Assessing the impact of atmospheric attributes on the effectiveness of solar irradiance for photosynthesis of urban vegetation in Attica, Greece

Photosynthetically active radiation (PAR) is a critical component driving the photosynthetic processes of plants. The content of global solar radiation (Rs) in PAR wavelengths, expressed as the PAR/Rs ratio, is an important attribute in meteorological and biological studies. Especially in the urban environment, which is affected by pollutants, urban materials, and elements, the solar radiation fluxes and their relations are highly influenced. In the present study, PAR/Rs is investigated in the Mediterranean urban environment of Attica, Greece. The temporal variation of the hourly ratio values is assessed in conjunction with several meteorological factors. The results indicate an annual PAR/Rs average value of 0.385 (SD = 0.039), with rather constant seasonal and monthly values and small variability, possibly attributed to the urban microclimate with small variability. Atmospheric clearness index and optical thickness were the most influential factors for the formation of the radiation ratio, which is also affected, to a lesser degree, by temperature, humidity, and related attributes. Aerosols, especially O 3 , NO 2 , and CO, also have impacts on the radiation ratio due to their preferred radiation absorbance at specific wavelengths. The results suggest important differences with similar studies in forest areas, underscoring the need for expanding the study in urban green areas, especially in Mediterranean cities.