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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.
