Preliminary assessment of the air quality effects of trees: An urban forestry-based SIP measure in Sacramento, California

Recent Environmental Protection Agency (EPA) guidance encourages innovative measures, such as strategic tree planting, to be considered in the State Implementation Plan (SIP) process to achieve air quality attainment. Since these measures don't require the same level of certainty as traditional measures (e.g. installation of smokestack scrubbers), a study was developed (phase 1) that relies on existing data and simplified models to develop initial estimates of the air quality effects associated with tree planting. This presentation documents a preliminary evaluation of the net effects of a large-scale urban forestry program on air quality for the Sacramento federal ozone non-attainment area. A more comprehensive research project is now underway to develop and refine the methods to better meet EPA requirements (phase 2).

This study includes 1) an assessment of current urban forest canopy and potential planting spaces, 2) a determination of the potential to fill those available places with new trees and/or prevent the loss of existing trees, 3) air quality modeling to account for proposed changes in canopy cover (CC) on air quality over the next 20 years, and 4) development of an EPA approved plan to achieve and verify proposed air quality improvements. While all aspects of the work will be touched on, we report here primarily on the initial urban canopy assessment and air quality effects. Air quality effects considered in Phase 1 were air temperature and rate of ozone formation; ozone NO₂ and PM10 deposition, changes in BVOC emissions, vehicular VOC and NO₂ emissions, avoided power plant emissions due to reduced cooling demand, and changes in VOC and NO₂ emissions from tree maintenance. The study period was the 3-day episode from 31 July - 2 August, a high ozone period captured during the summer 2000 Central California Ozone Study.

There are approximately 11 million trees in the urban areas of the Sacramento non-attainment area, and another 3 million trees in areas projected to become urbanized by 2028, based on a 1995 study of Sacramento's urban forest ecosystem. Canopy cover was estimated to be 14% in urban areas, and 5% in urbanizing areas. Five tree planting scenarios were evaluated. Each contained 1 million program trees planted by the year 2018, target date for local compliance with the 8-hour ozone standard. Program trees were used to either increase overall canopy cover (new tree), or take the place of an existing tree as either a replacement for a removal or a substitute for a tree to be planted in new development. Program tree species were selected to have a lower level of biogenic volatile organic compound (BVOC) emissions, so that replacement and substitute trees will result in reduced BVOC emissions. BVOC emission rates were calculated hourly for 20 years, using locally-derived tree growth data for each species. Reductions in ozone, NO₂ and PM10 concentrations were projected from deposition and power plant emissions (reduced building energy use due to trees) over the same period. Increased emissions associated with tree maintenance activities were estimated as well.

In 2018, reductions in BVOC emissions were projected to be greatest when all trees were replacements or substitutes, 1.07 tons/day (tpd). BVOC reductions were least when all trees were new (0.13 tpd). Pollutant uptake was projected to be greatest when all trees were new, and canopy cover increase the greatest. One million new, low-emitter trees are projected to increase canopy cover between 1 and 2 percent. Deposition was negligible when all trees were replacements or substitutes. Deposition was the second largest flux, after BVOCs, and ranged from reductions of 0.24 tpd for NO₂, to 1.2 tpd for PM10 and 1.5 tpd for ozone. VOC and NO₂ emissions reductions of 12 and 9 tpd, respectively, are required to meet emissions targets for 2018 of 107 and 78 tpd. Hence urban forestry-related reductions projected here associated with changing species mix and deposition are up to 8.9 percent of required BVOC reductions, and 2.7% of NO₂ reductions.