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Urban Forests, Environmental Quality and Human Health

Urban Tree Effects on Air Quality and Climate

[photo:] Power generating plant with five large smokestacksAir quality in urban areas is often degraded due to emissions from various sources (e.g., cars, factories, power plants) associated with urban development and high concentrations of people. In the United States, over 144 million people live in areas designated as non-attainment for ozone. In addition, carbon dioxide emissions, which contribute to global climate change, are considerable higher in urban areas. However, trees in urban areas have the ability to air quality and reduce greenhouse gas concentrations by both reducing emissions and directly removing pollutants from the atmosphere. Trees can reduce pollutant emissions by reducing building energy via tree shade, blocking winter winds and reduced air temperatures. Tree leaves (shade) and reduced air temperatures also affect the levels of ultraviolet radiation at ground level and human comfort.

Biogenic VOC Emission Estimated Rates for Common U.S. Tree and Shrub Genera (pdf)

Volatile Organic Compound (VOCs) emissions contribute to ozone and carbon monoxide formation. Tree species differ in the amount of VOCs emitted per gram of leaf mass. This document lists the estimated amount of Isoprene and Monoterpene emission for numerous genera. Emission rates are in micrograms of C/g of leaf dry weight/hr (standardized to 1,000 µmol m-2 s-1 of photosynthetically active radiation and 30oC). Some emission rates on list were not measured; rather they are median values of botanical relatives.

Use of Emerging Measures in a State Implementation Plan (SIP) (pdf)

In September, 2004, the U.S. Environmental Protection Agency (EPA) released a document titled "Incorporating Emerging and Voluntary Measures in a State Implementation Plan (SIP)" that details how new measures, which may include "strategic tree planting" can be incorporated in SIPs as a means to help meet air quality standards set by EPA. This document details how urban forestry fits within this new measure.

Cumulative tree effects on air quality

Urban vegetation can also directly and indirectly affect local and regional air quality by altering the urban atmospheric environment. The four main ways that urban trees affect air quality and reduce greenhouse gases are:
[photo:] Pedestrian walks on Boston sidewalk shaded by oak treesTemperature reduction and other microclimatic effects
Removal of air pollutants
Emission of volatile organic compounds (VOC) and tree maintenance emissions
Energy effects on buildings

(See summary paper -pdf for more details.)

Research focuses on understanding the cumulative and interactive effects of these four factors, and also focuses on:

  1. Quantifying the current urban forest structure in many cities and its effects and value related to pollution removal, carbon storage and sequestration, VOC emissions, and energy conservation. Estimates are made at the local to national scale.
  2. Developing tools and models to allow users to calculate the effects of local tree populations on pollution removal, carbon storage and sequestration, VOC emissions, and energy conservation
  3. Quantifying the cumulative effect of trees on ozone concentrations
  4. Quantifying carbon content in urban soils (link to soils)
  5. Developing programs to predict the spatial effect of changes in tree cover on ground-level air temperatures
  6. Modeling tree effects on human comfort
  7. Quantifying the effects of trees on ultraviolet radiation
  8. Monitoring tower measurements of carbon dioxide flux in Baltimore as part of the Baltimore Ecosystem Study

Research  Studies

Last Modified: 03/31/2009