Urban Forests for Clean Air
Researching the relationship between our trees and the local atmosphere
As far back as elementary school, we were taught that trees absorb carbon dioxide, turning it into wood and oxygen. For most of us this is the end of the story. But if you want to plant millions more trees, like the Sacramento Tree Foundation is dedicated to do, there is a bit more reading to do.
Some trees emit hydrocarbon gasses and it is these emissions that cause air scientists to wonder about the actual air quality benefit of trees. This is an important question: given the many pluses and the one minus, are trees a net benefit to air quality.
In the fall of 2005, the Sacramento Tree Foundation teamed up with the US Forest Service, the Sacramento Area Council of Governments and the Sacramento, El Dorado, Placer and Feather River Air Districts to scientifically study the effect of trees on air pollution. We received a grant of $725,000 and started work in the spring of 2006. The joint project, known as the Urban Forests for Clean Air demonstration project, is divided into three phases and will take about two years to complete.
The objective is to research and model the net benefits of trees to air quality. The plan is to understand how trees affect the air, decide which trees make the biggest improvement in our air quality and where to best invest in our urban forest to improve the air. These studies could have far reaching implications for other cities affected by our nation's worsening air quality.
Needless to say, it's an exciting way to enter our 25th year. The first phase of the Urban Forests for Clean Air project is to develop initial estimates of tree effects on air quality using existing science and simplified, mathematical models of air chemistry. This includes predicting reasonable changes to the type and number of trees that will make up our urban forest in the future.
The second phase is to build new urban forest computer models for the many ways chemicals in the air react with each other and sunlight to make pollution.
The third phase of the project is to report the results so that policy makers and the public can make better-informed tree choices.
So far, the investigation shows great news. In completing the first phase of the study, we have found that we can make a significant difference to regional air quality by expanding our tree canopy. The numbers are impressive. A smart and extensive tree planting campaign can remove seven tons of hydrocarbons, three tons of ozone and almost one ton of nitrogen oxides. Reaching these quantities won't be easy, though. Over the next 15 years, the Sacramento region will need to plant 5 million new trees and replace 2.4 million trees that will die during that time frame.
Tree hydrocarbon emissions are a question mark. Hydrocarbons from trees are not a significant health concern by themselves, but since they conspire with bright sunlight and nitrogen oxides to make ozone, they probably aren't helping our air quality. Whether they are a major part or a minor part of our air quality problem, we don't yet know. Air chemistry isn't as simple as equating more hydrocarbons with more ozone. To be prudent, though, we should assume that lowering hydrocarbons from trees wouldn't hurt and this means planting trees that are low emitters.
Best Trees, Best Places
Tree emissions vary greatly from one type of tree to another. It is also notoriously hard to figure out what a tree actually emits. Even the same tree can emit completely different amounts under the same conditions on a different day.
It may be many years before this science stabilizes. In the meantime, researchers have begun to categorize trees as low, medium, or high emitters using the best available data and admitting that it will probably change. It is tempting at this point to include lists of good and bad trees, but this would be misleading. The preliminary results of our study suggest that any tree, perhaps even the highest of emitters, is better than no tree at all.
Part Of The Solution
The Sacramento area has a serious and stubborn ozone pollution problem. Over the years, a great many things have been done to reduce this problem, but our region hasn't yet reached national and state air quality goals. Finding more ways to improve the air we breathe is proving difficult. The early results of our study show that trees, like most of us learned in grade school, will be a critical part of Sacramento's air quality solution.