 Good afternoon or good morning to everyone here online, I'm Mende Chan from the University of Hong Kong. And I'd like to say thanks to FAO and the Royal Forestry Department of Thailand Government for organizing this virtual meeting, giving us this opportunity to feel connected with each other and also meeting up with some old friends and making new friends in the still difficult times. And thank you Simone for your kind invitation and very happy to take this opportunity to talk about how urban forests can contribute to cleaner cities. As you were emphasized by Simone already in his opening talk, we need to enhance our urban forests so that diverse benefit can be garnered for improving citizens' quality of life and achieving sustainable development goals. The major ecosystem service or one of the major benefit that we can expect is air pollutant removal so that cities can be made cleaner and also healthier. Actually urban air pollution is a momentous global issue, maybe not at this moment. The World Health Organization has identified air pollution actually as the greatest environmental risk to human health. According to a recent study, 90% of the world's urban population live in cities exceeding the air quality standards as specified in their national or regional guidelines. And also an estimate suggested that all the air pollution can claim about 4.2 million lives each year worldwide. It's a huge number. Particularly, air pollution is of great concern in many Asian cities. In the cities, elevated pollutant concentrations and a large amount of potential sufferers converge. If we take PM 2.5 as an example here, each are very common urban air pollutant penetrating deeper into cavities and tissues of human body and affecting seriously human health. As you can see from this global map of PM 2.5, the highest concentrations are found in central and southern Asia. If we look at Chinese cities, urban air pollution has become a top concern. In a national survey conducted several years ago, nearly 80% of the respondents said that air quality or air pollution is a big problem affecting their health and daily life. This would be even further deteriorated by the projected urban population growth, increasing urbanization, coupled with impact on climate change, on atmospheric conditions and weather variability. We all understand that the myriad policy, technological and culture changes are required, definitely, for the curtailment of emission of this air pollutants at the source. Beyond that, clearly the mitigation of ongoing ambient air pollution is urgently needed. But it is very challenging because of the large volume of air into which the pollutants have been dispersed into, in comparison with the surface area, to which any potential solutions may be applied. But of course it is essential in order to reduce citizens' exposure to air pollution and in order to improve public health. Urban trees and urban forests offer a very cost-effective nature-based solution to cleaning up air pollutants. A number of physical chemical processes on all above-ground structures of the plants help cleaning air pollutants. I want to mention two major pathways, including deposition onto plant surfaces and also the stomata uptake when possible. For the particulate madhouse, as shown here by this diagram, the majority of this particulate madhouse can be removed by urban trees or urban forests from the ambient atmosphere through deposition and dispersion. When crossing into trees and plants, concentrated clouds of minuscule particulates or tiny particles can get dispersed and trapped in plant surfaces. So diluted by the air, leeward of the urban forest or woodland, dispersion and deposition effects can significantly reduce PM concentrations and sometimes even modify the composition of particulates. When it rains, the trapped particles would be washed away by water into the urban genus system. GAIS's air pollutant will normally be removed through dry deposition primarily by uptake via stomata. Here you can see those stomata are tiny openings. The plant in leaves, not only leaves actually, also stems and other plant organs. As illustrated by this diagram, GAIS's pollutants can penetrate the leaf cell walls and then enter the leaves internal structures through stomata and then maybe adsorbed by water fume. To form acids here, as you can see, through different chemical reactions with the inner leaf chemicals. There is a original literature on the removal capacity of urban forests or urban trees for different air pollutants on the basis of field measurement or simulation models. As shown by the speaker, the major pollutants analyzed in those papers are particulate matters in different sizes like PM2.5, PM10, following by nitrogen dioxide and ozone. So many other air pollutants have also received increasing attention in scientific research such as the sulfur dioxide, nitrogen oxide and so on and so forth. And at present, studies are mostly found in the United States and China. Pretty scarce in other Asian cities and almost absent in African countries where actually the air pollution is also a very significant issue. The removal capacity of particulate matters by urban trees or urban plants have been extensively investigated. But empirical results very greatly, as you can see from this figure, many factors actually will determine the removal rate, such as species, site conditions or the distance between the source and the trees. And also the pollutant concentrations, composition and configuration of plants in urban forests, local material logical conditions and many other factors. For the uptake rate, I mean the geysers air pollutant uptake, besides those influencing factors, many other factors may also matter, like the turbulent diffusion above and within the tree canopy, urban forest canopy, the photosynthetic activity and trigger pressure, diffusion close to the individual component of vegetation and even within stomata and reeds of chemical reaction between geysers, pollutant and leaf surface, and also the reed of solution inside of the urban plants. Of course, the fact of evidence so far we have suggests that planting more trees will increase pollutant removal, that's for sure. Urban planners and policy makers have, I think already recognized that there's important message and urban trees can be regarded as significant things for air pollution. And for sure, more trees with larger leaves and the bigger canopies, if we can precisely measure the canopy size using the message suggested by Steve, right, and the larger trees in general can track and uptake more pollutants, therefore more effective in air pollutant mitigation. But a fact is that the most polluted areas inside of cities are those places with very limited space for planting. So therefore, I think the most important message we need to consider when forming or formulating relevant policies greenery plants is that we need to find appropriate trees or right trees for right places. So to enhance the magnitude of air pollutant removal, a key to success lies in understanding the species characteristic and site condition, the interaction between the two can help us to evaluate appropriately or accurately the removal of air pollutant. For tree characteristics, it's necessary for us to think about, for example, trees' shape and porosity, leaf surface morphology, the availability and complexity of the cuticular vexes, arrangement of stomata or the availability and arrangement of stomata, presence of tiny hairs, which could be very effective in tracking the particulates. And in the meantime, I think site conditions should also be carefully evaluated and taken into consideration, particularly like the soil air pollutant source, the direction of wind and many other factors, then appropriate species can be selected and used in right places. I still have another key factor. I believe it will be very important. This factor is associated with the comprehensive evaluation of air pollutant removal capacity and other cold benefits, and also possible these services. Because most of time improving air quality is usually not the only objective of city managers. I think even according to the soil action plan, we have diverse multiple objectives. So other tree properties, such as the heat mitigation potentials, stress tolerance and water use efficiency here, and the shading, the emission potentials, and many other factors. We need to consider them jointly instead of individually. So, in doing so, synergistic combination of other benefits is possible, so that the co enhancement can be planned and practically achieved. Additionally, a number of study actually highlight that the interaction between individual plants and ambient air quality. Very complicated. Many species can have both positive and negative impact. So effective plant selection and for air pollution mitigation and also comprehensive evaluation of species for their benefit and detrimental impact can facilitate true scale understanding of the balance between the benefit that we would like to have and the detrimental aspect of vegetation at different levels from species, individual species to urban forest and even the whole green landscape within a city. So, before closing, I would like to highlight that urban trees have long been and unanimously regarded as the lungs of urban ecosystem. And it's a concept. All stakeholders and policy makers fully recognize because trees can absorb carbon dioxide and emit oxygen if activated. And we need to take it once that. We need to also recognize urban trees as leaders of urban ecosystems, where the majority of population are living, and we anticipate even more and more population will move into urbanized areas. As I have already mentioned at the beginning of this talk, it means that a lot of air proteins will be discharged or released into the atmosphere. So there is an urgent need for us to recognize the function of urban trees or urban forest as leaders of urban ecosystems for the region that they can effectively remove toxic air pollutants and make cities cleaner and healthier. So, I will stop here and leave a little bit time for any questions if any audience would like to raise.