Jul
17

Brown Carbon and Aerosols

The title of my project can be a bit misleading. While I am working with one brown carbon compound (4-Nitrophenol) in isolation, brown carbon almost never lives as a single molecule in the atmosphere. It is far more likely to exist in a cluster of other atmospheric molecules called an aerosol. These bundles of chemicals are what I am approximating in my studies of brown carbon.

Aerosols are broadly defined as small particles in the atmosphere. They can be composed of a wide variety of different chemicals depending on where they form. Aerosols over the sea will have sea salt and organics from biological activity, while aerosols formed over land will have dirt, inorganic compounds, and organics from plant emissions and human activity. These chemicals form clusters around a central nucleation seed, which can be a dust or salt particle. Aerosols can also be primary or secondary. Primary aerosols are emitted into the atmosphere as particles, as in human pollution or volcanic eruptions. Secondary organic aerosols (SOA) are formed in the atmosphere either when water soluble chemicals dissolve into cloud water or through oxidative processes causing chemicals to condense onto existing aerosols.

Brown carbon is one component that can make up a large portion of certain aerosols, typically those emitted by biomass burning or fuel combustion. It’s interesting to us because it’s photo-reactive in the visible light range, which introduces some atmospheric complexity. A lot of organic components of aerosols are photo-reactive as well, which leads me to one of the main questions of our experiments: how quickly are aerosol components being removed from the atmosphere?

There are several mechanisms for the “removal” of compounds from the atmosphere.  Aerosols typically have a lifetime of ~1 week in the atmosphere, but during this week they experience a lot of physical and chemical change. Organic compounds in aerosols can be broken down by sunlight or chemical reactions. When light absorbing organic compounds (like many of those in aerosols) are exposed to light, they absorb energy and go into an excited state. Most of the time when this happens the molecules are able to “relax” back into the ground state, but sometimes the energy from the light can cause bonds to break. This is how light degrades aerosols and brown carbon. Another big aerosol sink is OH radicals. OH radicals are highly reactive compounds in our atmosphere that can come from reactions with ozone (see figure) or from within the aerosols itself. These OH radicals will react with organic aerosols to further break bonds and speed up the effects of photolysis.

Ozone reacts with itself in acidic conditions to create OH radicals.

Ozone reacts with itself in acidic conditions to create OH radicals.

 

This is where aerosol lifetimes get complicated. Because they are a conglomerate of many different compounds, they are subject to decomposition from a number of sources. Aerosols are removed through the atmosphere via photolysis and reaction with OH radicals from the atmosphere, but they can also create OH radicals themselves as they are photolyzed. This can speed up the process even further. Studies have shown that the OH radicals produced within aerosols can access inner sections of the particle that outside OH radicals cannot reach, and that this effect significantly decreases an aerosol’s active lifetime. A lot of these studies conclude that aerosols have a strong effect on their own aging and the aging of other aerosols.

In my experiments we are testing the ability of OH radicals produced within organic aerosols to decrease the lifetime of another photosensitive compound (the brown carbon). We photolyze 4-Nitrophenol alone, 4-Nitrophenol with fresh (OH radical producing) SOA, and 4-Nitrophenol with aged SOA. Our results are indicating that the aged SOA does not continue to affect the lifetimes of other compounds, which suggests that SOA may not be contributing to the degradation of other aerosols throughout its entire week-long lifetime. This is the big picture, but a lot of my days are spent working out the small bugs in our methodologies! More on those in a post later. As always, thanks for reading and please leave any questions or comments down below! I love any chance to explain more.

Comments

  1. This is such a clear description of the process you are studying, and left me with a much clearer picture of aerosol “behavior” and breakdown. Thanks!!!!

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