This is a summary of a seminar presentation given by Dr. Pierre Herckes on March 1.

Abstract: Clouds and fogs play an important role in processing atmospheric particulate matter and gas phase compounds and affecting their lifetimes. Trace atmospheric compounds can be integrated into cloud and fog droplets, react chemically in the aqueous phase and be removed by precipitation. Hence fogs and clouds can act as cleanser of the atmosphere by incorporating pollutants and removing them. Nevertheless droplets can also increase particulate pollution by transforming gas phase compounds into nonvolatile species, which remain in the atmospheric particle phase after evaporation of the droplets. From a cloud physics perspective, scavenged gases and particles can modify the surface tension and hence cloud droplet activation and growth. Atmospheric droplets, particles and gases form a complex multiphase system, which is challenging to study.

This presentation will give an overview of the basic principles involved in fog and cloud chemistry and the tools to study them in the field. While past
research focused on inorganic chemistry, mainly cloud and fog acidification and sulfur oxidation, current research focuses on organic compounds and how clouds process them. This talk will present results from recent field studies, including in Northern Arizona, and discuss our current understanding of the organic matter in fog and cloud droplets. While a large part of the small molecular weight compounds is accounted for at a molecular level, very little is known about the structure, properties or origin of the high molecular weight material.

The abstract gives a good introduction to what Dr. Herckes spoke about. I’ll just touch on some of the main points. He mentioned that the chemistry that takes place in a cloud droplet depends on the size of the droplet. This intuitively makes sense. A larger droplet will have a smaller concentration of solute than a smaller droplet would. The size of droplets also produces different oxidation effects. (Bator and Collett 1997) He did not specify what those were, and as a non-chemist, I’m not exactly sure what that means. He also said that solute can affect the lifetime of the could. We know that the number of cloud condensation nuclei can affect cloud lifetime (Albrecht 1989), but it’s interesting that the CCN material can too.

I also was wondering how does the location of the experiments bias the results. Most of the experimental setups were in the Western United States. Is the aerosol content similar in other places in the World? He mentioned that the Eastern and Western portions of the United States have very different aerosol chemical characteristics. How much can be reasonably generalized from this experiment?

Does the chemistry affect the dynamics of the clouds? For this experiment, the clouds sampled were fog, so they had very little vertical development. By could the chemical composition of the CCN change the light absorbing characteristics of the cloud such that it lead to differential warming and change the circulation of the clouds?

He showed a graph of the scavanging efficiency versus liquid water content. It only had four points, so not much can be said about it. But it looked as if the scavanging efficiency was a function of lwc. As lwc increased, so did the scavanging efficiency. This sort of makes sense. As lwc increases, so does the total amount of liquid water in the air to scavange for aerosols. But, it’s the larger droplets that are the best scavangers - not only do they have a larger projected area, they have a larger gravitational settling rate (terminal velocity). So the best way to increase scavanging efficiency would be to have a lot of large droplets. But how does the spectral dispersion affect the scavanging efficiency? If the cloud drop spectrum has a lot of middle sized particles, I would think the scavanging efficiency would be different than a spectrum that has the same liquid water content but has more larger sized particles and less smaller particles.

Lastly, this study only looked at fog because it is easy to measure with ground based instruments. Can the chemistry associated with clouds be extrapolated to other cloud types?

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