VOCs participate formation of secondary organic aerosols formation and cloud condensation nuclei and this affects to the amount of reflected solar radiation. Biogenic volatile organic compounds (BVOC) released from the vegetation will react with OH, O3, Cl and NO2 radicals by forming reaction products which condense into the aerosol particles or form a cloud nuclei by interacting with biogenic secondary organic aerosols (BSOA). The abundance of cloud nuclei formation is creasing faster in the future since rising temperature and CO2 concentration increase BVOC and BSOA emissions from the vegetation. The increased cloud formation increases the ratio of reflected radiation. PAR radiation can be measured with fotons.
The circulation of the Earth in relation to the Sun generates a strong annual cycle for radiation and temperature that further affects other environmental factors such as soil moisture and air humidity. Environmental factors affect physiological processes and therefore, identification and understanding of their nature is needed in order to understand and predict the biological actions in the forest.
The variation in solar radiation during a day or a year drives the changes in temperature and air humidity: air temperature follows the seasonal and diurnal changes in solar radiation, whereas the minimum temperature of the day, for example, drives the air water content. Variation in CO2 concentration is an exception. Light drives the functioning of terrestrial ecosystems, which in turn affects the carbon dioxide concentration. However, light is not pursuing its variation, because house heating in the northern hemisphere during winter and other fossil fuel emissions affect CO2-concentrations.