Known as the ‘detergent’ of the atmosphere, the hydroxyl radical (OH) is
a key oxidant involved in the removal of major air pollutants such as nitrogen
oxides and many ozone-depleting substances such as methane from the air. It was
assumed by scientists that about 25% more OH exists in the atmosphere in the northern hemisphere where most of the air pollution-related human
activities take place, as compared with the southern hemisphere. The formation
of OH is stimulated in the northern hemisphere as it ‘cleans’ the atmosphere by reacting with and breaking
down the air pollutants. Because of the short lifetime (~1s) and low
concentrations of OH in the atmosphere, the direct measurement of OH quantity is
extremely challenging.
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| ESA / Thomas Reiter |
Contrary to this assumption, a sciencedaily article described the
findings of a recent Nature publication, which pointed out that the atmospheric OH
concentrations in both hemispheres are
at the same level. In this study, indirect estimates of OH concentrations were
made with methyl chloroform measurement data from two atmospheric monitoring
networks and an airplane that flew from pole to pole. This approach of
surrogate was tested and confirmed by an atmospheric model, which found a tight
linear relationship between the simulation results of methyl chloroform and OH.
As explained in the article, methyl chloroform is almost exclusively broken
down by OH, indicating a negative correlation between the two substances. Since
the OH radical is a major ‘cleaning agent’ of the atmosphere, this new finding
also means that the atmosphere’s self-cleaning capacity in the polluted
northern hemisphere was overestimated previously.
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| A graph showing how NH/SH OH ratios were estimated from observed NH-SH methyl chloroform gradients |
Overall, the article did a nice job
providing a concise yet clear description of the findings in the Nature
publication. I would give kudos to it for communicating lucidly the relationship between OH and methyl chloroform, which is the essence of
many abstruse models and simulations. In addition, the article provided some background
information that was not mentioned in the Nature publication, such as the
stimulation of OH by some air pollutants (eg. nitrogen oxides), as well as the
exclusive removal of methyl chloroform by OH. While this helps illustrate the
research findings, the credibility of this information remains questionable.
For these reasons, I would give the article a score of 8.5/10.
Links
Sciencedaily article:
http://www.sciencedaily.com/releases/2014/09/140911125049.htm
Nature publication:
http://www.nature.com/nature/journal/v513/n7517/full/nature13721.html
Interesting article, thanks for the summary, Yundi. Did the article discuss the process for measuring the methyl chloroform during the pole-to-pole flight?
ReplyDeleteYes, the research article did include this in its Methods section (not the news article, as you can imagine :p). The flights carried the Advanced Whole Air Sampling system. From this system, the methyl chloroform data were measured at hourly intervals using an analytical method called gas chromatography-mass spectrometry (GC MS).
DeleteI appreciate the rating at the bottom!
ReplyDeleteSo in summary they found a considerably less amount of methyl chloroform in the northern hemisphere? Then used this to determine that the OH was less as well? Probably a basic question but it's 8am and I have no idea what's going on (:
Also did you find anything particularly lacking in the summary? Was it necessary to understand the models and simulations in depth to understand the core material?
Thank you for the question! I dig through the research article again and found which I hope is a more accurate answer to your question. Although the decrease in CH3CCl3 amount is a good proxy for OH concentrations, the applied methodology was not as simple as that. To make it easier, let’s first agree on the following abbreviation rules.
DeleteNH=northern hemisphere/ic, SH=southern hemisphere/ic, CH3CCl3=methyl chloroform
It ‘s easy to understand that the NH atmosphere has a bigger reserve of CH3CCl3, even after this ozone depleting substance has been phased out and partially cleaned out of the atmosphere. And thus, there exists a (most likely positive) concentration gradient of CH3CCl3 between the NH and SH. What the researchers did was they simulated the NH and SH CH3CCl3 concentrations in atmospheric models based on several spatially distinct OH distributions, and plotted the NH-SH CH3CCl3 gradients against the NH/SH OH ratios, generating four linear relationships on a same graph since four different data sources were used. The observed NH-SH CH3CCl3 gradients were located on the lines, corresponding to the estimated NH/SH OH ratios. You can find the graph generated from the research as included in the reedited blog :)
Apparently all these methodologies and analyses were missing in the news article summary, but I think it's pretty justifiable for it to do so, meaning not necessarily need to include all the complex details on models and simulations. I would say the summarized information provided in the news article is straightforward and enough for the readers to understand the research fundings, except if you have a specific interest in exercising your brain with the air models you may go further into the research article :D
DeleteI understand the rationale behind using methyl chloroform as an inverse measure of OH, but do we know that introduction of methyl chloroform into the atmosphere is independent of latitude?
ReplyDeleteNeither of the articles mentioned whether the emission is latitude independent, but I would personally guess yes, and since the chemical has already been banned, ideally no more introduction of it would happen and the existing amount in the atmosphere has probably been well-mixed and distributed more evenly during recent years. However, I do think the research design has to some extent ruled out the impacts (if any) of latitude-concerned OH distribution on the research outcomes, as the methyl chloroform data were collected from sources at different locations including a pole-to-pole flight that technically covered all most latitudes.
DeleteI am more interested in the "atmospheric model" you mentioned in the summary. Is there more information about it in either article? Who built it and is it creditable?
ReplyDeleteThe research article mentioned the model they used was the JAMSTEC Atmospheric Chemistry Transport Model, which was initially developed by Japan Agency for Marine-Earth Sciences and Technology. I would love to believe that this is a pretty credible model.
Delete