Jun
25

Project Overview and Meeting the Team

Hello all! Over the next few days I will be posting about my first few weeks of research here in the lab. It’s currently week 4 out of 10, which I think is a good stopping place to reflect on what we have been doing in the O’Brien group and where we are heading next. This first post will be a broad overview; subsequent posts will dive more into the specifics.

There are four of us in the lab this summer: Corey, Tanner, Hongmin, and myself. The projects we are working on are different aspects of aerosol chemistry, but all of the work is collaborative so I will mention these characters often. Corey works on the gas side of things (he makes aerosols for us), Hongmin is studying complex aerosol mixtures, and Tanner and I are continuing my work on brown carbon in aqueous bulk. 

The system that I have been studying (and will continue to study!) is atmospheric brown carbon. Brown carbon is a category of organic compounds that are created by biomass burning and other unclean combustion. They absorb light in the visible region which gives them a yellowish-brown color (which is why they are called brown carbon!). These compounds are noteworthy because we really don’t know that much about their activity in the atmosphere. These guys can go into the air, form organic clusters called aerosols, and oxidize into what is then called a secondary organic aerosol (SOA). SOA can be formed from any semi-volatile organic and can be anthropogenic or biogenic. These can then do chemistry with other compounds, change in size, change in reactivity, absorb light, and even scatter light. They can exist in dry and wet environments, which changes the chemistry as well. What makes this even more complicated is that there is a huge number of different brown carbon molecules that can react and interact differently in the atmosphere. These complexities mean that we don’t know how long these compounds spend in the atmosphere or how they affect atmospheric warming and cooling. 

Here's a good overview of VOC (volatile organic carbon) emissions. Biogenic sources like pine trees and other plants emit a lot of volatile organics, which can go into the atmosphere and oxidize into SOA (secondary organic aerosol) of varying sizes (on the left) and hygroscopicity (how soluble the aerosol is in water). Both of these variations have effects on cloud formation, which in turn has effects on net warming or cooling as clouds reflects solar radiation.

Here’s a good overview of VOC (volatile organic carbon) emissions. Biogenic sources like pine trees and other plants emit a lot of volatile organics, which can go into the atmosphere and oxidize into SOA (secondary organic aerosol) of varying sizes (on the left) and hygroscopicity (how soluble the aerosol is in water). Both of these variations have effects on cloud formation, which in turn has effects on net warming or cooling as clouds reflect solar radiation. (picture from Zhao, D.F.; Buchholz, A.; Tillmann, R.; Kleist, E.; Wu, C.; Rubach, F.; Kiendler-Scharr, A.; Rudich, Y.; Wildt, J.; Mentel, T.F. Nature Communications 20178 (1).

 

My project focuses on how different components of aerosols may interact in the atmosphere when wet. I’m looking specifically at 4-Nitrophenol, a type of brown carbon, and -pinene SOA. We are seeing how the presence of SOA and other compounds affect the lifetime of 4-Nitrophenol when exposed to sunlight. We don’t know how long these chemicals persist in cloudwater in the atmosphere, and it’s important to consider how the presence of other atmospheric compounds may affect this. SOA may give off radicals when photolyzed that accelerates the photobleaching of brown carbon. This could mean that levels of brown carbon are highest in the atmosphere over night and drop during daylight hours. I’ll post more later about some of our processes and questions we have about these complex systems. Please feel free to comment below and let me know if you have any questions! If any of this is unclear I would love to clarify. Thanks for stopping by!

Comments

  1. ldolvin says:

    I’ll start with brown carbon sources. Brown carbon enters the environment both as primary emissions from unclean combustion and as secondary products formed in the atmosphere. First, the unclean combustion: this is basically referring to any burning that releases organic compounds. Biomass and biofuel burnings are the largest primary sources of brown carbon. Biomass refers to burning wood and vegetation, usually from forest fires or controlled burns. India and some countries in south/southeast Asia also have a lot of brown carbon emissions because people rely on wood and waste burning for fuel. Biofuels are the classic fossil fuels like gas, oil, etc. Car emissions are very brown carbon heavy. Second, the formation in the atmosphere: nitrogen compounds and organic molecules can get together in the atmosphere to form brown carbon. There are a lot of different mechanisms for formation because there are thousands of different organic compounds and thousands of possible products. Brown carbon is a very big category.

    The concentration of SOA in the atmosphere varies widely between locations, time of day, season, etc. Los Angeles at rush hour will have a lot more SOA than a field in Kansas at night. Maybe from 10-100 ppb? It really depends on what sources are nearby. I have to admit that I don’t know a ton of facts about day/night cycles, but I do know that sunlight is a big contributor to the “removal” of brown carbon aerosol. Removal in this sense means aging the aerosol so that it is no longer contributing to atmospheric chemistry. Past this point we still don’t know if the dead aerosol stays dead until it leaves the atmosphere through physical deposition processes, or if there are ways it can be reconstituted. A lot of people are working on the day/night cycle right now but it’s a bit outside of my purview.

    A lot of your questions are questions that people in the field are actively pursuing. You’re right in pointing out the challenges in characterizing a system this complex. Yes, there are a lot of different individual molecules that may be doing things slightly different ways, but because they have similar sources and reaction pathways we can generalize their behaviors a bit. I’m hesitant to get into modeling details because I don’t have a lot of direct experience with it, but I would say the research in this area is more concerned with characterizing processes than specific chemical species. Our predictions of aerosol lifetimes, light absorbing, and light scattering have huge error bars and really don’t match up with what we observe in the actual atmosphere, which indicates that there are processes going on that we still don’t understand.

    Thank you for the great questions, I’m actually going to use them in my next post!

  2. jfmorris says:

    Hi Lydia!
    I have many questions! I’m sure many of them are pretty basic though as I am not a chemist. I also don’t know if there’s a proper order for me to ask them in, but I’m just going to go for it. What kinds of combustion count as unclean combustion? Is brown carbon a gas or solid particulates? Do we know the major sources of brown carbon entering the environment? Do we have an idea of the concentration of SOAs in the atmosphere? If the levels of brown carbon in the atmosphere are higher overnight than during the day, does that mean that brown carbon is un-photobleaching or that the atmosphere is absorbing new brown carbons every night? 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? 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? Why 4-Nitrophenol as opposed to another?
    I know this is probably a lot, and feel free to not answer these questions. I’d love to chat about it sometime. Your project sounds really interesting, and I’m looking forward to reading all about it!

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