Dow Texas Operations at Freeport is located in the Houston-Galveston-Brazoria (HGB) non-attainment area for federal air quality standards for ground-level ozone, a pollutant that negatively affects human well-being and ecosystem health. Ozone is not emitted directly, but forms through photochemical processes in which precursors interact in the presence of sunlight. To come into compliance with federal ozone standards, the state of Texas has imposed limits on emissions of ozone precursors by large industrial sources in the eight-county HGB area. These limits result in substantial costs to regulated sources of precursor emissions in the form of technology upgrades, production process changes and penalty fees.

 

What role does the value of nature play in your analysis? How is this analysis using/advancing conservation science and tools?

 

Reforestation may provide a new compliance approach for industrial emitters while generating a slew of additional benefits for conservation and people. Several studies in the United States have documented the beneficial effects of forests on urban air quality. Trees directly remove atmospheric pollutants such as respirable particulate matter, ozone, sulfur dioxide, carbon monoxide and nitrous oxide through interception on, or uptake through, leaf surfaces. The U.S. Forest Service’s widely-used and peer-reviewed UFORE (Urban Forest Effects) model allows estimation of the quantities of six major air pollutants removed by trees in a given location. Urban forests also reduce the so-called urban heat island effect, lowering ambient air temperatures through evaporative cooling and the shading of buildings, roads and parking lots. This reduces evaporative emissions of volatile organic compounds (VOC), an important ozone precursor, and reduces space cooling needs, which in turn can lead to reduced power plant emissions of VOC, nitrous oxides — the other key ozone precursor — and other key air pollutants.

 

The U.S. Environmental Protection Agency (EPA) has formally identified tree planting as a measure states can use in their State Implementation Plans (SIP) to achieve compliance with federal air quality standards. However, to date, no state has incorporated tree planting into its SIPs. As a result, the potential and comparative costeffectiveness of the use of tree planting by industrial sources for air-quality compliance purposes remains unexplored.

 

We will use the UFORE model to estimate the quantities of ozone and NO2 a Dowfinanced reforestation project in the HGB area would be expected to remove directly, and the quantities of VOC that project would emit. This analysis entails the selection of planting sites and tree species based on ecological suitability, cost, conservation value and species-specific VOC emissions, and the prediction of the canopy structure to which the model is applied. Quantities of NO2 removed and VOC emitted by the project, adjusted for uncertainties as required by EPA, can be translated reasonably straightforwardly into potential corresponding ozone precursor credits/offsets (NO2) or debits (VOC) for Dow. In addition, ozone removal by the planted trees effectively is equivalent to avoided emissions of the precursors that would have produced this ozone. Since the quantity of ozone removed by trees exceeds the quantities of precursors removed (NO2) or emitted (VOC), it is the dominant determinant of the quantities of precursor credits a reforestation project would generate. Estimation of these equivalent avoided precursor emissions requires knowledge of local atmospheric chemistry, as ozone formation may be either VOC or NO2 (or sunlight) limited. The expected costeffectiveness of reforestation for compliance purposes then is estimated using the projected cost of the reforestation project and the estimated precursor credits, and compared to that of standard control approaches.

 

We also quantify the value of human health benefits from air pollution removal by the reforestation project and the value of selected other ecosystem services, such as increased green space supporting recreation and visual amenities, and net removal of atmospheric carbon.

 

How could this analysis change how Dow does business?

 

The Texas regulatory authorities have expressed willingness to consider the inclusion of reforestation in their air quality compliance strategy if our analysis convincingly demonstrates that reforestation can be an effective tool for reducing concentrations of ozone precursors. If our analysis also shows that reforestation is costcompetitive with conventional precursor control options, Dow and others are likely to incorporate reforestation into their compliance strategy. Our results and methodology will be made freely available, allowing other states and companies to evaluate inclusion of reforestation into ozone compliance plans.

 

What are the potential conservation outcomes from this analysis?

 

Reforestation for air quality compliance purposes in the HGB area could support restoration of portions of the Columbia Bottomland Hardwood Conservation Area southwest of Houston, which has lost over 70 percent of its historic land cover. Reforestation in other non-attainment areas in Texas and in other states could result in large additional conservation benefits.

 

What is the biggest challenge you’ve encountered doing this work?

 

The biggest challenge is to ensure our resulting emission reductions, credits/offsets and cost-effectiveness estimates are realistic. Attaining that realism requires detailed discussions with air pollution authorities on several key methodological issues — above all, how to convert the ozone removed by the planted trees into its equivalent avoided precursor emissions, which in turn depends on the correct characterization of local atmospheric conditions at, and downwind of, the planting sites.

 

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By Timm Kroeger, senior environmental economist, Sustainability Science Team, The Nature Conservancy