Organic aerosol enhances boreal forest photosynthesis under cumulus clouds

Résumé fourni par la source

Abstract Boreal forest is an important source of atmospheric aerosol particles, making clouds more reflective in a warmer climate. However, little is known about the aerosol effect on the scattering of radiation under a cloudy sky, which can have a cooling effect by enhancing the carbon uptake of boreal ecosystems. Here we show observational evidence that higher natural aerosol loadings associated with warmer conditions increase diffuse radiation in the presence of cumulus clouds and further invigorate photosynthesis in the boreal forest. The enhancement of photosynthesis occurs in the morning and evening or under clouds with cloud fraction 0.4–0.8, when moderate vapor pressure deficit keeps stomata in foliage open and the diffuse fertilization effect is maximized. The total effect of high aerosol loading is a further increase of the daily carbon uptake under cumulus clouds by 12% of the possible diffuse fertilization enhancement in the ecosystem. Our results provide observational support for the atmosphere-biosphere photosynthesis-based feedback, not only under clear sky but also under cumulus clouds frequently observed over boreal forest in summer.

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