Atmospheric chemists at the Harvard School of Engineering and Applied Sciences (SEAS) have found that when it comes to secondary organic material in the atmosphere, there are two distinct breeds: liquids and jellies.

Secondary organic materials (SOM) are airborne particles that have begun to react with gases in the atmosphere. In the past 20 years’ research and climate modeling, these SOM particles have been assumed to drift as liquids. In a liquid phase, the organic materials would absorb other compounds like ammonia or ozone very easily and then progress through a series of chemical changes (known as chemical aging) to form particles that reflect or absorb sunlight, or form clouds.

Now, experiments at Harvard, using particles of α-pinene SOM and adipic acid, have shown that a drop in humidity can send these common aerosols into a jelly-like phase, in which they resist chemical aging almost entirely. The findings, published in the Proceedings of the National Academy of Sciences, may call for a revision of regional and global climate models.

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