Volume 58(1) - January 2009

The carbonaceous aerosol—a remaining challenge

by Karl Espen Yttri*, Cathrine Lund Myhre* and Kjetiltørseth*

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Introduction

car pollutionThe ambient aerosol level remains a major challenge in atmospheric science due to its ability to cause negative health effects and its ability to influence the radiative balance and, thus, the Earth’s surface temperature. Our knowledge of the mechanisms by which the effects can be explained, however, is still a matter of ongoing research. Moreover, our understanding of the atmospheric sources and sinks and of the physical and chemical properties of the aerosol is still incomplete.

(V. Torres Molinero)

A large part of our shortcomings in this area can be attributed to the carbonaceous fraction of the aerosol, despite having received substantial scientific attention during the last 15-20 years. This could be explained partly by the large number of species involved in the formation and transformation of the carbonaceous aerosol, and by the fact that current analytical capabilities are insufficient for complete qualitative and quantitative characterization. Also, the emissions to the atmosphere of primary carbonaceous particles and gas precursors of secondary carbonaceous aerosols are poorly known.

To improve this situation, increased knowledge of nearly every aspect of the carbonaceous aerosol is needed. This article briefly highlights central effects of the carbonaceous aerosol on health and climate and addresses some of the knowledge gaps related to future projections. It also addresses the need for further development of monitoring activities to reduce these knowledge gaps.

The carbonaceous aerosol and health effects

On a worldwide basis, the annual number of premature deaths caused by cardiovascular and pulmonary diseases following ambient air particulate matter (PM) exposure is estimated to be substantial at 800 000 (World Health Organization (WHO), 2002). Despite growing evidence that certain sources of particulate matter are more strongly related to negative health effects than others (Hoek et al., 2002; Laden et al., 2000), WHO still recommends the use of only one risk factor when assessing the health impacts of ambient particulate matter exposure. Thus, any major contributor to ambient particulate matter, such as the carbonaceous fraction, constituting 20-70 per cent of the mass concentration, is of major concern.

Recently, epidemiological studies have demonstrated a statistical association between the carbonaceous aerosol and cardiovascular emergency department visits. With emerging evidence of effects which can be directly associated with the carbonaceous fraction, the ability to assess exposure and effect to larger populations will improve.

The carbonaceous aerosol contains a large number of organic species, but the majority remains yet to be identified. However, the presence of well-known toxics, such as oxy- and nitro-polycyclic aromatic hydrocarbons and polychlorinated dibenzodioxins/furans have been reported. Nevertheless, the scientific community is still grappling with what causes the ambient aerosol toxicity.

In a recent study, McDonald et al. (2004) were able to pinpoint certain particulate organic species (hopanes and steranes) when addressing the lung toxicity of diesel and gasoline exhaust samples. This finding provides valuable insight into which sources and constituents of the complex carbonaceous aerosol are responsible for the lung toxicity of inhaled particles. Further, it supports the epidemiological studies pointing towards vehicular traffic as an important source of air pollution leading to premature mortality (Hoek et al., 2002; Laden et al., 2000; Metzger et al., 2004). Finally, it strengthens the general advice given by WHO that combustion-derived primary particles are particularly important as they “are often rich in transition metals and organic compounds, and also have a relatively high surface area”. As the international community prepares to enter a regime where renewable fuels will play a more important role, it should be kept in mind that WHO does not distinguish between the effects caused by particles from combustion of fossil fuel and those of biomass combustion (WHO, 2005).

The carbonaceous aerosol and climate effects

When studying aerosol impact on climate, the largest uncertainties by far are associated with the effects of the carbonaceous aerosol. It is also fair to argue that the carbonaceous aerosol is currently the most important with respect to aerosol effect on climate. This is mainly attributed to the black carbon part of the carbonaceous aerosol, which absorbs solar radiation in the atmosphere. According to Ramanathan and Carmichael (2008), this feature has made black carbon the second most important contributor to global warming after carbon dioxide. However, the climate effect of black carbon is uncertain and debated (Forster et al., 2007).

Elevated black carbon concentrations in areas with high solar radiation are a major contributor to the so-called brown clouds covering large regions, for instance in Asia (Ramanathan and Carmichael, 2008). Brown clouds have led to dimming of the Earth’s surface, warming of the atmosphere and perturbation of the hydrological cycle, possibly affecting the monsoon.

satellite image  

MODIS Terra satellite image including fire hot spots (red dots) over western Canada on 5 July 2004, 19:35 UTC. The grey colours indicate the smoke plumes from the boreal forest fires.
NASA http://modis.gsfc.nasa.gov

 

As pointed out in the Fourth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC) (Forster, 2007), black carbon in snow has a substantial impact on total radiative forcing of the atmosphere by absorbing more incoming sunlight. Furthermore, black carbon deposited on snow and ice could enhance glacier melt, not only in the northern high latitudes but also in non-polar regions such as the Himalayas and could subsequently cause secondary effects with respect to water supply in the densely populated regions draining the Himalayas.

These pronounced effects of black carbon on the regional and global climate and its short lifetime (1 week ± 1 week) compared to carbon dioxide, has led several to the conclusion that reducing emissions of black carbon is the most effective strategy to slow global warming (Bond, 2007; Hansen et al., 2000; Jacobson, 2002), while reduction of greenhouse gases are needed to stop the warming. Thus, it is vital to quantify and subsequently reduce uncertainties in the climate effects of black carbon to be able to work out effective and well-targeted abatement strategies.

In this respect, increased knowledge of sources and physical and optical properties of black carbon is particularly essential for the implementation of effective mitigations steps. In particular, the use of source specific tracers, radiocarbon analysis, and aerosol time-of-flight mass spectrometers has proven helpful resolving sources of black carbon.

Any attempt to reduce black carbon emissions will also benefit the goals set by WHO with respect to health, as toxicological studies (Donaldson et al., 2000) have linked adverse health effects to elemental carbon exposure. The organic carbon fraction of the carbonaceous aerosols could enhance the absorbing capacity of black carbon by a factor of 2-4 when acting as a coating (Bond et al., 2006; Fuller et al., 1999; Jacobson, 2001, Schnaiter et al., 2005). The organic carbon aerosol is also ultraviolet-absorbing due to the presence of so-called brown carbon. Furthermore, organic carbon aerosol plays a role in the cloud droplet formation, which once was thought to be affected only by the inorganic fraction of the aerosol.

Future perspectives of the carbonaceous aerosol

The need for understanding the carbonaceous aerosol will become even more important as emissions thereof from developing economies are expected to dramatically increase in the future. In addition, the relative importance of different sources that emit carbonaceous aerosols to the atmosphere could change dramatically in the future as we attempt to adapt to a carbon-neutral society, replacing fossil fuel by renewable fuels. It is also speculated that global warming might have a similar affect by increasing the formation of secondary organic aerosols, following from atmospheric oxidation of gas-phase organic precursors.

While the switch to renewables will improve the situation with respect to carbon dioxide, the impact on the carbonaceous particulate air pollution level is uncertain. According to the International Energy Agency (IEA, 2007), close to 80 per cent of renewable energy sources are combustibles, of which 97 per cent is biomass. Projections made by the Energy Information Administration (2008) show that the consumption of biomass (renewables) is likely to increase by approximately 200 per cent between 2000 and 2020.

Future emissions of carbonaceous aerosols from the expected increase in biomass consumption will be critically dependent on the technologies used to transform biomass into heat and energy. Predictions made by the International Institute for Applied Systems Analysis for the CAFÉ (Clean Air For Europe) project point towards domestic heating and, in particular, wood burning as one of the major sources contributing to future loadings of particulate matter and black carbon in Europe. For large parts of Europe, emissions from residential wood burning are poorly regulated and combustion tends to take place in small installations with old technology, which promotes emissions of carbonaceous aerosols. In addition, the turnover time for wood stoves and fireplaces is rather long, which hampers the shift to new and cleaner technology.

A relatively local urban emission problem is transformed into a regional source of oxidized and presumably hydrophilic carbonaceous aerosols. The health consequences and climate effects of this oxidized material are almost certainly dramatically different from those of primary emissions. (Robinson et al., 2007)

A number of recent studies measuring levoglucosan, a unique tracer of carbonaceous aerosols from wood burning, have shown that it is present in the urban as well as the rural environment for a wide range of sites in Europe and in quite high concentrations. These include sites that were not expected to be particularly influenced by this source. For wood smoke particles, the physical and chemical characteristics will differ with combustion conditions and combustion appliance and this may affect the toxicity of the particles. Current knowledge on this matter is, however, very limited. While the presence of levoglucosan in winter points to carbonaceous aerosols from residential wood burning, the presence of levoglucosan in samples collected in summer has been associated with impacts from wild fires and agricultural waste burning. While agricultural waste burning is banned in most western European countries, it is common practice in large parts of the world.

In recent years, there have been several examples of emissions from wild and agricultural fires severely affecting the air quality in Europe (Saarikoski et al., 2007; Yttri et al., 2007), violating particulate matter limit values and raising the carbonaceous aerosol concentration by nearly one order of magnitude in certain cases. There are also examples of how wild and agricultural fires have affected the air pollution concentrations in the Arctic (Stohl et al., 2007), and it has been argued that boreal forest fires could be the major source of black carbon in the Arctic summer in years of high fire activity (Stohl et al., 2006; Stohl et al., 2007).

Concern has already been expressed regarding the consequences of large- scale conversion from gasoline to ethanol (bio-ethanol) with respect to ozone related health consequences. E85 (85 per cent ethanol and 15 per cent gasoline) may increase ozone-related cancer, mortality and hospitalization by as much 9 per cent in a major city such as Los Angeles, compared to 100 per cent gasoline, according to calculations made by Jacobson (2007).

Concern has also been attributed to oxidation of unburned ethanol as a source of acetaldehyde, which is a human carcinogen. Combustion of biofuels will inevitably change the organic constituent composition of the carbonaceous aerosol. While biofuels typically have higher oxygen content, more oxygenated species can be expected in the emissions. This fraction of the carbonaceous aerosol is the least explored, partly because of analytical limitations, and is thus an area of further investigation. Polar oxygenated compounds are the most water soluble species and thus potentially cloud-condensation-nuclei-active.

Recent developments in analytical chemistry have provided evidence that biogenic secondary organic aerosols (BSOA) contribute substantially (60 per cent) to the organic fraction of the atmospheric carbonaceous aerosol, even in the urban environment (Szidat et al., 2006). This confirms what has long been stated: that biogenic secondary organic aerosols are one of the major missing sources of the carbonaceous aerosol. It is hypothesized that global warming causes an increase in the concentration of biogenic secondary organic aerosols, due to a rise in emissions of biogenic volatile organic gaseous compounds that subsequently oxidize to form particulate matter in the atmosphere. In addition, biogenic secondary organic aerosol formation may be further propelled by temperature- dependent reaction rates, as the atmospheric global warming increases. Arguments raised by Robinson et al. (2007) suggest that anthropogenic secondary organic aerosols might also be more abundant than previously expected, because of the oxidation of low-volatility products that evaporate from primary carbonaceous aerosol with atmospheric dilution. This suggests that the majority of the population is exposed to secondary organic aerosols, even in urban areas. As stated by Robinson et al. (2007): “A relatively local urban emission problem is transformed into a regional source of oxidized and presumably hydrophilic carbonaceous aerosols. The health consequences and climate effects of this oxidized material are almost certainly dramatically different from those of primary emissions”. Primary biological aerosol particles have typically been ignored when assessing the sources of the carbonaceous aerosol. However, a few recent studies have shown that primary biological aerosol particles may account for a substantial 30-40 per cent of the organic fraction of the carbonaceous aerosol in moderately anthropogenically influenced regions (Winiwarter et al., 2008(a); Winiwarter et al., 2008(b); Yttri et al., 2007). Selected primary biological aerosol particles may be active as both cloud condensation nuclei and heterogeneous ice nuclei and thus can contribute to cloud formation. The heterogenic nature of this source makes it difficult to predict how it will respond to climate change.

Attempting to reduce global warming by reducing black carbon emissions requires targeting all major sources but, in particular, in regions of special concern; i.e. where emissions of black carbon have a strong climate effect. Examples are the growing economies in Asia such as those of China and India, which together account for 25-35 per cent of the world’s total black carbon emissions (Ramanathan and Carmichael, 2008). Another is northern Eurasia in winter and spring, which is the major source region of the Arctic lower troposphere (Barrie et al., 1986; Sharma et al., 2006; Stohl et al., 2006). Emissions within the Arctic itself should be reduced to a minimum, as they have a disproportionately large effect. This could prove difficult, as various anthropogenic activities are likely to increase as the sea ice retreats. An opening of the North-west Passage would probably increase shipping activity, as would further oil and gas exploration, as currently seen in the Barents Sea.

Another major challenge could be boreal forest fires in Siberia (Russian Federation), Canada and Alaska (USA), which are beyond human control. An increase in the frequency of forest fires has been postulated as one of the consequences of global warming and this could further escalate the melting of sea ice and snow in the Arctic. During the severe air pollution event that affected the European Arctic in spring 2006—which was caused by agricultural fires in eastern Europe—Stohl et al. (2007) nicely demonstrated how the disproportionate warming of the Arctic recruited new areas in the mid-latitudes as source regions of Arctic air pollution. This event could serve as an early warning of what could happen more frequently in the future if the Arctic warms more rapidly than the mid-latitudes. It also shows that the practice of agricultural waste burning should be banned.

Crop residues are a carbon-dioxide- neutral energy reserve that could add a valuable supplement to the total energy consumption; open field burning is thus a waste of resources. With the world population growing by 1 per cent per year during the period 2005-2030 (IEA, 2008) a proportional increase in food production and thus crop residues should follow, which could further enhance the problem of emissions from agricultural waste burning. For Ukraine, which has the highest European values for energy-crop potential it has been suggested that the wheat yields have the potential to double (FAO, 2003; Ericsson and Nilsson, 2006; Sciare et al., 2008). Thus, this is a non-negligible future source of carbonaceous aerosols.

car pollution   In a recent paper in Nature, Robinson et al. (2007) suggest that anthropogenic secondary organic aerosols might be more abundant than previously expected, because of oxidation of low volatility products that evaporate from primary carbonaceous aerosol with atmospheric dilution. This suggests that the majority of the population is exposed to secondary organic aerosols, even in urban areas.

During the last decades in Europe and North America, the anthropogenic emissions of ammonia, nitrogen oxides and non-methane hydrocarbons have been stabilized and those of sulphur dioxide have been significantly reduced. This has led to a relative increase in the importance of carbonaceous versus inorganic aerosol species. A further increase of carbonaceous substances, be it from the use of fossil or biofuels or from more frequent boreal forest fires, will increase the importance of mitigating their sources in the years to come.

How could monitoring networks meet the challenge of carbonaceous aerosols from a multitude of sources?

Long-term monitoring (> 10 years) of the carbonaceous aerosol is typically not available, although with a few exceptions (Scharma et al., 2006). This is partly due to the lack of a standardized approach of how sampling and subsequent chemical analysis should be performed. Substantial artifacts can be introduced during sampling of the carbonaceous aerosol, which can both grossly over- and underestimate its organic fraction and great analytical challenges are associated with splitting the organic fraction and the elemental carbon/black carbon fraction (McDow and Huntzicker, 1990; Schmid et al., 2001). Thus, data from various monitoring networks are hardly comparable. In Europe, effort is now being made to create a unified protocol for how to sample and chemically analyse carbonaceous aerosols in the rural environment for the European Monitoring and Evaluation of the Long-range Transmission of Air Pollutants in Europe/WMO Global Atmosphere Watch joint supersites through the European Supersites for Atmospheric Aerosol Research project.

In 2008, the level of sophistication needed to allocate various sources contributing to the ambient carbonaceous aerosol concentration is not met by any air-quality monitoring network, at least not on a continuous basis. To do so, component speciation must be widened and more sophisticated on- and offline instruments must be taken into service. Obviously, such requirements are not in line with easy-to-operate, low-cost instrumentation, but should rather be aimed at selected supersites within a network.

Alternatively, dedicated campaigns could be conducted. This approach has the strong advantage that it combines the efforts made by research groups with those conducted by national agencies. A recent example is the intensive campaigns undertaken by the European Monitoring and Evaluation Programme, of which some are also co-located with the campaigns of the European Integrated Project on Aerosol Cloud Climate Air Quality Interactions project. Here, specific measurements are being made during autumn 2008 and winter/spring 2009, in order to allocate sources of carbonaceous aerosols. Similarly, there are efforts providing long-term data also in North America. Unfortunately, the global coverage of sites measuring carbonaceous aerosols is very limited. In particular, the equatorial regions, Asia and the boreal regions are under-sampled. Typically, limitations originate in a lack of domestic competence, finances and infrastructures, but an increasing number of funding opportunities for capacity transfer might improve the situation in the years to come.

Similarly, the analytical capabilities have strongly improved during the last few years. One important improvement has been the implementation of various tracers, such as 14C, levoglucosan, cellulose, sugars and sugar alcohols, in source apportionment studies. Continued use of such tracers but also aerosol time-of-flight instruments will inevitably improve our understanding of the carbonaceous aerosol. Aerosol phase measurements should be backed up by simultaneous measurements of the likely gas-phase precursors to the carbonaceous aerosol, including biogenic volatile organic compounds, anthropogenically emitted volatile organic compounds, their degradation products and compounds such as formaldehyde and glyoxal (Simpson et al., 2007). For the latter components, even space-borne capacities exist, allowing the provision of regional concentration patterns using a single instrument.

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* Research scientist, Norwegian Institute for Air Research

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