Preprints
https://doi.org/10.5194/egusphere-2024-1582
https://doi.org/10.5194/egusphere-2024-1582
24 Jun 2024
 | 24 Jun 2024
Status: this preprint is open for discussion.

Impact of shrub branches on the shortwave vertical irradiance profile in snow

Florent Domine, Mireille Quémener, Ludovick Bégin, Benjamin Bouchard, Valérie Dionne, Sébastien Jerczynski, Raphaël Larouche, Félix Lévesque-Desrosiers, Simon-Olivier Philibert, Marc-André Vigneault, Ghislain Picard, and Daniel C. Côté

Abstract. In the Arctic, shrubs are expanding and are covered by snow most of the year. Shrub branches buried in snow absorb solar radiation and therefore reduce irradiance. This reduces photochemical reaction rates and the emission of reactive and climatically active molecules to the atmosphere. Here we monitored irradiance at selected wavelengths using filters at 390±125 nm and >715 nm in snow-covered Alnus incana (gray alders) shrubs in the boreal forest near Laval University and on nearby grassland during a whole winter by placing light sensors at fixed heights in shrubs and on grassland. Irradiance in shrubs was greatly reduced at 390 nm and much less at 760 nm, where ice is much more absorbent. We performed radiative transfer simulations, testing the hypothesis that shrub branches behave as homogeneous absorbers such as soot. At 390 nm, dense shrub branches are found to reduce irradiance similarly to about 140 ppb of soot. For the >715 nm wavelengths, insufficient data and the greater ice absorption do not allow accurate conclusions. Noting that photochemically active radiation is mostly in the near UV and blue, we calculate that a high branch density will reduce photochemical reaction rates integrated over the whole snowpack by about a factor of two. This may affect the composition of the lower Arctic atmosphere in winter and spring in numerous ways, including a lower oxidative capacity, lower levels of nitrogen oxides and modified secondary aerosol production. Climatic effects are expected from these compositional changes.

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Florent Domine, Mireille Quémener, Ludovick Bégin, Benjamin Bouchard, Valérie Dionne, Sébastien Jerczynski, Raphaël Larouche, Félix Lévesque-Desrosiers, Simon-Olivier Philibert, Marc-André Vigneault, Ghislain Picard, and Daniel C. Côté

Status: open (until 15 Aug 2024)

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Florent Domine, Mireille Quémener, Ludovick Bégin, Benjamin Bouchard, Valérie Dionne, Sébastien Jerczynski, Raphaël Larouche, Félix Lévesque-Desrosiers, Simon-Olivier Philibert, Marc-André Vigneault, Ghislain Picard, and Daniel C. Côté
Florent Domine, Mireille Quémener, Ludovick Bégin, Benjamin Bouchard, Valérie Dionne, Sébastien Jerczynski, Raphaël Larouche, Félix Lévesque-Desrosiers, Simon-Olivier Philibert, Marc-André Vigneault, Ghislain Picard, and Daniel C. Côté

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Short summary
Shrubs buried in snow absorb solar radiation and reduce irradiance in the snowpack. This decreases photochemical reactions rates and emissions to the atmosphere. By monitoring irradiance in snowpacks with and without shrubs, we conclude that shrubs absorb solar radiation as much as 140 ppb of soot and reduce irradiance by a factor of two. Shrub expansion in the Arctic may therefore affect tropospheric composition during the snow season, with climatic effects.