A new Portland State University study that maps Portland's air pollution patterns highlights the air-cleansing benefit of urban trees. But an economic analysis of the detailed pollution map shows Portland's air quality may be more hazardous than previously thought.
PSU's interdisciplinary Trees and Health Research Team, working with community volunteers and students, placed sensors at 144 locations around Portland to monitor levels of nitrogen dioxide (NO2). The brown noxious gas is an EPA-regulated major air pollutant that can exacerbate respiratory health problems such as asthma.
Comparing air pollution measurements to detailed maps of pollution sources and tree canopy cover, the team developed a model to predict NO2 levels at a resolution of 200 meters, approximately two to three city blocks.
In contrast, the Oregon Department of Environmental Quality currently monitors NO2 in Portland using only two sensors, including a "maximum source" monitor adjacent to a high-traffic freeway, according to the draft 2014 Oregon Annual Ambient Air Monitoring Network Plan.
Anthony Barnack, ambient air monitoring coordinator for the DEQ, explained that Portland's NO2 levels are well below federal EPA standards, so the agency focuses its monitoring efforts on pollutants approaching EPA limits, including ozone and PM2.5 - microscopic particulate matter.
Using a simple model that applies a uniform average air pollution value across the Portland region, the trees and health team estimated that NO2 imposes $34 million of economic harm annually, through missed school days, hospital stays and emergency room visits due to respiratory health problems.
But a more detailed analysis based upon the team's landscape-level model actually places this economic harm closer to $46 million.
"NO2 is a good marker for combustion exhaust," said team member Linda George, professor of environmental science at PSU, "so we can predict similar spatial variability of other combustion-related pollutants," including microscopic particulate matter that can cause cardiovascular disease and lung cancer.
Portland's leafy canopy offers at least a partial solution. "Urban trees take up NO2 an order of magnitude more than other models have shown," said George. The research team's landscape-level model indicates that trees can reduce NO2 by about 15 percent on average, depending on the quantity of mature trees within a 400-meter radius.
Putting the pieces together, the researchers valued the NO2-reducing benefits of trees at $6.6 million annually. But if one were to account for the benefits due to reduction of other air pollutants, said George, "this is just the tip of the iceberg."
Team member Vivek Shandas, an associate professor of urban studies and planning at PSU, expressed concern about the large variations in air quality from neighborhood to neighborhood that the model predicts. "If you were able to know at the street level or neighborhood level what the air quality is," says Shandas, "you might choose to walk your kid to school on a different route."
The solution is not as simple as merely planting more trees, however. Certain tree species emit compounds that react with NO2, potentially increasing ozone and particulate matter pollution.
The research team is working to determine which species provide the most benefit, to help homeowners, neighborhood groups and city planners make decisions in augmenting Portland's urban forest.
Team members are optimistic that the results can be helpful in guiding intentional cityscape design to reduce unanticipated air pollution burdens upon certain communities.
The Portland Bureau of Transportation is updating its Transportation System Plan, including efforts to evaluate and improve public health outcomes of transportation choices.
According to spokeswoman Diane Dulken, "the city's goal is to make transportation 70 percent reliant on methods other than single occupancy vehicles." This holistic approach to transportation management is expected to reduce all combustion-related emissions including NO2.
The PSU scientists hope to extend their analysis to other cities across the nation.
"This information will help us inform the discussion about how cities are built and how it matters to the people who live there," said Shandas. "It also highlights potential environmental justice issues as a majority of the most mature trees - the most valuable for reducing air pollution - are located in wealthy neighborhoods."