Scientists determine activation barrier in ammonia-sulfuric acid clusters that could lead to cloud formation

May 24, 2013
The two-step sequential ammonia-sulfuric acid loss pathway shows the presence of barriers for addition of ammonia and sulfuric acid to the clusters. Current atmospheric particle models do not consider these barriers, which may limit their precision.

( —Ammonia must overcome an energy barrier to join sulfuric acid and water to create clusters that can lead to cloud formation, according to scientists at Pacific Northwest National Laboratory and the University of Delaware.

The team used surface-induced dissociation, which breaks apart under controlled conditions. They found that when the clusters fragmented, they either lost an ammonia molecule followed by a sulfuric acid molecule, or lost the two molecules simultaneously. The energy required to dissociate a cluster is higher than the energy of the final products. This energy requirement implies that there is an energy or activation barrier that must be overcome for an ammonia molecule to join the cluster and help it grow. The research also suggests that the more conventional and simple-to-calculate diffusion rate should not be assumed to be the growth rate.

Acidic and basic gases released by gasoline-powered engines, coal-fired power plants, and other sources form clusters of sulfuric acid, water, and ammonia. These clusters can come together to create and grow clusters, which eventually grow large enough to be particles that can serve as the seeds for clouds. Because clouds influence the climate, scientists are developing the most accurate models possible to analyze the impact of regulations and different scenarios. This work paves the way for climate and cloud models with improved accuracy.

The team evaluated the dissociation of two positively charged ammonium bisulfate clusters, which are similar to clusters found in the atmosphere. Surface-induced dissociation of these ammonium bisulfate clusters indicated two unique pathways for cluster fragmentation:

1. a two-step pathway, where a cluster initially loses an ammonia molecule then a sulfuric acid molecule
2. a one-step pathway that proceeds via the loss of an ammonium bisulfate molecule.

The team compared experimental data to of cluster dissociation thermodynamics and determined that the loss of either an ammonia molecule or an ammonium bisulfate molecule is greater than the corresponding thermodynamic value. These results suggest the presence of an activation barrier for ammonia incorporation into molecular clusters as they grow, which may impact cluster distributions in the atmosphere.

The team is continuing to address the formation and growth of the - clusters, including examining the height of the activation barrier, the effect of polarity, and the role of water. Further, the team is investigating whether an activation barrier can exist in larger clusters, and if so, how does it affect the 's reactivity.

Explore further: New insights into cloud formation

More information: Bzdek, B. et al. 2013. Fragmentation Energetics of Clusters Relevant to Atmospheric New Particle Formation, Journal of the American Chemical Society 135(8):3276-3285. DOI: 10.1021/ja3124509

Related Stories

New insights into cloud formation

March 5, 2012

Clouds have a profound effect on the climate, but we know surprisingly little about how they form. Erika Sundén, researcher at the University of Gothenburg, Sweden, has studied how extremely small cloud particles can ...

Cloud seeds and ozone holes

July 30, 2012

New findings on the growth of ice clusters in Polar Stratospheric Clouds could help clarify the process of ozone depletion in the atmosphere.

Hubble sees a unique cluster: One of the 'hidden 15'

April 22, 2013

( —Palomar 2 is part of a group of 15 globulars known as the Palomar clusters. These clusters, as the name suggests, were discovered in survey plates from the first Palomar Observatory Sky Survey in the 1950s, ...

Recommended for you

Detecting HIV diagnostic antibodies with DNA nanomachines

October 7, 2015

New research may revolutionize the slow, cumbersome and expensive process of detecting the antibodies that can help with the diagnosis of infectious and auto-immune diseases such as rheumatoid arthritis and HIV. An international ...

Organic semiconductors get weird at the edge

October 6, 2015

As the push for tinier and faster electronics continues, a new finding by scientists at the University of British Columbia (UBC) and Monash University could help inform the design of the next generation of cheaper, more efficient ...

New polymer creates safer fuels

October 1, 2015

Before embarking on a transcontinental journey, jet airplanes fill up with tens of thousands of gallons of fuel. In the event of a crash, such large quantities of fuel increase the severity of an explosion upon impact. Researchers ...


Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.