If proof were ever needed that scientists rarely ‘switch off’, the following post is inspired by a lunchtime treat called Topfenbällchen, which I had after my presentation at the recent EGU General Assembly. A few of us remarked about how the donut-like dessert resembled an aerosol particle with a ‘coating’ but the question is what does this actually mean and what impact does this have on our atmosphere?
Aerosol particles are often referred to as separate species in a way that suggests they can be treated independently from each other when we assess their impact and/or how best to mitigate them. In the real atmosphere though, this is very rarely the case. Focusing on the image above, we see the jam centre of the Topfenbällchen with the dough surrounding this jam ‘core’. It turns out that this is often the structure that we observe for real aerosol particles in the atmosphere. One particular example of this is black carbon (or soot), which tends to take on the role of the jam ‘core’, while other types of particle form a coating around it in the same manner as the dough.
Whose coat is that jacket?
These coatings are a result of the vast number of other species that are emitted with black carbon e.g. during combustion of fossil fuels or wood. Often the coating will arise due to gases that were co-emitted with the black carbon condensing around the core, in the same way that a water droplet will form on a bathroom mirror. To complicate matters even further, the condensing gas might not have been emitted with the black carbon, for example, emissions from agricultural fertilisers or natural forests can also play a role. This liquid or solid coating can then ‘trap’ the black carbon core in the same way that the dough holds in the jam in the donut.
Feeling the benefit
From a climate perspective, black carbon warms the Earth’s temperature. A recent review study by Bond et al. assessed the role that black carbon plays in the climate system and concluded that:
…black carbon, with a total climate forcing of +1.1 W m-2, is the second most important human emission in terms of its climate-forcing in the present-day atmosphere; only carbon dioxide is estimated to have a greater forcing.
According to the study, the best estimate of the radiative forcing by black carbon is approximately two-thirds that of carbon dioxide. As mentioned previously, black carbon is not a lone wolf and is typically associated with other species. These species can have very different qualities, including the ability to cool the Earth’s temperature. The study was quick to note this point:
Sources that emit black carbon also emit other short-lived species that may either cool or warm climate.
Taking these additional sources into account is extremely complex as we’re attempting to assess how one uncertain aspect (black carbon) interacts with a multitude of other uncertain aspects (other short-lived species). Undeterred though, the study concluded that the best estimate for the forcing from black carbon combined with its co-emitted species was slightly negative but with huge uncertainties. The estimates ranged from cooling or warming that was comparable to the amount of forcing by carbon dioxide. In essence, black carbon and its partners in crime could either largely offset the historical radiative effect of carbon dioxide or add on an extra layer of warming to it.
I’ll get my coat
It turns out that how the dough combines with the jam core is particularly important when assessing the impact of atmospheric aerosols on climate. This is an essential consideration when we examine how we might mitigate these particles, as this mixture is highly complex and difficult to control at the level of an individual species. For example, if we attempted to limit emissions of black carbon, we are highly likely to alter the rest of the chemical cocktail. As we have seen, we’re currently uncertain about how the results of this might pan out.
There is much more surrounding this topic but I’ll leave it for a later time. Did someone say cake?
Posted on April 19, 2013, in Aerosols, Biomass burning, Black carbon, Climate, Pollution and tagged Aerosol, Air Pollution, Biomass burning, Black carbon, Climate, Global warming. Bookmark the permalink. 4 Comments.