Impact of the 2021 La Palma volcanic eruption on air quality: Insights from a multidisciplinary approach.

The La Palma 2021 volcanic eruption was the first subaerial eruption in a 50-year period in the Canary Islands (Spain), emitting ~1.8 Tg of sulphur dioxide (SO2) into the troposphere over nearly 3 months (19 September-13 December 2021), exceeding the total anthropogenic SO2 emitted from the 27 European Union countries in 2019. We conducted a comprehensive evaluation of the impact of the 2021 volcanic eruption on air quality (SO2, PM10 and PM2.5 concentrations) utilising a multidisciplinary approach, combining ground and satellite-based measurements with height-resolved aerosol and meteorological information. High concentrations of SO2, PM10 and PM2.5 were observed in La Palma (hourly mean SO2 up to ~2600 µg m-3 and also sporadically at ~140 km distance on the island of Tenerife (> 7700 µg m-3) in the free troposphere. PM10 and PM2.5 daily mean concentrations in La Palma peaked at ~380 and 60 µg m-3. Volcanic aerosols and desert dust both impacted the lower troposphere in a similar height range (~ 0-6 km) during the eruption, providing a unique opportunity to study the combined effect of both natural phenomena. The impact of the 2021 volcanic eruption on SO2 and PM concentrations was strongly influenced by the magnitude of the volcanic emissions, the injection height, the vertical stratification of the atmosphere and its seasonal dynamics. Mean daily SO2 concentrations increased during the eruption, from 38 µg m-3 (Phase I) to 92 µg m-3 (Phase II), showing an opposite temporal trend to mean daily SO2 emissions, which decreased from 34 kt (Phase I) to 7 kt (Phase II). The results of this study are relevant for emergency preparedness in all international areas at risk of volcanic eruptions; a multidisciplinary approach is key to understand the processes by which volcanic eruptions affect air quality and to mitigate and minimise impacts on the population.

Radiative habitable zones in martian polar environments.

The biologically damaging solar ultraviolet (UV) radiation (quantified by the DNA-weighted dose) reaches the martian surface in extremely high levels. Searching for potentially habitable UV-protected environments on Mars, we considered the polar ice caps that consist of a seasonally varying CO2 ice cover and a permanent H2O ice layer. It was found that, though the CO2 ice is insufficient by itself to screen the UV radiation, at approximately 1 m depth within the perennial H2O ice the DNA-weighted dose is reduced to terrestrial levels. This depth depends strongly on the optical properties of the H2O ice layers (for instance snow-like layers). The Earth-like DNA-weighted dose and Photosynthetically Active Radiation (PAR) requirements were used to define the upper and lower limits of the northern and southern polar Radiative Habitable Zone (RHZ) for which a temporal and spatial mapping was performed. Based on these studies we conclude that photosynthetic life might be possible within the ice layers of the polar regions. The thickness varies along each martian polar spring and summer between approximately 1.5 and 2.4 m for H2O ice-like layers, and a few centimeters for snow-like covers. These martian Earth-like radiative habitable environments may be primary targets for future martian astrobiological missions. Special attention should be paid to planetary protection, since the polar RHZ may also be subject to terrestrial contamination by probes.

Coupling of climate change and biotic UV exposure through changing snow-ice covers in terrestrial habitats.

During the spring, when ozone depletion at the polar regions is at its maximum and consequently the environmental UV exposure is potentially high, many terrestrial communities are covered in snow and heterogeneous snow-encrusted ice that form near the edges of snowpack. Using field measurements and a theoretical radiative transfer model, we calculated the thicknesses of these covers that are necessary to reduce DNA-weighted dose to levels equal to or lower than those received later in the season in the absence of covers when there is no ozone depletion. This depth is approximately 4 cm for a 60% depletion of the ozone column, suggesting that even thin snow-ice covers are enough to completely cancel the biological effects of ozone depletion. Loss of snow-ice covers during early summer can be rapid. The maximum rate of retreat of snow cover measured during November at Mars Oasis, Antarctica (71.9 degrees S, 68.2 degrees W), was 44.1 cm/day, with a mean retreat of 15.4 cm/day. Climate warming might increase UV-radiation damage by melting UV-protecting terrestrial snow-ice covers earlier in the season, when ozone depletion is more severe. Conversely, climate cooling could increase UV-protection afforded to terrestrial communities by increasing the extent of snow and ice covers. Even if anthropogenic ozone depletion is eventually reversed, these data suggest the importance of climate forcing in determining UV exposures of terrestrial microbial communities in snow- and ice-covered environments.