2011 News & Events

The Ups and Downs of Climate and Fine-Particle Pollution Across the U.S.

22 June 2011

After several years under tightened regulations for fine-particle pollution from diesel vehicles and wood stoves, is the air any cleaner in the U.S.? And if so, do the changes have an impact on climate?

These questions are addressed by a new study led by Dan Murphy of CSD, published in May in Atmospheric Chemistry and Physics. The paper shows that fine-particle pollution decreased significantly – a whopping 30% for the annual average – in a broad region across the entire U.S. from 1990 to 2004. The results are good news for the protection of human health and the preservation of visibility in national parks, but the paper shows that the climate consequences are a mixed bag – with a net impact of increased warming.

Rather than focus on the very acute situations encountered in polluted urban environments, the authors wanted to take a broad-brush look at "background" pollution levels and examine the climate implications of the changes in particles.

So Murphy and colleagues from the Desert Research Institute, the National Park Service, NOAA's Air Resources Lab, and several universities analyzed 15 years of fine-particle data gathered at 50 sites in national parks, at monuments, and other remote sites throughout the U.S. The data included measurements of particles containing elemental carbon (EC), which absorb light and hence warm the Earth, as well as fine particles (PM2.5) that are reflective and hence have a cooling effect. The "IMPROVE" network (Interagency Monitoring of Protected Visual Environments) provided the long data record needed for the analysis.

graphical representations of trends in elemental carbon (EC) and fine particle mass other than EC (PM2.5)
Trends in elemental carbon (EC; left panel) and fine particle mass other than EC (PM2.5; right panel) at monitoring stations between 1 March 1990 and 29 February 2004. The marker size shows the magnitude of the trend, and the marker color and direction indicates whether the trend is an increase or a decrease. The only urban site, Washington DC, is excluded from the averages in the bottom right corner of each panel. Images: Dan Murphy, NOAA

The vast majority of sites, over 80%, had a decreasing trend in the annual average of both elemental carbon and fine-particle pollution during the 1990 to 2004 period. Most decreases were in the range of 1.5 to 6 percent per year. Broken down by seasons, the trends were largest in winter months (consistent, for example, with the new particulate emissions standards for wood stoves that were introduced in 1998). In the mountain western U.S. during summer, elemental carbon actually increased, likely due to summer wildfires.

The implications of these trends for climate are a balance between two opposite effects. The black elemental carbon particles absorb light and produce a warming. Non-absorbing particles such as sulfate and organic carbon in PM2.5 reflect light and hence lead to a cooling. Murphy et al. calculated the balance of these direct effects using climate and chemistry models, finding that the net result is a warming of 0.3 Watts per square meter over the contiguous U.S., and 0.014 W/m2 globally.

The authors point out that the results show that in climate discussions, the year chosen as a "baseline" for further reductions in fine-particle pollution makes a big difference; the Kyoto Protocol and some proposed U.S. legislation use a 1990 baseline, whereas other proposed U.S. legislation uses 2005 as the baseline. Furthermore, projections of health and climate effects will be very different depending on the year used to estimate fine-particle pollution emissions.

For human health, the trends found by Murphy et al. are nothing but good news. For climate... it depends on how you look at it. A large fraction of the climate benefits from reducing elemental carbon have already been gained – good news to be sure, but news that carries with it the realization that fewer benefits can be achieved from this relatively "quick fix" in the future.

D. M. Murphy1, J. C. Chow2, E. M. Leibensperger3, W. C. Malm4, M. Pitchford5, B. A. Schichtel6, J. G. Watson2, and W. H. White7, Decreases in elemental carbon and fine particle mass in the United States, Atmospheric Chemistry and Physics, doi:10.5194/acp-11-4679-2011, 2011.

  1. Chemical Sciences Division, Earth System Research Laboratory, National Oceanic and Atmospheric Administration
  2. Division of Atmospheric Sciences, Desert Research Institute
  3. School of Engineering and Applied Sciences, Harvard University
  4. Cooperative Institute for Research in the Atmosphere, Colorado State University
  5. Air Resources Laboratory, National Oceanic and Atmospheric Administration
  6. National Park Service, Colorado State University Foothills Campus
  7. Crocker Nuclear Laboratory, University of California Davis

Observations at national parks and other remote sites show that average elemental carbon and fine particle mass concentrations in the United States both decreased by over 25 % between 1990 and 2004. Percentage decreases in elemental carbon were much larger in winter than in summer. These data suggest that emissions controls have been effective in reducing particulate concentrations not only in polluted areas but also across the United States. Despite the reduction in elemental carbon, the simultaneous decrease in non-absorbing particles implies that the overall radiative forcing from these changes was toward warming. The use of a 2005 instead of 1990 as a baseline for climate-relevant emissions from the United States would imply a significantly lower baseline for aerosol emissions. The use of older data will overestimate the possibility for future reductions in warming due to black carbon controls.