From the Field

Reforestation for Ozone Removal: An Attractive Option

September 8, 2014

Smog blankets Houston, Texas. Ozone — together with fine particulates — contributes to smog formation. A new study documents the viability of reforestation for ozone control. Photo credit: Flicker user Rick Kempel under a Creative Commons 2.0 license.

When it comes to improving air quality in the United States, most of the low-hanging fruit is gone. There’s plenty of work still to do, but it’s getting more difficult and more expensive.

So the idea of using forests — nature’s air cleaners — to remove smog-causing ozone and its precursors has great appeal. Even more so if the approach could be implemented at lower cost than high-tech approaches.

Now new findings by a multi-disciplinary team of scientists — led by Timm Kroeger, an environmental economist at the Nature Conservancy — appear to validate this approach.

Kroeger and his colleagues modeled the uptake of ozone and NOx (which contributes to the formation of ozone) by a re-growing forest. They found that restoring deforested sites could be a less expensive alternative to the next tier of ozone abatement technologies — especially if companies could also earn carbon credits. Their findings appear this week in the Proceedings of the National Academy of Sciences.

“We looked at something that hasn’t been looked at,” said Kroeger, “which is reforestation of peri-urban areas – that is, replanting trees around the urban periphery — for ozone benefits. We put together a blueprint of what questions would need to be answered to move forward with this.”

A Novel Approach to the Persistent Ozone Challenge

Air quality in the United States has increased dramatically since the 1970s when Congress passed the Clean Air Act and automakers introduced the catalytic converter. But ground-level ozone continues to keep children out of school and send elders and those with respiratory ailments to the emergency room in dozens of U.S. metropolitan areas.

In the developing cities of China and Latin America, the situation is far worse. One estimate puts global ozone-related deaths at 152,000 per year.

Ozone forms when nitrogen oxide compounds (NOx) combine with volatile organic compounds (VOCs) in the presence of sunlight.  All three are required for the process, so whichever is in shortest supply determines the rate of ozone formation.

It’s the end product, ozone, that’s unhealthy. But the precursor, NOx, is what’s emitted from furnaces, factories and tailpipes. So it’s those emissions that are regulated. In regions where ambient ozone concentrations exceed EPA standards – designated “non-attainment zones” – additional pollution reduction measures are required.

Growing trees remove both ozone and NOx from the air. And U.S. Environmental Protection Agency policy encourages creative approaches to achieving a federally-set air quality goal, including tree-planting.

Knowing this, Nature Conservancy and The Dow Chemical Company scientists from Freeport, Texas — who got together in 2011 to see where they might find common ground for conservation and business — sought to test how companies could use reforestation to achieve a level of additional ozone avoidance that industry has found difficult and expensive to reach.

The Ozone Abatement Outcome is Good, but Details Matter

The study — conducted by researchers from the Conservancy, Dow and the University of Florida — combines air-quality and weather data with a forest growth and mortality model to estimate how much ozone and NOx would be removed by reforesting once-deforested areas.

They based the model on a 405-hectare site, currently dominated by grass and shrubs, located outside of Houston, Texas.

The researchers tested several hypothetical scenarios — varying planting densities, ratios of NOx to ozone formation, and cost reduction options. Since state and federal environmental agencies regulate emissions of NOx and not ozone, the researchers had to back-calculate how much NOx would have been required to form the ozone that was removed.

Their findings: Reforestation almost always matched or improved upon the cost of technological controls, unless a company would have to purchase land for reforestation.

With land trusts, conservation organizations and state and regional environmental agencies constantly seeking funds for reforestation, many sites are likely available through partnerships.

If companies could also  document a forest’s carbon removal and sell the credits — the California Air Resources Board has an active market for forest carbon projects, for example — the option might be especially financially attractive to industry.

With carbon credits, the cost of reforestation for ozone removal was estimated to be as small as less than 30% the cost of technological controls or purchased pollution allowances.

Locations with the potential to use reforestation for ozone abatement are shown in green. They have ozone levels that exceed federal standards, were once forested but are not currently, and have NOx-limited formation of ozone.
Locations with the potential to use reforestation for ozone abatement are shown in green. They have ozone levels that exceed federal standards, were once forested but are not currently, and have NOx-limited formation of ozone.

Pollution Control with Other Benefits

There are important considerations that affect the prospect for long-term success of the strategy on both the local and the regional level. The researchers chose a site located on bottomland that had once been forested for several reasons. Previously forested locations offer a good (but not perfect) filter for reforestation success.

Bottomland forests are among the most diverse in the region, which makes the co-benefits attractive to conservation and land-protection organizations that might partner on such projects. Bottomlands are also less vulnerable to drought and fire — which could quickly put the captured NOx and ozone back into the air. But carbon markets have already developed a framework for dealing with that scenario. An insurance plan splits the predicted cost of the occasional total loss among all the participants in a pollution-credit market.

Across the United States, the approach has potential in thousands of sites totaling 153,527 km(almost the area of Georgia), but it’s worth bearing a few cautions in mind. Where forests would replace other valuable habitat – such as grasslands or wetlands — it may not be an ecologically attractive option.  Similarly, in areas with high land prices, ecologically suitable reforestation sites may not make financial sense. And local regulators need to be assured that “green-infrastructure” forest solutions will be as robust and reliable as traditional “gray-infrastructure” solutions.

Also, remember VOCs — those other compounds that contribute to ozone formation? They come from factories and tailpipes, but also from forests. Sites where ozone formation is driven more by VOCs than by NOx would not be good candidates for ozone abatement by reforestation.

Even with those cautions, pollution control that also offers improved biodiversity, erosion protection, recreation opportunities and improved water quality — while saving money — begins to deliver on the promise of natural infrastructure.

Correction, 6 October 2014: The original blog post stated that the approach has potential in thousands of sites that together are “almost the area of California.” This should have read “almost the area of Georgia” and has been corrected.

Opinions expressed on Cool Green Science and in any corresponding comments are the personal opinions of the original authors and do not necessarily reflect the views of The Nature Conservancy. 

Join the Discussion

Please note that all comments are moderated and may take some time to appear.


  1. Hi Theresa,

    Great point. The national map just shows general enabling conditions (in terms of potential for trees to abate ozone, and general suitability for tree planting), and does not account for regional climatic variations like drought, nor microclimatic specifics that could complicate the value proposition at an individual site. So in short: we would certainly not recommend planting trees in California at the moment. If EPA does eventually allow this kind of reforestation to meet ozone abatement requirements, we would want to do the same kind of analysis we did in Texas in any other potential sites, at which point we would see whether a general potential translates into a real opportunity. It is possible that by then the drought will have abated, but it also likely that some of the areas on the map with potential (as identified by a coarse national analysis) are not actually workable.