Jul
01

A Bit of Q&A

In this post I want to take the time to answer some questions I got from readers on my first post (linked here: http://honorsfellows.blogs.wm.edu/2019/06/25/project-overview-meeting-team/). I loved the feedback because it’s hard to cover everything at once and these questions helped show me what I need to include when talking about my project in the future. Thanks for everyone who contributed! Without further ado, the Brown Carbon FAQ:

Brown Carbon Sources

Q: Do we know the major sources of brown carbon entering the environment?

Q: What kinds of combustion count as unclean combustion?

And Q: Do we have any understanding of how much of these compounds occur naturally vs coming from human activities?

We do know the major sources of brown carbon! Brown carbon is produced in two ways: primary emissions and secondary formation. The primary emissions come from unclean combustion, which basically means any burning that gives off organic products. This includes burning of wood, coal, oil, gas, and waste. When these products burn they give off black carbon (which we recognize as soot), and brown carbon. Forest fires and controlled burns are one big source of brown carbon. There are also a lot of brown carbon emissions in places like India and Southeast Asia where many people burn wood and waste for energy. This means that brown carbon comes from both natural sources and human activities. These are usually differentiated using the terms “biogenic” and “anthropogenic”.

 

Relationship Between Brown Carbon and SOA

Q: What is the relationship between brown carbon and SOA?

Q: Are we concerned about the potential effects of all SOAs in the atmosphere, or primarily the derived from brown carbon?

SOA (secondary organic aerosol) is basically a soup of organic compounds clustered around a center seed. Brown carbon can be part of SOA or most of an SOA. It can also exist on its own, although it usually doesn’t stay that way in the atmosphere for very long. We are concerned about all SOAs in the atmosphere because all SOAs are affecting the climate and human health. All SOAs scatter and absorb light, although they may do so in varying degrees depending on their state and composition. They are also bad for air quality and for us to breathe. Brown carbon is of particular interest, however, because it’s involved in a lot of SOA and reacts with light in the visible region. This is why we study it, although we also study it because it’s something to study and somebody might as well do it!

 

Brown Carbon and SOA Effects

Q: Do we (as a society) or you (as the lab) have an idea of the general influence of these SOAs on the climate or the atmosphere?

Q: You say that different kinds of brown carbon can have widely different lifespans and affects on the atmosphere in terms of warming or cooling. Is there a way to know if particular attributes of brown carbon lead to particular atmospheric effects? Are the complex interactions too much to make a mapping like that?

We actually don’t have an idea of the general influence of the SOAs. There’s something called the radiative forcing budget which basically outlines what contributes to the net warming and cooling of the earth. Aerosols have the a very large error associated with them in the models because they are pretty complicated systems that are hard to characterize. We don’t even know if they are contributing more to net warming or net cooling. This is because they perform a lot of atmospheric functions, like seeding clouds that reflect light or by absorbing light themselves. These functions also vary based on the aerosol’s composition (whether or not it has brown carbon in it, for one!), relative humidity, size, age, and other factors. This does mean they are very complicated to model, especially since aerosols can be made up of thousands of compounds. The good news is that a lot of the compound are pretty similar and we can generalize their behavior into larger categories (like brown carbon!).

 

My Research

Q: Why 4-Nitrophenol as opposed to another?

It’s an easy proxy for other brown carbons! Since it’s soluble in water and has a fairly short photolytic lifetime it makes experimentation a little easier.

Q: How did you come to your research question?

Honestly, I came into the lab and was assigned to a project. I do find the atmospheric science really interesting though, and I like thinking about real systems that don’t necessarily follow the rules of clean chemistry. I also spent a really insane weekend reading papers and coming up with a proposal idea.

Q: How the heck do you make an aerosol or get a compound into an aqueous bulk?

Thanks for the great question mom! For the brown carbon, we actually bought 4-Nitrophenol salt which is the compound dried into solid form. We just dissolve this into water and dilute it to very very low cloud water concentrations, and voila!

The aerosol is a little bit trickier. We start with an aerosol precursor like a-pinene (which is emitted by pine trees!) and evaporate it into the gas phase. Then we zap it will light to oxidize it. This makes it a secondary organic aerosol. We pump it through a filter and it gets caught on the filter. Then we soak the filter in organic solvent which gets the SOA off the filter and into the solvent. Then we evaporate the solvent so that there is dried SOA on the bottom of the vial, and FINALLY we rehydrate it with water!

Q: How good of a model is lab work compared to the atmosphere?

It’s not excellent, but it does give us certain information. We are a lot cleaner than the atmosphere, but that means we can isolate specific variables. Part of what we are studying is how brown carbon behaves in “dirtier” environments, like with other SOA compounds that may speed up its degradation. We also study it in large volumes of water, which is not how it exists in the atmosphere. A lot of labs are working on droplet chemistry, doing experiments in tiny tiny droplet volumes that more accurately represent the surface area to water ratio you find in the atmosphere. It actually changes the chemistry a lot!

 

If you made it this far, thank you for reading! Join me next week for some actual updates on my actual project. If you still have unanswered questions (I do too) or thoughts in general I would love to chat in the comments below!

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