Transmissivity of solar radiation and the influence of diffuse radiation in different trees inside a tropical urban park

Trees are known to have a positive effect on the thermal environment of urban areas. However, different species have different impact in the local climate. In this work we present the results of a measurement campaign aiming to quantify the effect of different leaf structures on the solar transmissivity and the impact on mean radiant temperature as a relevant variable for thermal comfort. Six different trees have been assessed on different weather conditions. Sensors were deployed to measure air temperature, wind speed and direction, relative humidity, globe temperature and solar radiation (global and diffuse). Each day two trees were evaluated simultaneously with sensors under the tree crown and beside it (as a climatic reference site). The quantitative results show that the transmissivity of solar radiation through the tree's crown (varying between 9.4 % and 30.1 %) was influenced by the leaf structure and the sky view factor. Based on these, the impact of the direct and diffuse components of solar radiation on the tree transmissivity was different. A higher fraction of incoming diffuse radiation increased the transmissivity of solar radiation, especially for trees with low leaf area Index and high sky view factor. Overall, lower solar transmissivity, higher levels of incoming solar radiation and lower fraction of diffuse radiation caused a higher impact on mean radiant temperature under the tree crown, reaching a reduction of 27.0 °C. The outcomes of this work can influence decision-making in the context of improving thermal comfort through adequate management of greenery.