EPA Draft Report Concludes Climate Change May Significantly Increase Ground-Level Ozone in Areas of the US
|Change in ozone concentrations between present and 2050 in simulated September-October mean MDA8 O3 concentrations (in ppb), from one of the papers cited in the EPA synthesis report. (Nolte et al., 2007) Click to enlarge.|
Climate change has the potential to produce significant increases in near-surface O3 concentrations in many areas of the US by 2050, according to a EPA draft document released for public comment. The increases are in the range 2-8 ppb for summertime-average maximum daily 8-hour (MDA8) O3.
The results suggest that areas in non-attainment for ozone (O3) and areas just below the O3 National Ambient Air Quality Standards (NAAQS) “should begin to consider the impacts of climate change as they develop their attainment and maintenance strategies, even for near-term planning horizons. In other words, they may need to account for a “climate penalty” imposed on their control policies.”
Ground-level ozone is created by chemical reactions between oxides of nitrogen (NOx) and volatile organic compounds (VOC) in the presence of sunlight and hot weather. Ground-level ozone is the primary constituent of smog.
Breathing ozone can trigger a variety of health problems including chest pain, coughing, throat irritation, and congestion. A National Research Council (NRC) report published in April found that short-term exposure to current levels of ozone in many areas is likely to contribute to premature deaths, and added that the evidence is strong enough that EPA should include ozone-related mortality in health-benefit analyses related to future ozone standards. (Earlier post.)
In March, EPA established a new primary 8-hour standard for ozone of 0.075 parts per million (ppm) [75 ppb], and a new secondary standard set at a form and level identical to the new primary standard. The previous primary and secondary standards were identical 8-hour standards, set at 0.08 ppm. Because ozone is measured out to three decimal places, the standard effectively became 0.084 ppm; therefore, areas with ozone levels as high as 0.084 ppm were considered to have met the 0.08 ppm standard, due to rounding.
The new standard is at the higher end of options proposed by EPA staff scientists in a paper submitted last year, and falls above the standard recommended by scientific and medical groups, including the Clean Air Scientific Advisory Committee (CASAC) which assists the Administrator of the EPA. (Earlier post.)
In its Fourth Assessment Report (AR4) in 2007, the Intergovernmental Panel on Climate Change (IPCC) found that “future climate change may cause significant air quality degradation by changing the dispersion rate of pollutants, the chemical environment for ozone and aerosol generation and the strength of emissions from the biosphere, fires and dust. The sign and magnitude of these effects are highly uncertain and will vary regionally.”
The EPA’s Office of Research and Development (ORD) Global Change Research Program, in partnership with EPA’s Office of Air and Radiation (OAR) and several Regional offices, began an assessment effort to increase scientific understanding of the multiple complex interactions between climate and atmospheric chemistry.
The ultimate goal of this assessment, according to the EPA, is to enhance the ability of air quality managers to consider global change in their decisions through improved characterization of the potential impacts of global change on air quality.
The draft of the interim report—a synthesis of research on the impact of climate change on ground-level ozone—is part of the first phase of an integrated assessment framework.
The report is focused primarily on the air quality impact of global climate change, and largely does not address the relative importance of climate vs. anthropogenic emissions of air pollutants or the effectiveness of air quality management efforts. Future reports will focus on those dimensions, according to EPA.
The EPA also emphasized that the report does not represent and should not be construed to represent any Agency determination or policy.
While the report found that simulation results for certain regions of the country (for example, a loosely bounded area encompassing parts of the Mid-Atlantic, Northeast, and lower Midwest) tended to show agreement in results, other regions, notably the West Coast and the Southeast/Gulf Coast, show conflicting results.
The EPA’s findings also indicate that, where climate-change-induced increases in O3 do occur, “damaging effects on ecosystems, agriculture, and health will be especially pronounced, due to increases in the frequency of extreme pollution events.”
Other broad findings of the report include:
Climate change has the potential to push O3 concentrations beyond the envelope of natural interannual variability in many regions of the US. In addition, it has the potential to lengthen the O3 season.
Air quality managers may need multi-year simulations to support the development of long-term air quality control strategies, to capture the effects of both natural meteorological variability and climate-induced changes, the EPA suggested. Air quality managers may also need to plan to extend the season over which they monitor O3 concentrations and be prepared to issue air quality alerts earlier in the spring and later into the fall.
Increasing global near-surface humidity associated with climate change has significant potential to decrease O3 concentrations in remote areas with low ambient NOx levels. In other words, changes in O3 concentrations as a result of climate change will depend, in part, on whether an area is clean or polluted, and/or on the degree of influence of air masses from adjacent clean or polluted areas.
The potential impact of climate change on PM is less well understood than that on O3. Preliminary results from the modeling studies show a range of increases and decreases in PM concentrations in different regions and for different component chemical species in the same region.
Climate change has the potential to impact a number of meteorological variables important for O3. Whether changes in these variables lead to increases, decreases, or no change in O3 concentrations in a given region depends on whether the effects of these individual changes on O3 act in concert or compete with each other.
There is disagreement across models of the effects of climate change on the summertime mid-latitude storm tracks, with implications for the simulated frequency and duration of synoptic stagnation events and resulting extreme O3 episodes.
Climate change has the potential to increase biogenic emissions of O3 precursors, but significant uncertainties remain about the impact of these emissions changes on O3 concentrations in a given region. Increases in lightning NOx production may also be a factor in future O3 changes.
Specific configuration choices made in the development and application of the integrated global-to-regional climate and air quality modeling systems used in this assessment are key determinants of simulated future US regional air quality. The unique characteristics of the climate change problem present significant challenges for uncertainty analysis of air quality impacts.
Preliminary sensitivity tests suggest that the impacts of climate change on future US regional O3 concentrations are potentially significant compared to future anthropogenic emissions changes, but these relative impacts are highly sensitive to the detailed assumptions about the magnitude and spatial distribution of emissions changes.
These results highlight the need for emissions scenarios with greater regional detail, consistency between global and regional assumptions, and consistency between greenhouse gases and precursor emissions. Meeting this need is a major focus of Phase II of the assessment effort.