Abstract. The effect of aerosol loading on solar radiation and the subsequent effect on
photosynthesis is a relevant question for estimating climate feedback
mechanisms. This effect is quantified in the present study using ground-based
measurements from five remote sites in boreal and hemiboreal (coniferous and
mixed) forests of Eurasia. The diffuse fraction of global radiation
associated with the direct effect of aerosols, i.e. excluding the effect of
clouds, increases with an increase in the aerosol loading. The increase in
the diffuse fraction of global radiation from approximately 0.11 on days
characterized by low aerosol loading to 0.2–0.27 on days with relatively
high aerosol loading leads to an increase in gross primary production (GPP)
between 6 % and 14 % at all sites. The largest increase in GPP
(relative to days with low aerosol loading) is observed for two types of
ecosystems: a coniferous forest at high latitudes and a mixed forest at the
middle latitudes. For the former ecosystem the change in GPP due to the
relatively large increase in the diffuse radiation is compensated for by the
moderate increase in the light use efficiency. For the latter ecosystem, the
increase in the diffuse radiation is smaller for the same aerosol loading,
but the smaller change in GPP due to this relationship between radiation and
aerosol loading is compensated for by the higher increase in the light use
efficiency. The dependence of GPP on the diffuse fraction of solar radiation
has a weakly pronounced maximum related to clouds.
Simulation of the Unexpected Photosynthetic Seasonality in Amazonian Evergreen Forests by Using an Improved Diffuse Fraction-Based Light Use Efficiency Model
