Field data collection and modelling
We obtained road data, including data on highways and smaller streets, from Google Maps, using ArcGIS 9.3. Field data were collected from June to September 2017. Six percent of all street segments were included during data collection for each sub-district, in accordance with theI-Tree Street User Manual , which requires that sample inventories include 3-6% of all street segments to achieve a standard error of ~10%. Street segments were randomly selected from the road data set from each sub-district. For each tree, the main attributes we collected were DBH (diameter at breast height), crown size, health condition, LandUse and LocSite which were asked for i-tree street to assess air purification.