Where the people are: Current trends and future potential targeted investments in urban trees for PM10 and temperature mitigation in 27 U.S. Cities

Urban trees reduce respirable particulate matter (PM10) concentrations and maximum daytime summer temperatures. While most cities are losing tree cover, some are considering ambitious planting efforts. Maximizing PM10 and heat mitigation for people from such efforts requires cost-effective targeting. We adapt published methods to estimate the impact of a decade (2004-2014) of tree cover change on city-level PM10 and heat mitigation in 27 U.S. cities and present a new methodology for estimating local-level PM10 and heat mitigation by street trees and tree patches. We map potential tree planting sites in the 27 cities and use our local-level PM10 and heat mitigation methods to assess the population-weighted return on investment (ROI) of each site for PM10 and heat abatement for nearby populations. Twenty-three of the 27 cities lost canopy cover during 2004-2014, reducing estimated city-level PM10 removal by 6% (168 tons) and increasing city-level average maximum daily summer temperature by 0.1 °C on average across cities. We find large potential for urban reforestation to increase PM10 and heat abatement. Intra-city variation in planting site ROI - driven primarily by differences in population density around planting sites - exceeds four orders of magnitude, indicating large scope for targeting to increase PM10 and heat abatement from reforestation. Reforesting each city's top 20% ROI sites could lower average annual PM10 concentrations by >2 μg/m3 for 3.4-11.4 million people and average maximum daily summer temperatures by >2 °C for 1.7-12.7 million - effects large enough to provide meaningful health benefits - at a combined annual cost of $102 million.