Global Corrections to Reference Irradiance Spectra for Non-Clear-Sky Conditions

Photochemical reactions
in surface waters play important
roles
in element cycling and in the removal of organic contaminants, among
other processes. A central environmental variable affecting photochemical
processes in surface waters is the incoming solar irradiance, as this
initiates these processes. However, clear-sky incident irradiance
spectra are often used when evaluating the fate of aquatic contaminants,
leading to an overestimation of contaminant decay rates due to photochemical
transformation. In this work, incident irradiance satellite data were
used to develop global-scale non-clear-sky correction factors for
commonly used reference irradiance spectra. Non-clear-sky conditions
can decrease incident irradiance by over 90% depending on the geographic
location and time of the year, with latitudes above 40°N being
most heavily affected by seasons. The impact of non-clear-sky conditions
on contaminant half-lives was illustrated in a case study of triclosan
in lake Greifensee, which showed a 39% increase in the triclosan half-life
over the course of a year under non-clear-sky conditions. A global
annual average correction factor of 0.76 was determined as an approximate
way to account for non-clear-sky conditions. The correction factors
are developed at monthly and seasonal resolutions for every location
on the globe between 70°N and 60°S at a 4 km spatial resolution
and can be used by researchers, practitioners, and regulators who
need improved estimates of incident irradiance.