Why are flames yellow?

What is it about?

Soot causes flames to glow yellow. It is composed of many tiny particles made of carbon. Whilst the soot is in the flame it glows yellow because it is hot. Often, some soot particles are able to escape from flames. For example, think of the clouds of black exhaust being emitted from old vehicles. This is soot. Soot is a harmful pollutant and is known to have a significant detrimental impact both on human health and climate change. Despite the significance of soot pollution on our lives, our scientific understanding of what causes soot to form remains incomplete. In this paper, we explore a new idea that we hope may resolve some of this mystery.

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Why is it important?

The paper investigates the potential role played by polycyclic aromatic hydrocarbons - the constituent molecules of soot - in the formation of new soot particles. The polycyclic aromatic hydrocarbons may be flat or curved. We have previously shown that curvature of polycyclic aromatic hydrocarbons leads to an electric polarization, meaning that one side of the molecule carries a negative electric charge whilst the other side of the molecule carries a positive electric charge. In this new work, we present the results of theoretical calculations to explore the smallest molecules that could possess such a polarity and computed the ability of such curved polycyclic aromatic hydrocarbon molecules to stack on top of each other. The calculations show that the energy required for curved and non-curved polycyclic aromatic hydrocarbon molecules to form stacks is very similar. However, the calculations also show that this picture changes very dramatically if you introduce a charged species. The presence of a charged species results in the curved molecules being able to stack much more easily.

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