Guest commentary by Loretta Mickley, Harvard University
Every summer over much of the United States, we brace ourselves for heat waves. During these periods, the air turns muggy and usually smoggy. After a few days, a cold front moves in, sweeping away the pollution and ending the heat. Given that we are on a path towards global warming, atmospheric chemists are asking how climate change could affect air quality. Will warmer temperatures mean more pollution during these episodes? Will episodes last longer? Most importantly, what effect will changes in air quality have on human health?
Recently the National Resource Defense Council (NRDC) released Heat Advisory, a report warning that surface air quality could suffer greatly as a result of climate change. In response, a group called the Pacific Research Institute (PRI), together with another group called United for Jobs, published Air Quality False Alarm, a detailed criticism of the NRDC forecast. PRI argues, among other things, that anthropogenic emissions in the U.S. will drop sharply in coming decades. In their view, air pollution will become a thing of the past, no matter what happens to climate.
What’s the story here? First, a little background on ozone and particulate matter (PM), two major components of smog. Surface ozone is formed from a mix of natural and anthropogenic precursors like nitrogen oxides and volatile organic carbon. We have measurements of surface ozone dating back to the late 1800s which imply that ozone in some regions has increased 2-5 times due to emissions of ozone precursors from cars, industry, and power plants. As for PM, there are many different kinds – e.g., organic carbon, soot, and sulfate-ammonium-nitrate. Some kinds of PM, like soot, are directly emitted into the air, but other kinds condense from gas-phase molecules. Like ozone, PM has both natural and anthropogenic ingredients.
Many factors govern the severity and timing of pollution episodes. An obvious factor is the magnitude of precursor emissions. But there are meteorological factors, like how stagnant the surface air is and whether it’s clear or cloudy, warm or cool. The summer of 1998, for example, saw a record number of ozone exceedances averaged over New England. That summer was also the warmest on record for that region. The hot summer that Europe endured in 2003 was also a summer of high pollution levels for that continent. But the cool summer in the U.S. that same year meant that the we saw low levels of pollution.
So how will pollution evolve over the coming decades as climate changes? The easy answer is: oh, the warmer temperatures mean greater pollution! But it’s more complicated than that. Then there are other meteorological factors to consider. As the surface temperatures rise, will the depth of the boundary layer increase, diluting the pollutants within it? Maybe stronger surface winds will carry all the pollution away. What about changes in cloud cover or rainfall?
To tackle issues of this complexity, modelers often turn to sensitivity studies. A sensitivity study is one in which you change just one or two variables, and keep everything else constant. By taking the problem apart in this way, you can isolate the effect of one or two factors at a time.
In one sensitivity study, Aw and Kleeman  imposed a 5ºC increase in temperature over the Los Angeles basin, but kept all other meteorological variables (like windspeed) constant in their model. Ozone in the region increased by 10-15%, but concentrations of sulfate-ammonia-nitrate PM decreased by 10-15%. That’s because ammonia condenses less readily at high temperatures. This is an interesting result. But in the real world, stalled high pressure systems, like the one over the Midwest and Northeast last week (April 18-20), can lead to both warm temperatures and high PM. With clear skies and weak winds, PM can accumulate over the source regions. As the climate changes, not only could temperatures change, but also the behavior of these high pressure systems.
In my research group, we tried a different sensitivity study [Mickley et al., 2004]. We devised our model experiment to test just the effect of changing wind patterns on pollutant concentrations. What we found was that the severity summertime regional pollution episodes in the Midwest and Northeast U.S. increased significantly by 2050, relative to present. Also, the average length of an episode increased from 2 to 3-4 days. Why did this happen? Our model forecast a 20% decline in the frequency of cold fronts sweeping into the U.S., so stagnation events in the model persisted longer. That allowed both gas-phase and PM pollution to build to higher concentrations.
Another model study [Hogrefe et al., 2004] focused on the effect of climate change on just surface ozone. The authors found that even with emissions of ozone precursors in the model held at 1990s levels, the total number of “exceedance days” increased by about 60% over the eastern U.S. (An exceedance day is a day in which ozone averaged over 8 hours exceeds the EPA threshold of 84 ppb.) Because of the complexity of the study, Hogrefe et al.  could not diagnose precisely all the meteorological changes (temperature? circulation patterns?) contributing to the increased surface ozone in their model. But they did find that one factor accounting for about half the increase was enhanced emissions of natural ozone precursors, which are temperature-sensitive.
One of the biggest unknowns, of course, is how anthropogenic emissions will evolve in the future. The Clean Air Act has led to tremendous improvements in air quality since the 1970s. But even if our emissions do decline, the consequences for air pollution are uncertain. Fiore et al.  have shown that decreases in U.S. emissions may be offset by increases elsewhere in the world. Specifically, rising methane emissions elsewhere in the world could significantly enhance background levels of ozone over the U.S., leading to as much pollution in 2030 as we saw in the mid-1990s.
So there’s a lot more to be learned about the links between climate and pollution. Since both surface ozone and PM have adverse effects on human health, understanding the link is important.
Aw, J., and M.J. Kleeman, Evaluating the first-order effect of intraannual air pollution on urban air pollution, J. Geophys. Res., 108, 4365, 10.1029/2002JD002688, 2003.
Fiore, A.M., D.J. Jacob, B.D. Field, D.G. Streets, S.D. Fernandes, and C. Jang, Linking ozone pollution and climate change: The case for controlling methane, Geophys. Res. Lett., 29, 1919, doi:10.1029/2002GL015601, 2002.
Hogrefe, C., B. Lynn, K. Civerolo, J.-Y. Ku, J. Rosenthal, C. Rosenzweig, S. Gaffin, K. Knowlton, and P.L. Kinney, Simulating changes in regional air pollution over the eastern United States due to changes in global and regional climate and emissions. J. Geophys. Res., 109, D22301, doi:10.1029/2004JD004690, 2004.
Mickley, L. J., D. J. Jacob, B. D. Field, and D. Rind, Effects of future climate change on regional air pollution episodes in the United States, Geophys. Res. Let., 30, L24103, doi:10.1029/2004GL021216, 2004.