Modelling air purification
The i-Tree Streets Model (i-Tree, 2018) was used to assess air purification benefits in metropolitan Shenyang. We first used the method proposed by McPherson (2010) to select the best city match, i.e. to pick a “best fit” climate zone for this Asian city. The best match city for Shenyang was Queens, New York, so the “US northeast climate zone” was chosen for the modelling. To exam the effects of population density, city sprawl (urban-suburban gradient) and district development plans, i.e. government policy on the distribution of air purification ecosystem services provided by street tree assemblages , we regrouped the field data of sub-districts in 3 ways: (1) by population density (districts with population density > 10,000 persons/km2 were grouped as P1, districts with 1,000-10,000 persons/km2 were assigned to P2, and districts with < 1,000 persons/km2 were placed in group P3), (2) by beltways (sub-districts within the first beltway were grouped as B1, sub-districts between the first and the second beltway were assigned to B2, and the sub-districts between the second and the third beltway were places in B3 (Figs 1B and 1C), (3) by districts (the districts named HG, HP, S, T, Y, HN and D respectively). We got 13 groups in total. We then ran the i-Tree Street Model on each group of field data, individually and got average tree benefit of air purification (ATB, $/tree) and total benefit of air purification for each group.