Arboriculture & Urban Forestry 33(1): January 2007 3 Air Quality Benefits The hourly pollutant dry deposition per tree was expressed as the product of deposition velocity Vd 1/(Ra + Rb + Rc) (where Ra, Rb, and Rc are aerodynamic, boundary layer, and stomatal resistances), pollutant concentration C, canopy pro- jection area CPA, and a time step. Hourly deposition veloc- ities for ozone (O3), nitrogen dioxide (NO2), sulfur dioxide (SO2), and particulate matter of <10 m diameter (PM10) were calculated using estimates for the resistances Ra, Rb, and Rc for each hour throughout a “base year” (Scott et al. 1998). Hourly meteorological data and pollutant concentrations were obtained from local monitoring stations in the Denver met- ropolitan area for 1999, when pollutant concentrations were near average. Energy savings result in reduced emissions of criteria air Figure 1. Predicted size of green ash and pistache trees based on street tree data from Fort Collins, Colorado, and Modesto, California. a biomass equation for an urban green ash, a congener (Fraxi- nus velutina ‘Modesto’) (Pillsbury et al. 1998) was used. Carbon dioxide released through decomposition of dead woody biomass was based on an average annual tree removal rate of 0.8%. CO2 released as a result of tree maintenance activities was estimated based on annual consumption of gasoline and diesel fuel as 0.14 kg/cm dbh (0.78 lb/inch dbh) (McPherson et al. 2003). Reductions in building energy use result in reduced emis- sions of CO2. Emission reductions were calculated as the product of energy savings and CO2 emission factors for elec- tricity and heating. Heating fuel was natural gas, whereas the fuel mix for electrical generation was 78% coal, 16% hydro, 5% natural gas, and 1% other. The value of CO2 reductions was $0.017/kg ($15/ton CO2) based on the average of high and low estimates by CO2e.com (2002). Table 1. Monetized value of benefits for the single-tree example in Fort Collins and the multiple-tree example in Davis. Benefits CO2 O3 Electricity ($/kWh) Natural gas ($/MJ) Runoff avoided ($/m3 ($/kg) ($/kg) PM10 NO2 ($/kg) ($/kg) Volatile organic hydrocarbons ($/kg) Aesthetics/other ($/m2 ) ) Ft. Collins 0.78 0.068 2.85 0.017 6.77 11.31 6.77 10.69 232.71 Davis 0.12 0.061 0.460 0.033 8.48 9.84 8.48 3.32 553.60 pollutants (volatile organic hydrocarbons [VOCs], NO2, SO2, PM10) from power plants and space-heating equipment. These avoided emissions were calculated using utility- specific emission factors for electricity and heating fuels. Emissions of biogenic volatile organic compounds (BVOCs) from the green ash were included in the analysis because of their impact on ozone formation. The hourly emis- sion rates of isoprene (0.04 g/g/hr), monoterpene (0.04 g/ g/hr), and other VOCs (0.12 g/g/hr) were adjusted for tem- perature (monoterpene) or for sunlight and temperature (iso- prene) (Winer et al. 1998). This approach did not account for the benefit associated with lowered summertime air tempera- tures and the resulting reduced hydrocarbon emissions from anthropogenic and biogenic sources. The monetary value of tree effects on air quality should reflect the value that society places on clean air as indicated by its willingness to pay for pollutant reductions. Lacking specific data for Fort Collins, air quality benefits were mon- etized as damage values (Table 1) using regression relation- ships between emission values, pollutant concentrations, and population numbers (Wang and Santini 1995). This regres- sion provides estimates of the costs of damages to human health resulting from air pollution. Stormwater Runoff Reductions A numeric interception model accounted for the amount of annual rainfall intercepted by trees as well as throughfall and stem flow (Xiao et al. 2000). The volume of water stored in tree crowns was calculated from tree crown leaf and stem surface areas and water depth on these surfaces. Hourly me- teorological and rainfall data for 1998 in Fort Collins were used because total rainfall was close to the average annual amount (458 mm versus historic mean of 452 mm, 18.3 in versus 18.1 in). Stormwater reduction benefits were priced by estimating costs of controlling stormwater runoff with reten- tion/detention basins in Fort Collins. Total expenditures for retention/detention basin land acquisition, construction, and annual maintenance and operation costs for 20 years were ©2007 International Society of Arboriculture
January 2007
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