization elastic lidar (532 and 808 nm, CIMEL CE376). We evaluate the vertical distribution and temporal evolution of a variety of aerosols observed in the subtropical North Atlantic region, covering from Saharan dust outbreaks and volcanic aerosols to fresh and aged wildfires, observed between August 2021 and August 2023, using a modified two-wavelength Klett inversion method to derive the aerosol backscatter and extinction coefficients from CE376 lidar observations. We assessed the performance of the CE376 lidar during an intercomparison campaign with an MPL-4B lidar (MPLNET) while collocated at the facilities of the Izaña Atmospheric Research Centre (Canary Islands, Spain). Both instruments depicted the vertical aerosol structure similarly. The main differences were attributed to errors arising from the determination of the overlap function and the depolarization calibration in each instrument and the larger effect of the solar background on the CE376 system during daylight. Absolute differences in the volume depolarization ratio (δ^v) were 0.3 % reduced to 0.2 % when only nighttime data was used. The measurements of particle linear depolarization (δ^P), extinction Ångström exponent (EAE (532/808)) and attenuated colour ratio (ACR (808-532)), provided by the combination of the two channels of the CE376, allow us to describe the composition and size of the studied aerosols. Fresh Saharan dust particles were the largest particles found in this study with non-spherical morphology and traveling in a well-mixed layer, exhibiting the lowest EAE (532/808) and highest ACR (808-532) and δ^P (532) > 0.15. The smaller particles with quasi homogeneous morphology were attributed to sulphate aerosol from the initial stage of the Cumbre Vieja volcano eruption and aged Canadian wildfires traveling across the Atlantic, showing the lowest δ^P (< 0.08) and the highest EAE (532/808) (>1). Middle values of these retrieved parameters were associated with the heterogenous mixture of ash, soot, and charred vegetation from fresh local forest wildfires. The retrieved properties demonstrate the excellent performance of the CE376 micro-lidar as a tool for continuous monitoring and characterisation of the temporal and vertical distribution of atmospheric aerosols.

We present a characterization of the optical properties of different aerosol types based on data collected using a compact dual-wavelength depolarization elastic lidar (532 and 808 nm, CIMEL CE376). We evaluate the vertical distribution and temporal evolution of a variety of aerosols observed in the subtropical North Atlantic region, covering from Saharan dust outbreaks and volcanic aerosols to fresh and aged wildfires, observed between August 2021 and August 2023, using a modified two-wavelength Klett inversion method to derive the aerosol backscatter and extinction coefficients from CE376 lidar observations. We assessed the performance of the CE376 lidar during an intercomparison campaign with an MPL-4B lidar (MPLNET) while collocated at the facilities of the Izaña Atmospheric Research Centre (Canary Islands, Spain). Both instruments depicted the vertical aerosol structure similarly. The main differences were attributed to errors arising from the determination of the overlap function and the depolarization calibration in each instrument and the larger effect of the solar background on the CE376 system during daylight. Absolute differences in the volume depolarization ratio (δ^v) were 0.3 % reduced to 0.2 % when only nighttime data was used. The measurements of particle linear depolarization (δ^P), extinction Ångström exponent (EAE (532/808)) and attenuated colour ratio (ACR (808-532)), provided by the combination of the two channels of the CE376, allow us to describe the composition and size of the studied aerosols. Fresh Saharan dust particles were the largest particles found in this study with non-spherical morphology and traveling in a well-mixed layer, exhibiting the lowest EAE (532/808) and highest ACR (808-532) and δ^P (532) > 0.15. The smaller particles with quasi homogeneous morphology were attributed to sulphate aerosol from the initial stage of the Cumbre Vieja volcano eruption and aged Canadian wildfires traveling across the Atlantic, showing the lowest δ^P (< 0.08) and the highest EAE (532/808) (>1). Middle values of these retrieved parameters were associated with the heterogenous mixture of ash, soot, and charred vegetation from fresh local forest wildfires. The retrieved properties demonstrate the excellent performance of the CE376 micro-lidar as a tool for continuous monitoring and characterisation of the temporal and vertical distribution of atmospheric aerosols.