Pablo González-Sicilia

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.

Volcanic emissions from the Tajogaite volcano, located on the Cumbre Vieja edifice on the island of La Palma (Canary Islands, Spain), caused significant public health and aviation disruptions throughout the eruption (19 September–13 December 2021, officially declared over on 25 December). Nonetheless, it is considered the most significant volcanic event in Europe over the past 75 years due to the substantial amount of SO2 released into the atmosphere. The Instituto Geográfico Nacional (IGN), the authority responsible for volcano surveillance in Spain, implemented extensive operational monitoring to track volcanic activity and to provide a robust estimation of the volcanic plume height using a video-surveillance network. In parallel, the State Meteorological Agency of Spain (AEMET), in partnership with other Spanish ACTRIS (Aerosol, Clouds, and Trace Gases Research Infrastructure) members and collaborating institutions, conducted an unprecedented instrumental deployment to evaluate the impacts of this volcanic event on atmospheric composition. This effort included a network of aerosol profilers surrounding the volcano. A total of four profiling instruments were installed on La Palma: one MPL-4B lidar and three ceilometers. Additionally, a pre-existing Raman lidar on the island contributed valuable data to this study. These efforts are undertaken due to the importance of monitoring volcanic plume height in terms of air quality (necessary for the implementation of effective civil protection policies), volcanic activity surveillance (for tracking and forecasting eruptive behaviour), and, from a scientific perspective, for improving our understanding of the climatic and radiative impacts of this type of aerosol. In this study, the eruptive process was characterised in terms of the altitude of the dispersive volcanic plume (hd), measured by both IGN and AEMET-ACTRIS, and the altitude of the eruptive column (hec), measured by IGN. Modulating factors such as seismicity and meteorological conditions were also analysed. The consistency between the two independent and complementary datasets (hd,IGN and hd,AEMET) was assessed throughout the eruption (mean difference of 258.6 m). Our results confirmed the existence of three distinct eruptive phases, encompassing a range of styles from Strombolian explosive to effusive activity. While these phases have been characterised in previous studies, the results of the present work provide complementary information and novel insights from an alternative observational approach, which may be of use in future volcanic crises and will be applied to operational surveillance during such events. A subsequent comparison of hd,AEMET with the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) aerosol layer height product (ALHCALIOP) revealed a systematic underestimation by the satellite product, with a mean difference of 392.2 m. Finally, the impact of using hec in estimating SO2 emissions from the NASA MSVOLSO2L4 satellite-based product was evaluated. When a fixed (standard) plume altitude of 8 km was used instead of the observed hec, the total SO2 emission was significantly underestimated by an average of 56.2 %, and by up to 84.7 %. These findings underscore the importance of accurately determining the volcanic plume height when deriving SO2 emissions from satellite data.

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.

In a world increasingly impacted by climate change and natural hazards, atmospheric monitoring networks are essential for informed decision-making. During the 2021 La Palma eruption, we integrated surface and ground-based remote sensing measurements from global atmospheric network instruments, complemented by rapidly deployed sensors, to monitor volcanic gas emissions up to 140 km from the source. We used direct-sun measurements from low-resolution (EM27/SUN) and high-resolution (IFS-125HR) Fourier transform infrared (FTIR) spectrometers. On La Palma, the EM27/SUN was combined with a differential optical absorption spectroscopy (DOAS) instrument. We present new FTIR retrieval methods to derive the SO₂, CO₂, CO, HF, and HCl relative abundance in the plume from both low- and high-resolution solar absorption spectra. Using Sentinel-5P TROPOspheric Monitoring Instrument (TROPOMI) data, we derived SO₂ fluxes and estimated total emissions of 1.8 ± 0.2 Mt SO₂, 19.4 ± 1.8 Mt CO₂, 0.123 ± 0.005 Mt CO, 0.05 ± 0.01 Mt HCl, and 0.013 ± 0.002 Mt HF over the course of the eruption. These results are consistent with the mass balance derived from petrologic degassing estimates. This study demonstrates that high- and low-resolution FTIR and DOAS spectrometers, integrated within global monitoring networks, can provide quantitative constraints on volcanic gas composition and fluxes over large distances. Such capabilities are directly applicable to volcanic crisis monitoring, complementing dedicated networks, satellite observations and supporting improved assessments of volcanic impacts on the atmospheric composition at regional scales.

Volcanic emissions from the Tajogaite volcano, located on the Cumbre Vieja edifice on the island of La Palma (Canary Islands, Spain), caused significant public health and aviation disruptions throughout the volcanic event (19 September – 13 December 2021, officially declared over on 25 December). The Instituto Geográfico Nacional (IGN), the authority responsible for volcano surveillance in Spain, implemented extensive operational monitoring to track volcanic activity and to provide a robust estimation of the volcanic plume height using a video-surveillance network. In parallel, the State Meteorological Agency of Spain (AEMET), in collaboration with other members of ACTRIS (Aerosol, Clouds, and Trace Gases Research Infrastructure) in Spain, in collaboration with other institutions, carried out an unprecedented instrumental deployment to assess the atmospheric composition impacts of this volcanic event. This effort included a network of aerosol profilers surrounding the volcano. A total of four profiling instruments were installed on La Palma: one MPL-4B lidar and three ceilometers. Additionally, a pre-existing Raman lidar on the island contributed valuable data to this study. In this study, the eruptive process was characterised in terms of the altitude of the dispersive volcanic plume (h_d), measured by both IGN and AEMET-ACTRIS, and the altitude of the eruptive column (h_ec), measured by IGN. Modulating factors such as seismicity and meteorological conditions were also analysed. The consistency between the two independent and complementary datasets (h_d,IGN and h_d,AEMET) was assessed throughout the eruption (mean difference of 258.6 m). Our results confirmed the existence of three distinct eruptive phases, encompassing a range of styles from Strombolian explosive to effusive activity. While these phases have been characterised in previous studies, the results of the present work provide complementary information and novel insights from a different scientific perspective, which may be of use in future volcanic crises and will be applied to operational surveillance during such events. A subsequent comparison of h_d,AEMET with the Cloud-Aerosol Lidar with Orthogonal Polarization (CALIOP) aerosol layer height product (ALH_CALIOP) revealed a systematic underestimation by the satellite product, with a mean difference of 392.2 m. Finally, the impact of using h_ec in estimating SO₂ emissions from the NASA MSVOLSO2L4 satellite-based product was evaluated. When a fixed (standard) plume altitude of 8 km was used instead of the observed h_ec, the total SO₂ mass was significantly underestimated by an average of 56.2 %, and by up to 84.7 %. These findings underscore the importance of accurately determining the volcanic plume height when deriving SO₂ emissions from satellite data.