Cirrus cloud formation may be affected by wet exhaust soot

The ability of soot particles to adsorb water has been examined by Russian scientists in an effort to determine their involvement in cloud formation.

Black carbon, or soot particles from aircraft exhaust are one of the main constituents of background aerosols in the upper atmosphere. Carbonaceous particles can act as ice-nuclei for cirrus cloud formation, but exactly how much ice-nucleating ability soot particles have is uncertain.

To be incorporated into clouds, particles in the atmosphere must be able to adsorb water onto their surfaces, but soot particles have traditionally been found to be hydrophobic in nature. This has led to the widespread assumption that freshly emitted soot particles are poor ice nuclei, and become involved in cloud nucleation only after their surfaces are covered by soluble sulfur-containing compounds such as sulfuric acid.

To throw more light on the processes involved, Olga Popovicheva and colleagues from Moscow State University examined the wettability of aircraft combustor and kerosene flame soots. They found that soot could be a relatively good water adsorber, even without any soluble sulfur-containing compounds, and they identified a specific mechanism for water adsorption on to active sites on the particle surface. This allows water to condense on the insoluble soot particles even in unsaturated conditions. Aircraft exhaust soot particles may therefore play a bigger role in cirrus cloud formation than was previously thought.

Popovicheva hopes the work will develop in the future to form the basis of a global model to estimate aircraft-induced climate change.

Rowena Milan