The datasets analysed in this study were obtained at the Izaña Observatory (IZO), operated by the Izaña Atmospheric Research Center (IARC) of the Spanish State Meteorological Agency (AEMET) (http://izana.aemet.es, last access: 15 April 2025). IZO is located on the island of Tenerife (Canary Islands, Spain (28.3° N, 16.5° W)), at an altitude of 2400 m a.s.l. (Fig. a), and is typically situated above a quasi-permanent temperature inversion layer. This persistent atmospheric feature limits the vertical transport of locally generated pollutants from lower altitudes, ensuring that the measurements collected at the site are mostly representative of free-tropospheric conditions . Consequently, IZO offers an exceptional environment for both in situ and remote sensing observations of trace gases and aerosols. The combination of a stable total ozone column, extremely low water vapour content, reduced aerosol loading, and the frequent occurrence of clean, cloud-free skies makes the observatory an ideal reference site for calibration and validation activities (more details in ).

In August 2023, an extensive wildfire on the island of Tenerife provided a unique natural laboratory for investigating the atmospheric effects of biomass-burning aerosols. Although IZO remained physically unaffected by the fire, its proximity to the burning area offered an exceptional opportunity to monitor the resulting smoke plume and examine its evolution under free-tropospheric conditions. This situation enabled the direct characterisation of the optical, microphysical and radiative properties of wildfire-derived aerosols, thus yielding valuable insight into their short-term influence on the regional radiation budget and atmospheric composition.

Climatic conditions in the Canary Islands during August 2023 created a highly favourable environment for wildfire development. According to AEMET , this month was exceptionally dry, with a mean temperature of 25.4 °C and a positive anomaly of +2.3 °C compared to the 1991–2020 reference period, ranking as the warmest August since 1961. A heatwave between 10 and 14 August pushed temperatures close to 40 °C, while relative humidity fell below 20 % and wind gusts exceeded 30 km h⁻¹ in Tenerife, as recorded at the AEMET station in Candelaria (463 m a.s.l.), near to the origin point of the subsequent wildfire. Although the fire ignited one day after the maximum alert had been lifted-coinciding with notably milder conditions (27.3 °C maximum temperature; 75 % minimum humidity, ), the severe desiccation of the forest fuel over prior weeks was decisive in enabling its rapid spread. Supporting this, the base of the subsidence-induced thermal inversion remained below 600 m a.s.l. from 11 to 19 August, maintaining a warm and dry free troposphere above the island .

During the period 15–31 August 2023, a pronounced aerosol episode was detected at IZO on 17 and 18 August (Figs.  c, d and ). The AOD values increased sharply across all measured wavelengths (340–870 nm), reaching maxima of 4.81 and 3.63 at 340 and 500 nm, respectively, on 17 August, and 4.10 and 2.25 at 340 and 500 nm, respectively, on 18 August. Simultaneously, AE >2 on both days (2.06 on 17 August and 2.04 on 18 August) (see Table ), indicating a predominance of fine-mode particles. These features are characteristic of biomass-burning aerosol intrusions and are comparable to those reported by during the extreme wildfire episode in Greece in August 2021, where AOD values up to 3.6 at 500 nm, AE up to 2.4 (440–870 nm), and fine-mode fractio (FMF) values around 0.98 were observed. Therefore, the following study focuses on the events recorded on 17 and 18 August.

A more detailed characterisation of the aerosol optical and microphysical properties during this episode is presented in Fig. . Spectral AOD and AE evolution is presented in Fig. a. The AOD decomposition at 500 nm during the event (Fig. b) shows that the fine-mode component dominated the total column, with the FMF approaching unity. On 17 August, the high AE and FMF values (∼1.5 and >0.8, respectively) indicate a fine-mode-dominated aerosol population, typical of freshly emitted smoke. As the event evolved (18–19 August), AE decreased below 0.8 and FMF below 0.4, revealing aerosol mixing with coarse dust particles within the plume. Figure c confirms this dominance, with data points from 17 and 18 August clustering in the region of high AE (>1.5) and high FMF (>0.8) (Fig. c), indicative of fine-mode aerosols from biomass burning . These values are similar to those reported by for the extreme wildfires that occurred in Greece in August 2021 and 2023, respectively, as well as to those reported by at the El Arenosillo site in southern Europe as a result of the 2023 Canadian forest fires.

The vertical distribution of aerosols derived from lidar observations (Fig. d) reveals that the fresh smoke layer extended up to 4 km a.s.l. on 17 and 18 August, with enhanced backscatter coefficients (not shown) and low volume depolarization ratios (δv) of 0.05–0.10 during the most intense fire periods (grey areas in Fig. ). For the 17 August event (no product available for 18 August), δp values ranged from 0.05 to 0.12. These values are consistent with those reported by for fresh smoke, who found δp values around 0.05. According to these authors, such low δp values indicate the dominant presence of spherical particles, mainly composed of a solid soot core coated with a liquid sulfate shell .

An elevated layer reaching 7–8 km a.s.l. appeared on 19 August, characterised by high δv and δp values (up to 0.34 and 0.36, respectively), corresponding to the Saharan dust plume transported to IZO. The influence of coarse mineral dust particles is also consistent with the low AE values observed in Fig. 4a, b, and c. After 19 August, both AOD and backscatter intensity decreased rapidly, marking the end of the episode. These results agree with those reported by , who found similar values for both fresh aerosols from the same fire event and for mineral dust measured at Izaña using a dual-wavelength depolarisation elastic lidar (Cimel CE376). This elevated layer reached the altitude of Izaña between 05:00 and 09:00 UTC on 19 August and again after 12:00 UTC on the same day.

This local event was also characterised using in situ surface measurements, complementing remote-sensing observations. PM₁₀ and PM_2.5 concentrations experienced a substantial increase compared to their typical levels. In particular, PM₁₀ and PM_2.5 (Fig. a) reached peak concentrations of 712.19 and 594.64 µg m⁻³, respectively, representing an increase of approximately 700 µg m⁻³ above typical values for the station .

Five significant peaks were identified in the time series (Fig. ), with one peak excluded from TEOM data (Fig. a) due to relative humidity filtering (Sect. ). These peaks occurred on three consecutive days: two on 17 August (late morning and afternoon), one on 18 August afternoon, and two on 19 August, one at midnight and the other encompassing from the morning to midday. The peaks on 17 August morning and 18 August afternoon (shaded areas in Fig. ) coincided with fire events identified from columnar properties (Fig. ). The elevated PM₁₀ and PM_2.5 concentrations within these periods indicate direct impacts from wildfire smoke plumes at the observatory, reaching maximum values of 485.19 and 167.80 µg m⁻³ for PM₁₀ and 401.53 and 116.26 µg m⁻³ for PM_2.5 (17 and 18 August). The eBC concentrations reached record values for the station with peaks of 14.74 and 10.31 µg m⁻³ for the PM₁₀ and PM₁ size cuts on 17 August, and 4.69 and 7.81 µg m⁻³ on 18 August. Correspondingly, the SAE, PM_2.5 / PM₁₀ ratio, and SSA, which provide complementary information on aerosol size and optical properties, reached maximum values of 1.93, 0.83, and 0.98 on 17 August, and 2.21, 0.69, and 0.96 on 18 August, respectively. These values collectively confirm the dominance of fine, light-scattering wildfire-originated particles during the selected events.

The temporal evolution of ground-level measurements from 17 to 20 August (Fig. ), revealed the successive entrance and exit of smoke-dominated air masses. Their progressive mixing with desert dust was reflected by oscillations in the PM_2.5 / PM₁₀ ratio from ∼ 0.8 to ∼ 0.3, accompanied by a decrease in the SAE from values above 2 to below 0.2. This behaviour was also evident in columnar-integrated measurements on 17 and 18 August. In contrast, the comparison between column-integrated and in situ measurements on 19 August revealed several complications. First, the midnight event could not be measured in the atmospheric column due to the limited availability of lunar photometry data, which is only available between the first and last quarters of the lunar cycle . Second, it is important to emphasise that the detection of short-lived aerosol events may differ between in situ and remote-sensing (column-integrated) techniques because each method samples different atmospheric layers, and local conditions at the station level may not be representative of the column-averaged aerosol load. This discrepancy was evident on 19 August, where the morning event showed fine particles at the surface level, with eBC concentrations peaking at 12.33 and 13.00 µg m⁻³ in PM₁₀ and PM₁ fractions, and SAE reaching 2.19 (Fig. b and c), contrasting with the columnar measurements of FMF and AE (Fig. a and b), which indicated coarser particles with values below 0.4 and 0.5, respectively. The lidar data (Fig. d) clarify this discrepancy where low δv values at altitudes below 3 km a.s.l. revealed a distinct aerosol layer near the surface for both events on 19 August. The consequence of this pronounced aerosol layering above the station is that surface observations detected the arrival of smoke-dominated air masses, whereas column-integrated observations indicated the dominance of coarser desert dust particles.

This local wildfire also had a direct impact on in situ atmospheric concentrations of CO₂, CH₄, and CO at IZO, which showed higher-than-usual values between 17 and 19 August (Fig. ). During the event, CO₂, CH₄, and CO concentrations increased significantly and reached peaks of approximately 520 ppm, 2800 ppb, and 12 000 ppb, respectively, well above typical values at IZO (410–430 ppm for CO₂, 1800–2100 ppb for CH₄ and 50–200 ppb for CO). These marked increases (about 25 % for CO₂, 35 % for CH₄ and nearly two orders of magnitude for CO) are fully consistent with the presence of intense biomass-burning plumes over the observatory during the event, as confirmed by collateral smoke transport identified in Sentinel-5P TROPOMI CO observations (Fig. d).

The temporal evolution of the in situ measurements reveals a strong correspondence between peaks in particulate matter (PM₁₀, PM_2.5), eBC, and greenhouse gases (CO₂, CH₄, CO) (Figs.  and ). All maxima occurred concurrently with the periods of highest columnar AOD and strongest backscatter in the lidar profiles. This simultaneity provides robust evidence that the observed perturbations in aerosol and gas concentrations originated from the same wildfire plume. These independent datasets confirm the coherent atmospheric signature of fresh biomass-burning emissions, reinforcing the interpretation of a strongly localised and intense smoke influence at Izaña during the 17–19 August events.

The influence of aerosols on spectral irradiance becomes evident when examining the spectra measured during the two days with the highest AOD (17 and 18 August 2023; Fig. a). Figure  illustrates the pronounced variability observed in the GHI_λ, DNI_λ, and DHI_λ components, highlighting the impact of these high-turbidity episodes, which lead to substantial spectral attenuation and angular redistribution of solar radiation. During the most intense smoke events, daily GHI_λ was reduced by 21 %–27 %, DNI_λ by 72 %–99 %, while DHI_λ irradiance increased by 72 %–75 % compared with clean-sky conditions at IZO. These results are similar to those observed during the 2021 Greece wildfires, with decreases of 10 %–20 % in daily GHI_λ .

These measurements represent one of the few detailed spectral characterisations (300–1100 nm) of extreme biomass-burning aerosol episodes at a high-altitude observatory. The exceptionally large AOD values observed at IZO (up to 3.6 at 500 nm) place this event among the most intense smoke episodes documented.

The spectral radiative effects of biomass-burning aerosols were analysed at the times when the AOD at 500 nm (Fig. a, c, Table ) reached its maximum values on each study day: 11:56 UTC (SZA 22.7°, AOD 3.63) on 17 August and 15:46 UTC (SZA 39.1°, AOD 2.25) on 18 August. Figure  shows the spectral distribution of irradiance, radiative forcing (ΔF; W m⁻² nm⁻¹), spectral AOD determined from EKO RSB measurements and radiative forcing efficiency (ΔFeff; W m⁻² nm⁻¹ AOD⁻¹). These large AOD values confirm the strong presence of smoke particles over the site, which substantially altered the radiative fluxes and their spectral distribution. Clear differences between the two days, as well as among the various SZA conditions, highlight the temporal evolution of the aerosol optical properties and their influence on surface radiation. The ΔFeff was calculated using Eq. (), based on the spectral AOD retrieved at the EKO RSB wavelengths.

The differences between the two days reflect both the varying aerosol load and the changes in solar geometry, with the higher AOD and smaller SZA on 17 August enhancing the radiative impact. Slight variations in aerosol optical properties, including potential differences in fine-mode dominance or absorption, may also contribute to the observed contrast.

The spectral irradiance of GHI_λ, DNI_λ, and DHI_λ exhibits a clear contrast between the measurements obtained under clean conditions (dashed lines) and those affected by fire events (solid lines) (Fig. a, e). On both days, the high AOD caused a strong attenuation of DNI_λ irradiance, mainly below 700 nm, accompanied by an enhancement of the DHI_λ component. The stronger impact between 450 and 460 nm, with ΔF values of −1.69 and -1.59 W m⁻² nm⁻¹, respectively, indicates the dominance of fine-mode smoke particles which scatter efficiently short wavelengths. A pronounced attenuation of the DNI_λ spectral irradiance and enhancement of the DHI_λ component are observed, reflecting the strong scattering capacity of biomass-burning aerosols. This enhanced scattering capacity is consistent with the high SSA values obtained from the surface measurements (Fig.  in Sect. ). The resulting spectra of ΔF are negative for the DNI_λ and GHI_λ components and positive for the diffuse one, indicating a net surface cooling effect dominated by scattering processes. The magnitude of the ΔF is larger on 17 August (AOD500nm=3.63) than on 18 August (AOD500nm=2.25), consistent with the higher aerosol load and the smaller solar zenith angle. The ΔFeff spectra (Fig. d, h) exhibit similar shapes for both days, with maximum efficiency in the visible region (400–800 nm), suggesting that the optical properties of the aerosols, likely dominated by biomass-burning particles, remained relatively constant throughout the episode.

The integrated spectral radiative forcing values (ΔDF and ΔDF^eff) determined from Eq. () are shown in Table . The results are consistent with the spectral behaviour discussed in Fig. . The total ΔDF^GHI_λ (300–1100 nm) reached −395 W m⁻² on 17 August and −299 W m⁻² on 18 August, confirming a strong surface cooling effect during the biomass-burning episode. The forcing was dominated by the DNI_λ component (∼−700 and −609 W m⁻², respectively), while the DHI_λ component partially compensated for this loss with positive values (∼+250 and +174 W m⁻²), as expected under intense scattering conditions.

The band-integrated ΔFeff exhibited a distinct spectral dependence, peaking in the visible range (−77 to −99 W m⁻²AOD⁻¹ for the GHI_λ component), consistent with the strong scattering capacity of fine-mode smoke particles. Near-IR efficiencies were slightly lower (−66 to −85 W m⁻²AOD⁻¹), while UV contributions remained minor (∼−10 W m⁻²AOD⁻¹). For the DNI_λ component, ΔFeff reached up to −211 W m⁻²AOD⁻¹, whereas the DHI_λ component showed positive values (∼+50–80 W m⁻²AOD⁻¹), highlighting the enhanced scattering under high aerosol load. Integrated over 300–1100 nm, total ΔFeff reached −154 W m⁻²AOD⁻¹ on 17 August and −194 W m⁻²AOD⁻¹ on 18 August, indicating a slightly higher radiative efficiency on the latter day despite lower AOD. These findings confirm that biomass-burning aerosols at IZO produced a marked shortwave surface cooling dominated by visible-range scattering processes.

Spectrally, for both GHI_λ and DNI_λ spectral radiation, the visible range (400–700 nm) contributed the most to the total forcing (∼ 60 %–65 %), followed by the near-IR range (∼ 22 %–25 % for GHI_λ and ∼ 28 %–31 % for DNI_λ) and the UV range (∼ 13 % for GHI_λ and ∼ 7 % for DNI_λ) for the two days. This distribution matches the maximum in the incident solar irradiance and the high scattering efficiency of fine-mode smoke particles in the visible region. ΔFeff also reflects the larger radiative impact per unit AOD on 18 August, despite the lower aerosol load, suggesting a slightly enhanced absorption or differences in aerosol optical properties and solar geometry. Overall, these integrated values confirm that biomass-burning aerosols at IZO produced a pronounced shortwave radiative cooling dominated by scattering processes, with maximum efficiency in the visible range.

Besides the instantaneous spectral and band-integrated values, daily radiative forcing was also computed to assess the accumulated shortwave impact of the smoke-laden days. On 17 August, the total daily forcing reached −56 (GHI_λ), −162 (DNI_λ), and +70 W m⁻² (DHI_λ), while on 18 August the corresponding values were −44, −137, and +60 W m⁻², respectively. The daily forcing efficiencies were −77, −313, and +138 W m⁻² AOD⁻¹ for the GHI_λ, DNI_λ, and DHI_λ components on 17 August, and −84, −332, and +145 W m⁻² AOD⁻¹ on 18 August. These values are consistent with previously reported daily-scale radiative impacts of biomass-burning aerosols. For example, reported global radiative forcing values ranging from −13 to −60 W m⁻² and efficiencies between −80 and −40 W m⁻² AOD⁻¹ during the 2012 Siberian wildfires.

In addition to the instantaneous spectral and band-integrated forcing values discussed above, the temporal evolution of the broadband shortwave radiative forcing was analysed over the two smoke-affected days in order to evaluate the diurnal behaviour of the aerosol perturbation (Fig. ). The time series shows a clear enhancement in the magnitude of the forcing during periods of strongest smoke influence, with the largest cooling occurring around local noon, when solar irradiance is at its maximum. On 17 August, the forcing exhibits a pronounced peak between approximately 11:30 and 13:30 UTC, coinciding with the period of highest aerosol loading observed at the station. On 18 August, the forcing remains significant over a broader time interval, with a maximum around 15:30 UTC, consistent with the later arrival of the densest smoke plume (Fig. a).

Although the magnitude observed at IZO lies among the highest documented due to the exceptional aerosol load, this comparison reinforces that the radiative response observed at IZO is consistent with the upper bounds of biomass-burning forcing reported in the literature.