1. Increasing Canopy Cover in Urban Areas
David J. Nowak
USDA Forest Service
Northeastern Research Station
Syracuse, NY

2. Overview
Program Benefits
Trees and Air Quality
Issues

3. Tree Canopy Cover Affects:
Air and surface temperatures
Building energy use
Ultraviolet radiation loads
Climate change
Air quality
Water quality & flows
Social & psychological well-being
Aesthetics…

4. Temperature reduction
Removal
Emissions
Energy Conservation

5. Literature - Atlanta Case Study
Ozone conc. (ppb)
Maximum: June 4,
20% reduction in natural hydrocarbon emissions
Photochemical effect (2o C increase)
Biogenic emission effect (2o C increase)
Anthropogenic emission effect (2o C increase)
14% increase in maximum ozone concentration
due to loss of vegetation
(Cardelino and Chameides, 1990)

6. Los Angeles Basin Study
Air quality impacts of increased urban tree cover may be locally positive or negative with respect to ozone
Net basin-wide effect of increased urban tree cover is a decrease in ozone concentrations if the additional trees are low VOC emitters
(Taha, 1996)

7. Urban Trees and Ozone in the Northeastern United States
Increased urban tree cover: Reduced ozone (O3) in urban areas (-1 ppb daytime)
Physical effects of trees on pollution removal, air temperature, wind speed and boundary layer height are important
Tree removal of NOx lead to increased O3 at night (loss of NOx scavenging of O3)
Tree VOC emissions had no detectable (<1 ppb) effect on O3
(Nowak, Civerolo, Rao, Sistla and Luley, 2000)

8. New York City Area Study
10% increase in urban tree cover
Reduced 1-hour maximum O3 by ~4 ppb (132 ppb to 128 ppb)
8-hour maximum O3 by ~1 ppb
Little difference in maximum reductions between 10% and 30% tree cover increase
Very significant impact
37% reduction in amount needed to gain attainment
Additional tree cover will remove thousands of tons of air pollutants per year
(Luley et al, 2000)

9. Issues Remaining Emissions reductions
But trees emit; VOC / NOx equivalents
Land Use Change (bigger issue than trees)
Canopy preservation
Monitoring / verification / enforcement
Programs vs. tree cover
Flexible SIP program
Maintenance program

10. Questions?