An illuminating look into black carbon

Mar 01 2012
VERTIC Blog >> Environment

Hugh Chalmers, London

Short-lived pollutants such as black carbon do not typically enjoy the same attention given to greenhouse gases in multilateral climate change negotiations, despite making a significant contribution to global warming. But recent news suggests that when nations focus on these pollutants, they can agree to powerful mitigation measures in a relatively short time. With financial support from a small number of nations, the UN Environment Programme (UNEP) will soon implement a programme aimed at tackling these pollutants at their source; the inefficient burning of fuels. According to UNEP, if implemented widely enough this programme alone could halve the global temperature rise projected for 2050. Despite this potential, all financial support for this ‘second front in the fight against global warming’ has come from outside the dominant multilateral climate change negotiating forum. Is there a way to monitor the global levels of black carbon, and if there is how might it widen and improve the support for this new front?

A burning question
Black carbon (BC) is a product of the incomplete burning of fossil fuels, wood, and other biomass. Given enough air these fuels can combust completely, converting all the stored carbon into carbon dioxide (CO2). In practice there is never enough air present and combustion converts the stored carbon into a mix of other products such as carbon monoxide, organic carbons and black carbon. This mix of carbon products is what we know as soot. As its name suggests, BC gives the soot its dark colour. This light-absorbing property is also what makes it so damaging to the environment.

As BC absorbs sunlight it heats up, warming itself and its immediate environment. On entering our atmosphere, BC warming can influence cloud formation and disrupt rain patterns. After a short period of time BC mixes with other particles and becomes soluble in water, allowing it to fall as rain where it can become embedded in snow and ice. Here BC warming dramatically accelerates the rate at which ice melts, further influencing the global climate. It is now estimated that over 100 years, one gramme of BC has between 100 to 2,000 times the warming potential of one gramme of C02.

While the damage that BC can do to the environment is abundantly clear, it is not so obvious exactly how much black carbon exists in the global environment at any one time. Building such a global inventory for BC is not very easy. But doing so could galvanise more nations into taking action against BC and allow these efforts to be targeted at the areas of greatest need.

From the bottom, up….
Most contemporary efforts to develop such an inventory focus on the rate at which BC is emitted, rather than directly measuring existing quantities. This is not a simple process. Different forms of imperfect combustion create different quantities of carbon products, so estimates must be made as to the fraction of fuel that is converted to BC. One must also have accurate information as to the rate at which each type of combustion occurs throughout the globe. Access to such information is limited, particularly from developing states, who are thought to account for over three-quarters of BC emissions. Even among the developed states within the Arctic Council, accurate information is sometimes hard to come by, and considerable uncertainty remains in their emissions estimates.

One attempt at a global inventory of BC based on 1996 fuel-use data demonstrates just how severe these uncertainties can be. Although they estimate that approximately eight tonnes of BC are emitted per year, they acknowledge that the actual figure could lie anywhere between 4.3 and 22 tonnes per year. Such ‘bottom-up’ approaches of monitoring can produce a highly-detailed picture of how and where BC is emitted. However, they are extremely reliant on sparse fuel consumption information and do not tell us much about where the BC goes and how long it stays there. To get a better picture of the global stocks of BC, the researchers of the above inventory agree that more direct ‘top-down’ methods must be included.

…or from the top, down?
At a local level, there are a number of monitoring networks which sample the air and analyse collected particles to determine levels of atmospheric BC. In the mid-latitudes of the Northern Hemisphere sampling networks such as the UK Black Carbon Network provide good estimates as to local levels of black carbon. The majority of these samplers operate by collecting particulate matter from the air in filters. The level of light absorbed by these particles is then measured and an estimate is made as to how much absorption can be attributed to BC.

If these local measurements of atmospheric BC are compared to local data on emissions, it is possible to draw limited conclusions as to the distribution and transport of BC in the atmosphere. However, these networks are not sufficiently dispersed to give a good idea as to the global distribution of BC. Emissions from North America and Europe are frequently transported up to the Arctic, where the effects of deposited BC are severe and the monitoring networks few in number.

According to the Arctic Council’s Arctic Mapping and Assessments Programme (AMAP), there have been only a few long-term observations of BC in the Arctic which focused only on the Western Arctic. Employing similar sampling techniques to those described above, these observations unfortunately give neither a full picture of BC levels in the Arctic nor an idea of long-term trends. Monitoring the level of BC deposited in snow and ice faces similar challenges. According to AMAP, even fewer attempts have been made to monitor deposited BC. In all cases, these observations came to their conclusions by using similar filter-based detection techniques on melted samples of collected snow. Lacking a suitably broad range of samples, AMAP has argued that ‘it is a challenge to generalize findings from these campaigns.’

The highest ground
When one considers the inherent challenges of collecting numerous samples from the hazardous and changeable environment of the Arctic, this lack of a suitable sample base becomes understandable. One way around this problem is to find a way to measure the level of deposited BC remotely. Scientists around the globe have already turned to satellite technology to monitor global levels of CO2, could this ultimate form of ‘top-down’ monitoring be used to create a global inventory of BC?

The Japanese GOSAT satellite has been orbiting Earth from pole to pole since 2009 and has been able to build up a detailed and long-term picture of global CO2 distributions. Within a year of its launch the satellite was producing detailed CO2 maps, identifying individual hotspots in previously unsuspected locations. Similar satellites aimed at detecting atmospheric BC are either fully operational or in the development stage. Both the European Space Agency’s ENVISAT satellite and the North American Space Agency’s (NASA) TERRA satellite are capable of identifying the weak infrared signature produced by warming atmospheric BC. Had its launch not failed, NASA’s Glory satellite would also be monitoring atmospheric BC by observing the size, quantity and shapes of atmospheric aerosols.

Importantly, there are also suggestions that these satellites can be used to identify warming BC embedded in snow and ice. Scientists from the Norwegian Space Centre’s PRODEX project have been testing the usefulness of satellite information against physical samples of BC in the Arctic. Although their conclusions are tentative, they show a level of promise for the satellite-based detection of BC.

The scientists tested satellite-based BC monitoring by identifying two sites in the Arctic island of Svalbard which would normally have similar local climates and BC levels. However, one of the sites was carefully chosen as being close to the coal mining settlement of Barentsburg, where coal dusts and vehicle exhausts raise the BC level in the surrounding area. As these sites were imaged by the TERRA and ENVISAT satellites, a clear difference in the infrared signals from both sites emerged over the course of a year. Although their study does not take into account numerous other anthropogenic influences on the satellite signal, the differences were found to be consistent with the change in sampled BC levels at both sites.

Monitoring-induced vertigo?
Unfortunately neither satellite-based monitoring techniques are sufficiently advanced to give an accurate picture of global BC distributions. The PRODEX programme may well be on to something, but their work is only established at a conceptual level. And while satellite-based monitoring of atmospheric BC has come a long way, it alone cannot derive the amount of BC detected. A global inventory of BC must be developed from a combination of all the techniques described above. But as all these techniques have their strengths and their weaknesses, which are the best suited?

It is important to remember that the development of a global inventory of BC is a means to an end, namely the effective mitigation of BC warming. Perhaps then the answer can be found from within the UNEP programme itself. According to their assessment of the impact of black carbon, the implementation of their supported mitigation measures ‘would be most effective if it were country- and region-specific’. If there is indeed a causal link between the methods used to monitor an issue and the manner in which the issue is addressed, then the global monitoring of BC levels would best serve its purpose if it were carried out in a similarly tailored manner. In regions such as the Arctic where the physical collection of samples is challenging and the available fuel combustion information is incomplete, satellite-based BC monitoring may soon prove extremely useful.


Last changed: Mar 01 2012 at 10:43 PM




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