Particulate matter (PM) air pollution is a serious public health issue for the United States. While there is a growing body of evidence that climate change will partially counter the effectiveness of future precursor emission reductions to reduce ozone (O3) air pollution, the links between PM and climate change are more complex and less understood. This paper discusses what we currently understand about the potential sensitivity of PM episodes to climate-change-related shifts in air pollution meteorology, in the broader context of the emissions and atmospheric chemistry drivers of PM. For example, initial studies have focused largely on annual average concentrations of inorganic aerosol species. However, the potential for future changes in the occurrence of PM episodes, and their underlying meteorological drivers, are likely more important to understand and remain highly uncertain. In addition, a number of other poorly understood factors interact with these likely critical meteorological changes. These include changes in emissions from wildfires, as well as atmospheric processing of organic aerosol precursor chemicals. More work is needed to support the management of the health and environmental risks of climate-induced changes in PM. We suggest five priorities for the research community to address based on the current state of the literature.

Research priorities for understanding the impacts of climate change on particulate matter air pollution due to shifts in weather patterns, emissions, and chemistry.

Particulate matter (PM) is one of the most pervasive air quality problems facing the United States, posing a major challenge for public health. PM is a complex mixture of anthropogenic, biogenic, and natural materials, suspended as aerosol particles in the atmosphere. Major components of PM include sulfate, nitrate, ammonium, organic carbon, elemental carbon, sea salt, and dust. The aerosols that make up PM may be emitted directly, in which case they are known as primary aerosols, or they may be formed as secondary aerosols from gas-phase precursors. Major aerosol precursors include SO2, NOx (≡NO + NO2), NH3, and volatile organic compounds (VOCs). Primary aerosols and precursors of secondary aerosols are emitted by a variety of processes and sources, including combustion, evaporation, agricultural activities, and natural processes. When inhaled, PM can lead to significant health problems, including asthma, chronic bronchitis, reduced lung function, irregular heartbeat, heart attack, and premature death (e.g., see U.S. EPA 2009c; Lave and Seskin 1973; Dockery et al. 1993; Pope et al. 2002; Sacks et al. 2011). In addition to its effects on health, PM has several other types of impacts. For example, PM reduces visibility in cities and national parks. In addition, certain PM species are extremely important climate forcers (Charlson et al. 1992; Forster et al. 2007; Bond et al. 2013).

In recent decades, U.S. environmental legislation, such as the Clean Air Act, has been highly successful in reducing the atmospheric burden of PM nationally, with corresponding positive effects on public health. Pope et al. (2009) attributed nearly five months of the 2.72-yr increase in U.S. life expectancy between 1980 and 2000 to reductions in average PM2.5 levels by 6.52 μg m−3 across the United States. While emissions of pollutants and precursors are the main determinant of ambient pollution concentrations, there has emerged a growing understanding that global climate change has the potential to make it more difficult to continue to achieve such air quality improvements (NRC 2001, 2004; Forster et al. 2007). In response to this challenge, the U.S. Environmental Protection Agency (EPA) has been leading a major effort to improve our fundamental understanding of the multiple, complex links between global climate change and regional U.S. air quality. This growing knowledge base provides a foundation for adapting the U.S. air quality management system to the long-term challenge of climate change. While anthropogenic emissions of PM and its precursors are expected to continue to decrease in the United States over the coming decades, understanding the extent to which the changing climate will affect strategies to improve air quality is an important aspect of this adaptation.

One of the most important aspects of air quality management is the mitigation of air pollution episodes. The potential for climate change to significantly impact PM concentrations during air pollution episodes, with corresponding negative implications for public health, is situated within the broader context of the dominant role that environmental extremes play in conversations about climate change adaptation and mitigation (e.g., see Field et al. 2012). For example, evidence suggests that extremes, such as very hot temperatures, are becoming much more frequent as a result of the changing climate (Hansen et al. 2012). Beyond hydroclimatic extremes, such as hurricanes, tornadoes, torrential downpours, heat waves, and droughts, Earth system extremes like harmful algal blooms, wildfires, air pollution episodes, and disease outbreaks also affect important sectors of the economy and the environment, impacting people where they live and work. Critical research questions for the scientific community relate to whether such extremes are changing, or may in the future change, in intensity, duration, frequency, timing, and spatial extent as a result of climate change, as well as the potential for the occurrence of unprecedented extremes.

This paper discusses what we currently understand about the potential sensitivity of PM episodes to climate-change-related changes in air pollution meteorology, in the broader context of the emissions and atmospheric chemistry drivers of PM. We reiterate the recommendations of Ravishankara et al. (2012) in proposing a research agenda to improve scientific understanding of PM in a changing climate, as a foundation for an improved ability to adapt to the impacts and to manage the risks of climate-induced changes in air quality.

BACKGROUND: CURRENT UNDERSTANDING OF CLIMATE CHANGE AND PM.

An important part of our background knowledge base for understanding the potential implications of climate change on PM is recently improved understanding of the links between climate change and ground-level ozone (O3) concentrations. A number of recent studies of the effects of climate change on ground-level O3 have shown that the changing climate could have significant impacts on O3 air quality, as synthesized in recent efforts (U.S. EPA 2009a; Weaver et al. 2009; Jacob and Winner, 2009). For example, the sensitivity of O3 to temperature has been explored in several modeling studies (e.g., Sillman and Samson 1995; Dawson et al. 2007; Rasmussen et al. 2012). Collectively, this work suggests that, all else being equal, climate-induced changes in temperature, cloud cover, biogenic emissions, and synoptic-scale circulation patterns pose a significant risk for increased O3 concentrations over large portions of the United States, with corresponding risks for human health (Post et al. 2012). This impact, as well as the additional precursor emissions decreases that may be needed as a result, was termed the “climate penalty” by Wu et al. (2008). It is likely that the United States is already experiencing this climate penalty, as shown in a study of 21 years of O3 and temperature observations across the rural eastern United States (Bloomer et al. 2009). This body of research was an important consideration in the EPA administrator's 2009 finding that current and projected greenhouse gas concentrations pose a threat to human health and welfare (U.S. EPA 2009b).

Some of this work also considered PM. Most notably, the review of the impacts of climate change on air quality by Jacob and Winner (2009), while focusing more on O3, did also examine initial results regarding the effects of climate change on PM. The studies summarized therein pointed to several of the same meteorological variables as the main drivers behind climate-induced changes in PM. For example, Racherla and Adams (2006) and Tagaris et al. (2007) considered the effect of changing precipitation on sulfate to be especially important, while the latter study also discussed boundary layer height and wind speed changes. Tsigaridis and Kanakidou (2007) suggested that temperature- and precipitation-induced changes in biogenic emissions of VOCs could increase organic aerosol concentrations appreciably over the United States, while Heald et al. (2008) pointed to changes in both biogenic emissions and direct effects of precipitation as potentially important drivers of changes in organic aerosol concentrations.

Additional analysis (see Fig. 1) shows links between meteorology and observed PM2.5 episodes at one monitoring site in Chicago, Illinois (in the U.S. upper Midwest), and one in Birmingham, Alabama (in the U.S. Southeast), from 2007 to 2011. For this analysis, a modified version of the Wang and Angell (1999) definition of stagnation was used with National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) Reanalysis 1 data (Kalnay et al. 1996) to classify days as “stagnant” or “nonstagnant.” In Chicago, PM episodes are largely a cold-weather phenomenon, resulting from stagnant days with low mixing heights and temperature inversions, whi le PM episodes in Birmingham are generally associated with stagnation, regardless of temperature bin (Fig. 1). In Fig. 1, the presence of stagnant conditions appears to have caused consistently greater PM concentrations in Birmingham, regardless of temperature, while stagnant conditions appear to have had the most impact on Chicago PM concentrations on cold days. While only about a one-quarter of days during this 5-yr period were classified as stagnant in this analysis, two-thirds of the episode days (with PM2 .5 concentrations greater than 35 μg m−3) in Fig. 1 met the criteria for being considered stagnant. Climate change has the potential to strongly affect these driving factors, with corresponding implications for the occurrence of PM episodes. Horton et al. (2012) examined how the changing climate may affect stagnation frequency. Using an ensemble of the phase 3 of the Coupled Model Intercomparison Project (CMIP3) models and the A1B scenario for the late twenty-first century, they calculated that the eastern United States would experience an increase in stagnant days in all four seasons. Both the average summer and the average autumn were calculated to have two more stagnant days per season than under late twentieth-century conditions.

Fig. 1.

Box plots of daily average PM2.5 concentrations in 5°C bins in (top) Chicago and (bottom) Birmingham for the 2007–11 period. Black boxes and whiskers represent days classified as stagnant, while red boxes and whiskers represent days classified as nonstagnant. Boxes and whiskers represent the minimum, 25th percentile, median, 75th percentile, and maximum PM2.5 concentrations in each bin. Chicago PM data are from the Mayfair Pumping Station monitoring site (17-031-0052), while Birmingham PM data are from the Wylam monitoring site (01-073-2003). Temperature and other meteorological data in the stagnation determination were from the NCEP–NCAR reanalysis. Measurements from dates surrounding the 4 Jul holiday have been removed due to the PM contribution from fireworks. The 500-mb wind speed criterion was not used in Chicago for determining stagnant days, since this criterion is more relevant for multiday warm weather stagnation episodes, and the precipitation threshold was raised to 10 mm in Birmingham due to the wet bias in the reanalysis in the Southeast in summer; both of these are departures from the Wang and Angell (1999) definition of stagnation.

Fig. 1.

Box plots of daily average PM2.5 concentrations in 5°C bins in (top) Chicago and (bottom) Birmingham for the 2007–11 period. Black boxes and whiskers represent days classified as stagnant, while red boxes and whiskers represent days classified as nonstagnant. Boxes and whiskers represent the minimum, 25th percentile, median, 75th percentile, and maximum PM2.5 concentrations in each bin. Chicago PM data are from the Mayfair Pumping Station monitoring site (17-031-0052), while Birmingham PM data are from the Wylam monitoring site (01-073-2003). Temperature and other meteorological data in the stagnation determination were from the NCEP–NCAR reanalysis. Measurements from dates surrounding the 4 Jul holiday have been removed due to the PM contribution from fireworks. The 500-mb wind speed criterion was not used in Chicago for determining stagnant days, since this criterion is more relevant for multiday warm weather stagnation episodes, and the precipitation threshold was raised to 10 mm in Birmingham due to the wet bias in the reanalysis in the Southeast in summer; both of these are departures from the Wang and Angell (1999) definition of stagnation.

These initial study findings, and the above-mentioned simple analysis, thus tend to paint a complex picture of the range and types of potential impacts of climate change on PM in the United States, in part because changes in the most relevant meteorological factors for PM (temperature, precipitation, and mixing) will often have competing impacts—and these impacts and interactions are difficult to diagnose by focusing on longer-term monthly, seasonal, and annual averages or by grouping various regions or PM species together. For example, Pye et al. (2009) simulated large seasonal and regional effects (on the order of several μg m−3) that mostly negated one another when averaged over the entire year and summed to account for total PM. Nevertheless, some common general conclusions emerged from these initial studies:

  • Very broadly, for sulfate, these earlier modeling studies consistently found that simulated multidecadal climate change led to concentration increases in the U.S. Northeast and decreases in the Southeast, assuming no changes to SO2 emissions, with less agreement for other regions. Climate-induced increases in sulfate could most often be associated with changes in oxidation due to warmer temperatures, whereas climate-induced decreases resulted from increases in wet deposition due to increases in precipitation.

  • By contrast, simulated climate change resulted in decreases in annual average nitrate concentrations over most of the country due to the effect of higher temperatures on nitrate partitioning, though precipitation and transport added significant second-order complexity to this simple temperature–nitrate relationship.

  • For carbonaceous aerosols, changes in temperature- driven partitioning, biogenic emissions, wet deposition, and synoptic-scale cyclones were all important.

As a whole, the studies summarized by Jacob and Winner (2009) suggested that climate-change-induced differences in model-simulated annual average total PM concentrations between the present day and the 2050s would be less than 1.0 μg m−3 in average PM concentration as a result of these competing changes in individual aerosol species. As indicated earlier, however, average values may not be a good metric for evaluating policy relevant impacts of climate change on PM, and episode analysis may be better. Additionally, these conclusions are subject to major gaps in our understanding of several critical factors with the potential to overwhelm these simulated changes. In particular, the influence of climate change on synoptic- and event-scale mixing and precipitation, the impacts of temperature changes on partitioning of primary and secondary organic aerosols, and the links between climate change and PM emissions from wildfires and dust events are not yet well captured in such studies. These gaps are also heavily intertwined with complicated anthropogenic factors related to development or farming, such as land use and land cover, and emissions of both greenhouse gases and traditional pollutants. We discuss these gaps and put forward an integrated climate and PM research agenda to address them.

RESEARCH OPPORTUNITIES FOR UNDERSTANDING CLIMATE IMPACTS ON PM.

The research summarized above suggests that there are several understudied links between climate and aerosol research that, if pursued, could significantly increase our understanding of the implications of climate change for PM in the United States, portrayed schematically in Fig. 2. Major elements include the meteorology of pollution episodes; natural emissions from wildfires, vegetation, and dust events; and organic aerosol modeling. These elements are heavily intertwined, and thus understanding the linkages between them and their links to anthropogenic emissions and human activities is also critical. For example, the meteorology of air pollution episodes is related to the meteorology that is most conducive to wildfires. Similarly, the emissions of biogenic VOCs are strongly related to meteorology (Guenther et al. 2006). Here we summarize current understanding in these areas to suggest a set of high-priority foci for future climate and PM research.

Fig. 2.

Conceptual model of the climate change impact on the PM problem. Several of the arrows could point in both directions to signify feedback; however, this figure is intended only to illustrate the impact of the changing climate on PM.

Fig. 2.

Conceptual model of the climate change impact on the PM problem. Several of the arrows could point in both directions to signify feedback; however, this figure is intended only to illustrate the impact of the changing climate on PM.

Meteorological drivers of PM episodes.

Recommendation 1: Understand the links among climate change, synoptic phenomena, local stagnation, and frequency of precipitation. Figures 1 and 2 suggest that studies of the impacts of a changing climate on PM episodes should consider changes in both winter and summer stagnation, in particular, on a regional basis. Such stagnation events, in turn, result from distinct synoptic-scale conditions that have strong, but as yet uncertain, links to climate change. While they did not simulate air quality, Bengtsson et al. (2006) projected decreased frequency of storm tracks under a late twenty-first-century A1B climate as compared to a twentieth-century climate in the upper Midwest in winter and in both the Midwest and Northeast in summer. The analyses by Lambert and Fyfe (2006) and Pinto et al. (2007) also showed decreases in cyclone frequency over the Northern Hemisphere in general, while the latter study also suggested that this is true over North America in particular. In their analysis of the CMIP5 GCMs, Chang et al. (2012) showed that these models generally predict a decrease in cyclone frequency over North America in winter and over much of the continent in summer, and that CMIP5 models show a stronger decrease in cyclone frequency than CMIP3 models.

Indeed, climate-induced changes in synoptic-scale weather patterns, such as midlatitude cyclones, frontal passages, and location and frequency of high pressure systems, have been suggested as major drivers of future changes in PM episodes (see, e.g., Leung and Gustafson 2005). The interannual variability in the frequency of cyclones has been shown by Tai et al. (2012) to be a strong driver of the interannual variability in PM2.5 concentrations in the Midwest. In their study, regional annual average PM2.5 concentrations were compared to the average amount of time between cyclones in a given year; years with less frequent cyclones had higher annual average PM2.5 concentrations, which has implications for the concentrations during and frequency of individual episodes. In their study focused on ozone, Zhu and Liang (2013) showed a strong link between the Bermuda high and the pattern of ozone concentrations over the eastern half of the United States. These studies would suggest that changes in synoptic phenomena could have major implications for pollution episodes.

However, the potential for changes in short-term and episodic PM concentrations as a result of climate change has only been considered by a small number of studies. Though few, these results suggest the potential for appreciable changes in short-term PM concentrations and episodes of PM under a future climate, as a result of the potential for climate change to impact synoptic meteorology. For example, Mickley et al. (2004) performed a modeling study in which an inert black carbon (BC) tracer was used to represent PM in a global-scale simulation of the time period from 1950 to 2052 in the Goddard Institute for Space Studies (GISS) GCM II. Emissions of BC were held at present-day levels throughout the simulation so that the effects of meteorologically driven transport could be isolated. The authors highlighted the importance of changes in pollution episodes, rather than seasonal mean concentrations, which changed little between the present and the 2050s. The simulation results showed an increase in the severity of summertime pollution episodes in the Northeast and Midwest by 2050, which occurred despite simulated increases in mixing depth over these regions, due to a decreasing trend in cyclones. As fewer cold fronts with clean air traveled across the Midwest and Northeast, episode severity increased accordingly.

In addition, the number of simulated PM episodes increased considerably from the present day to the 2050s in the study of January and July PM concentrations by Dawson et al. (2009), who held emissions at present-day levels and compared air quality under simulated present-day and future meteorology. The average area experiencing a 24-h-average PM2.5 concentration greater than 35 μg m−3 on any day in a given July increased by a factor of 6.4, indicating a major increase in episode extent, with the largest increases occurring in the Midwest and Ohio River valley, where sulfate is the dominant summertime PM component. These changes in simulated short-term PM concentrations also reflected an increase in stagnation over the Midwest. Additionally, in some areas, such as the Southeast, simulated PM concentrations increased despite increases in precipitation, indicating that other factors outweighed precipitation changes (or, alternatively, that total precipitation may not be the best metric for assessing potential impacts on PM and that other metrics, such as the number of days with precipitation, might be more appropriate). Changes in episodes under a 2050s “business as usual” scenario in California were examined by Mahmud et al. (2012); their simulations indicated that extreme events would be exacerbated in the future in the Central Valley, though changes in extremes in the Los Angeles area and changes in annual average concentrations were small.

Furthermore, we have Leibensperger et al. (2008), whose conclusions, while focused on O3, are likely applicable to PM as well. This study showed that the frequency of midlatitude cyclones has been decreasing across the Midwest and Northeast, thereby increasing the number of stagnation days each year, and the authors concluded that this has in large part countered the benefits of decreasing emissions of O3 precursors. Similarly, slowly migrating anticyclones and stagnating high pressure conditions, such as those associated with the Bermuda high, have been strongly linked with O3 episodes in the eastern United States (Comrie and Yarnal 1992), and evidence suggests that the Bermuda high is strengthening due to anthropogenic climate change (Li et al. 2011). If these links between stagnant conditions and O3 are considered in light of the observation by Tai et al. (2010) that concentrations of PM2.5 in the United States are on average 2.6 μg m−3 (compared to a 24-h PM2.5 air quality standard of 35 μg m−3) higher on stagnant days versus nonstagnant days, then the implication is that anticipated future changes in stagnation and synoptic-scale meteorology could have a large impact on episodic PM concentrations in the United States. Additionally, given the relationships between weather types, air pollution episodes, and associated health impacts (Hanna et al. 2011; Winner and Cass 2001), it would likely follow that changes in synoptic-scale meteorology and weather type would affect air pollution episodes and pollution-related health impacts.

Finally, while precipitation has frequently been examined in studies of the changing climate and its impacts on pollution concentrations, explorations of future climate scenarios' impacts on PM have generally reported changes in total precipitation in units such as millimeters per year (mm yr−1), rather than as a frequency. Changes in how often precipitation occurs, however, is likely to be an important driver of changes in PM episodes (Dawson et al. 2009). In addition, the frequency of precipitation is linked to wildfires, which in turn are a potentially important driver of climate-change-related changes in PM, especially PM episodes, as discussed in more detail in the “Emissions” section below.

Addressing this recommendation will also require addressing several methodological challenges presented by the modeling required to support such investigations. For example, Tai et al. (2012) showed that even the same general circulation model can predict quite different changes in cyclone frequency for different realizations of a given future emissions scenario, which would suggest that one realization of a particular scenario may not be adequate for estimating changes in cyclones and stagnation. Similarly, Manders et al. (2012) showed that different GCMs simulating the same future scenario can produce different impacts on regional O3 and PM.

The modeling of precipitation, even for the present day, is a difficult undertaking. And the downscaling of GCM output to the regional scale requires a careful consideration of technical issues, such as meteorological and chemical boundary conditions, and nudging approaches. However, progress (e.g., Bowden et al. 2012) has been made recently in determining how best to link these scales to capture the global-scale dynamics of the GCM (or reanalysis) while still making use of the finer spatial scale of the regional model.

Emissions.

Wildfires

Recommendation 2: Refine estimates of climate-change-related wildfire activity changes and their impacts on PM and PM precursor emissions. In addition to synoptic-scale meteorology, wildfires are also a major contributor to PM episodes, particularly carbonaceous aerosol concentrations in the western United States during summer (Park et al. 2003). As was summarized in the review by Keywood et al. (2013), the area burned by wildfires in North America is expected to increase dramatically over the twenty-first century, primarily due to warmer temperatures and precipitation changes. The changing climate has already led to higher large-wildfire frequency, longer wildfire durations, and longer wildfire seasons in the western United States (Westerling et al. 2006). The consequence for this on area burned could be dramatic; for example, Flannigan et al. (2005) estimated a doubling (from +74% to +118%) of area burned in Canada under a 3 × CO2 climate.

The links between the changing climate and changing PM emissions from wildfires show a rather consistent increase in wildfire-related PM under a changed climate for seasonal or annual averages of PM. For example, Spracklen et al. (2007) estimated that changes in wildfires have caused a 30% increase in summertime organic carbon aerosol concentrations in the western United States over the last 30 years. In subsequent work, Spracklen et al. (2009) simulated increases in May–October average concentrations of organic carbon aerosols of 40% and elemental carbon aerosol concentrations of 20% over the western United States due to changes in wildfires in a changing climate [between 2000 and the 2050s, using the Inter-governmental Panel on Climate Change (IPCC) A1B scenario]. Yue et al. (2013) suggest that the effect of wildfires will be most consequential for PM episodes, with a smaller effect on longer-term average concentrations. However, only a small number of studies have considered the changes in PM concentrations and PM episodes (frequency, severity, and duration) that might result from climate-induced changes in wildfires. Research on better quantifying the emissions and subsequent impacts on ambient PM concentrations from changing wildfires is necessary to improve adaptation planning for air quality. Such research would likely build on the foundational work linking wildfires to specific meteorological phenomena, such as the work of Lafon and Quiring (2012), who most strongly related wildfire activity and area burned to daily variability of precipitation. This research would suggest that the changing frequency of precipitation (discussed in the “Meteorological drivers of PM episodes” section), not just the changing amount of precipitation, could affect PM concentrations via wildfires. Another example is the research of Hessl et al. (2004), who linked wildfires in the Pacific Northwest to the Pacific decadal oscillation and the drought severity index, which shows the varying temporal scales of important meteorological drivers of fires.

Biogenic VOC emissions.

Recommendation 3: Better quantify how changing climatic conditions and CO2 concentrations will affect emissions of the biogenic VOC species that are PM precursors. Also, incorporate recent advances in the understanding of the chemistry of biogenic VOCs into studies of how the changing climate will affect PM concentrations. One of the ways climate change is expected to impact O3 concentrations is through changes in biogenic VOC emissions, especially increased emissions of the O3 precursor isoprene (Weaver et al. 2009). While isoprene has been thought to be a relatively minor precursor of PM, recent advances in the understanding of the oxidation of isoprene (and its oxidation products) in the aqueous phase (Ervens et al. 2008), suggest that the role of isoprene in forming organic aerosols may generally be underestimated in chemical transport models. In addition to isoprene, other biogenic VOCs are also important PM precursors. For example, monoterpenes and sesquiterpenes can be oxidized to form organic aerosols. How climate change will impact emissions of biogenic VOCs, such as α-pinene and β-pinene, which are also PM precursors, has not been well studied. Similarly, recent research (Horváth et al. 2012) has suggested that soils may be a source of terpene emissions, though this generally has not been taken into account in chemical transport models.

A link between increased temperature and increased biogenic VOC emissions has been included in representations such as the Model of Emissions of Gases and Aerosols from Nature (MEGAN; Guenther et al. 2006) and used in many studies to estimate increases in biogenic emissions resulting from climate change. However, substantial uncertainties remain. For example, isoprene emissions are affected in very complex ways by ambient CO2 concentrations (Rosenstiel et al. 2003; Monson et al. 2007; Possell and Hewitt, 2011; Sun et al. 2012), creating an unclear net effect of elevated CO2 on isoprene emissions. One recent study (Pacifico et al. 2012) of these competing effects suggests that the temperature-driven increase and CO2-driven suppression of isoprene emissions may essentially negate one another. The ambient CO2 concentration may also affect the sensitivity of isoprene emissions to temperature (Way et al. 2011). However, the effect on the changing climate on nonisoprene biogenic VOCs, which are thought to form aerosols more readily than does isoprene, has not been a major focus of research to date.

In addition, land cover will also change in the coming decades, driven by both climatic and human factors, but in ways that may be hard to anticipate; some plantation tree species, such as poplar, are high isoprene emitters (Wiedinmyer et al. 2006), so increases in their production may lead to increased biogenic VOC emissions that should be accounted for in studies of climate-related impacts on air quality (e.g., see Avise et al. 2009). Similarly, Berg et al. (2013) showed that invasive species, such as bark beetles, which are affected by climate change, can, in turn, affect biogenic emissions and the aerosols that form from them. As the flux and spatial pattern of biogenic emissions change, there will be effects on the concentrations of organic aerosol concentrations.

In the few modeling studies to date to address some of these questions, future changes in biogenic emissions, induced by both the changing climate and the changing land use and land cover, had significant impacts on biogenic aerosol concentrations (Heald et al. 2008; Chen et al. 2009; Lam et al. 2011). For example, Wu et al. (2012) projected that climate- and CO2-driven changes in land cover would result in a 10% increase in global secondary organic aerosol (SOA) burden between 2000 and 2050, and a 20% increase in SOA burden between 2000 and 2100 (following the IPCC A1B scenario), including increases in SOA concentration of several tenths of a microgram per cubic meter (μg m−3) over much of the southwestern and northeastern United States. However, these studies do not yet paint a consistent picture of the magnitude of this impact. Additionally, recent advances in isoprene modeling, such as improved aqueous chemistry treatments, have not yet been incorporated into these studies of the impacts of climate change on organic PM.

Drought and dust.

Recommendation 4: Estimate the effects of evolving precipitation patterns, especially changes in droughts, on the emissions and transport of the dust component of PM. Dust is an important constituent of PM, especially in the coarse fraction between 2.5 and 10 μm. Dust from Asia (Duce et al. 1980; Prospero 1979) and Africa (Prospero et al. 1970) can be transported over very long distances to the United States, though only a small fraction of the PM in the United States is attributed to long-range transport from other continents. However, the Task Force on Hemispheric Transport of Air Pollution mentioned changing source-to-receptor relationships for long-range transport as one aspect of climate change that needs further study (TF HTAP 2011). For example, the transport of African dust to southern Europe appears to be linked to synoptic-scale meteorological phenomena such as the North Atlantic Oscillation (NAO) (Cusack et al. 2012; Pey et al. 2012), so changes in the NAO could result in changing dust transport. Similarly, domestically generated dust is also related to meteorology; for example, Okin and Reheis (2002) related the strength of the ENSO anomaly to dust events in the southwestern United States. Given the expected changes in droughts over the coming decades, there could potentially be an appreciable impact on dust concentrations; for example, the transition to a more arid climate in the U.S. Southwest has been rather well established (Seager et al. 2007), though the consequences for airborne dust have not been quantified. Numerous basic scientific questions surround all of these potential pathways for altered dust contributions to PM concentrations in an altered climate.

Modeling of organic aerosol processing.

Recommendation 5: Incorporate recent advances in the modeling of organic aerosols, including those formed from biogenic VOC emissions, into studies of the effects of the changing climate on these aerosols. Changing biogenic VOCs emissions are linked to another uncertain aspect of the impacts of climate change on PM: atmospheric processing of organic aerosols. Similarly, advances in the modeling of aqueous chemistry (Carlton et al. 2008) suggest that there is an important link between cloud water and organic aerosol production, though the question of how climate change will affect this production pathway remains an open one. While recent advances in the understanding of the oxidation of organic aerosol precursors and the partitioning of organic aerosols between the condensed and vapor phases (Robinson et al. 2007; Jimenez et al. 2009) have led to more complete modeling of how temperature affects organic aerosols, these improvements have not fully been incorporated into studies of the impacts of climate change. The study by Day and Pandis (2011), which included the organic aerosol model improvements by Murphy and Pandis (2009), represents an important step in incorporating these developments into studies of climate change impacts on PM; Day and Pandis (2011) compared the effects of changing organic aerosol partitioning at higher temperatures and increased biogenic emissions (see also the “Biogenic VOC emissions” section) and found that increased aerosol concentrations due to increased biogenic emissions could greatly outweigh partitioning effects. While this study suggests that advances in the modeling of aerosol partitioning are less consequential for understanding climatic impacts than are advances in the understanding of biogenic emissions, this cannot yet be stated with a high degree of confidence.

CONCLUSIONS.

Research to date on the impacts of climate change on policy relevant to concentrations of ambient PM air pollution suggest that there are several critical but understudied links between climate and aerosol research that, if pursued, could significantly increase our understanding of the implications of climate change for PM in the United States. While emissions of aerosols and precursors will likely remain the biggest determinant of ambient PM concentrations, climatic changes can have important impacts on PM. Changes in land use and land cover, from farming or development, for example, will affect emissions important to both ambient PM concentrations and to climate change. The initial studies of climate change and PM examined here have largely excluded some of the key processes that could result in large climate-induced PM changes, including changing emissions from wildfires and dust, and atmospheric processing of organic aerosol precursors. In addition, original analysis presented here indicates that regional and seasonal consideration of meteorological episodes, rather than simply shifts in mean climate, is critically important for understanding climate change impacts on PM. The few studies that have examined how climate change is expected to impact PM pollution episodes and short-term concentrations of PM have provided preliminary evidence that climate change may exacerbate high PM episodes, but the meteorological variables to which these studies point as driving changes in pollution episodes require more attention in modeling studies of future climate. Potentially important aspects of this issue include stagnation, synoptic-scale meteorology, weather-type classification, and precipitation frequency.

In this paper we have summarized the current understanding in these areas to suggest a set of high-priority foci for future climate and PM research. These science needs are encapsulated in the following set of recommendations:

  • Recommendation 1: Understand the links among climate change, synoptic phenomena, local stagnation, and frequency of precipitation.

  • Recommendation 2: Refine estimates of climate-change-related wildfire activity changes and their impacts on PM and PM precursor emissions.

  • Recommendation 3: Better quantify how changing climatic conditions and CO2 concentrations will affect emissions of the biogenic VOC species that are PM precursors. Also, incorporate recent advances in the understanding of the chemistry of biogenic VOCs into studies of how the changing climate will affect PM concentrations.

  • Recommendation 4: Estimate the effects of evolving precipitation patterns, especially changes in droughts, on the emissions and transport of the dust component of PM.

  • Recommendation 5: Incorporate recent advances in the modeling of organic aerosols into studies of the effects of the changing climate on these aerosols.

We propose this set of recommendations as a research framework for organizing future investments in climate change–PM science, to build fundamental understanding of critical Earth system processes in an area of first-order societal importance.

ACKNOWLEDGMENTS

The authors thank Chris Nolte for his assistance with reanalysis data and comments on a previous draft, as well as Marcus Sarofim for his comments on a previous draft. The views expressed in this paper are those of the authors and do not necessarily represent the views or policies of the U.S. Environmental Protection Agency.

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