Comparing the modeled deposition of PM2.5 with the Eddy Covariance flux and SEM analysis of an urban forest in Naples

Trees can remove particles from the air through the physical deposition on the leaf surface. This process depends on pollution concentration and weather conditions as wind speed and precipitation, in addition to leaf characteristics. Wind speed increases at the same time the deposition velocity and the resuspension of PM deposited, instead, the rain washes off into the soil the particles accumulated on the leaf. The PM flux removed by trees has been modeled in the i-Tree Eco model considering the effect of wind speed on deposition velocity and resuspension and fixing a threshold of leaf washing (0.2 mm x LAI). However, the results of the model have not been validated with measured data and especially the washing threshold and resuspension classes based on wind speed still remain uncertain. In this study, we compared the modeled deposition of PM2.5 with the Eddy Covariance flux measured in an urban forest in Naples. The results of the model have been further validated by comparing the expected PM2.5 accumulations on the leaf (net flux integral) with the average PM load experimentally determined in the same site where the model input data (i.e., PM concentration, wind speed and rain) have been collected. The model and Eddy Covariance presented a good agreement in assessing the deposition flux on leaves but we show that also precipitation events higher than the threshold are not able to wash all particles accumulated on leaves as confirmed by the higher accumulation of PM2.5 measured with the SEM analysis. Furthermore, a wind speed above 20 m s-1 strongly affects the deposition because of the high resuspension back to the atmosphere. Finally, we highlight the importance of including a species-specific parametrization in the model to take into account the influence of leaf characteristics on the deposition velocity, resuspension, and leaf washing.