Anthony's Science Blog

Biogeochemistry Reading Group – Steve Brown

February 4, 2008 – 2:59 pm

Although we have not met since spring ’07, one of my favorite experiences as a graduate student at CU has been helping to organize and participate in the Biogeochemistry Reading Group. The idea of the group was to bring together students interested in biogeochemistry at CU and have them meet with one researcher each week who studies biogeochemistry or a closely related topic. These researchers were selected from within CU and from the various agencies based in Boulder such as NOAA and NCAR. At each meeting, we would read a paper by the researcher and then have a conversation with them about their paper, their research in general, and their life in science. I took notes at many of the meetings and I want to periodically on this blog share some of the insights that we gained during those conversations.

Steve Brown is an atmospheric chemist with NOAA and we met with him in January ’07. We read his paper in Science about the role of nocturnal processing of nitrogen oxides in the atmosphere, especially with regards to dinitrogen pentoxide (N₂O₅) and sulfur aerosols.

First, there were some basic definitions for us mostly biologists. N₂O₅ is an obscure compound, but it turns out to be really important in atmospheric chemistry. Something like 90% of NO_x ends up being N₂O₅ overnight. (Another important fact to know here for non-chemists is that the atmosphere is basically a giant chemical reactor that uses the 20% O₂ in the atmosphere to repeatedly oxidize compounds. The energy for this comes from the sun. So, NO_x (NO + NO₂) comes out of tailpipes and other combustion reactions and then gets oxidized to N₂O₅, NO₃ and other stuff.)

Aerosol means air and particles. Particles are a few tenths of a micron, they accumulate until they settle out. If you start with a dry particle and you increase the humidity, there will be a threshold at which it takes on a bunch of water at once and then grows. This is called “deliquesence.” The aerosols will shrink as humidity goes down until they lose all of their water at once. This is called “efflorescence.” Sulfur content will affect how these two processes occur.

Here’s the meat of Steve’s story, though it’s complicated, so I’m not sure I got it exactly right. Read the paper if you have doubts! Steve says that N₂O₅ is really stable in cold weather. (In general, the winter air problem is aerosols and the summer air problem is ozone.) Air pollution tends to be a lot more concentrated in winter because the boundary layer is a lot lower. Sulfate aerosols take up water very readily and organic aerosols do not. During the day, ·OH (hydroxide radical) plus NO₂ gives you nitric acid. At night, you get the series of equations in Steve’s paper that result in N₂O₅ and its breakdown. The nighttime reaction rates may be very high. At night, this process consumes ozone; during the day ozone is created. Thus, N₂O₅ may change the pattern of N deposition by manipulating the form of nitrogen. For example, N₂O₅ may allow the N to be tranported further since dry deposition is very efficient for HNO₃ but not for N₂O₅.

One of the most interesting things in this discussion was that Steve pointed out that his story shows how higher sulfate content in the atmosphere may actually help with ozone! But, he also said “we would never say that.” Presumably though, they would (and Steve did!) in fact say that because they believe it to be true based on their data. BUT, they are reluctant to say those particular words due to the likelihood that they will be quoted out of context and used as a tool by pollution-happy industry hacks. (I think atmospheric chemists have been sensitive to this type of thing ever since Ronald Reagan said “Trees cause more pollution than automobiles do.” Which is partially true only when they are provided the other half of the necessary reactants from human activities.)

Steve explained that his opinion was that the solution to the total problem is reductions in sulfur aerosols accompanied by reductions in NO_x. Most sulfur in the atmosphere comes from coal-fired power plants. In this country, that tends to occur in the Ohio River Valley, so that’s where Steve et al. did their study. Sulfur emissions were reduced a lot in the 80’s but they have leveled off. In the last 10 years, there has been a lot of progress on NOx emissions, but not as much on sulfur (though I think that sulfur is still better managed than nitrogen overall).

Finally, a few interesting and unanswered questions came up at the end of the discussion. 1. In the Denver Boulder area, what is the spatial extent of nitrogen compound plumes and in what form is the nitrogen in what location? 2. Do microbes participate in atmospheric chemistry? Microbes are about 1 micron in diameter, which is a pretty big particle.

Write in if you can answer the mystery questions (I’ll be pretty impressed if you can). Whew! That’s it. I am apparently a verbose-ass blogger. I may have to learn how to do more with less in future posts!

Update (2/20/08): Re: Question 2. See this story about microbes in clouds!