The Mysterious Oxidant X

With the IPCC getting ready to churn out yet another frightening report based on consensus science in 2013, it is interesting to note that many things have changed since the previous report (AR4). For example, oxidation is a major factor in atmospheric chemistry and can impact many environmental issues: stratospheric ozone loss, acidification of water and soil, air quality, cloud formation and, naturally, climate change. In the AR4 report the only atmospheric oxidation factors included were ozone (O3), the hydroxyl radical (OH) and the nitrate radical (NO3). In a recent scientific report, measurements from a Finnish forest revealed a previously unknown atmospheric oxidant that promotes production of sulfuric acid—one of the main precursors for the formation and growth of aerosol particles and clouds. Scientists are still unsure what this mysterious chemical compound is, they refer to it as oxidant X.

Anyone who has tried to understand the complexities of climate science has learned that there are a multitude of factors to consider—changing land use, cloud formation, orbital variation, atmospheric chemistry and aerosols to name a few. The formation of gaseous sulfuric acid (H2SO4) is a major contributing factor connecting atmospheric oxidation chemistry with the formation and growth of new aerosol particles. Aerosol particles, serving as nucleation sites for water condensation, are a major factor in cloud formation and clouds have a complex, and as of yet still poorly understood, impact on climate. OH has previously been assumed to be the only oxidizer that converts sulfur dioxide (SO2) to sulfuric acid but as with many things in climate science, that “fact” is changing.

In a letter appearing in the journal Nature, entitled “A new atmospherically relevant oxidant of sulphur dioxide,” the surprising results from a study by R. L. Mauldin III and colleagues are reported. Their work comprises a careful monitoring of OH and H2SO4 from above the forest canopy, deep in the forested wilds of Finland. In typically cautious scientific language, here is a description of their work and major finding, taken from the paper's abstract:

Here we present atmospheric observations from a boreal forest region in Finland, supported by laboratory experiments and theoretical considerations, that allow us to identify another compound, probably a stabilized Criegee intermediate (a carbonyl oxide with two free-radical sites) or its derivative, which has a significant capacity to oxidize sulphur dioxide and potentially other trace gases. This compound probably enhances the reactivity of the atmosphere, particularly with regard to the production of sulphuric acid, and consequently atmospheric aerosol formation. Our findings suggest that this new atmospherically relevant oxidation route is important relative to oxidation by the hydroxyl radical, at least at moderate concentrations of that radical. We also find that the oxidation chemistry of this compound seems to be tightly linked to the presence of alkenes of biogenic origin.

What is not obvious from the statement above is that this finding was unexpected and may have considerable impact on how atmospheric chemistry is modeled. The atmospheric concentration of the mysterious oxidant—which Mauldin et al. dubbed “X”—was found to exceed that of OH. Noticeably, the concentration of X showed no clear daily cycle, remaining active in the evenings and at night. By comparison, OH activity is only significant during the daytime. The continued conversion of sulfur dioxide to sulfuric acid at night was one of the first clues that something else was involved other than OH.

“Until now, the general consensus has been that the rate at which sulphur dioxide (SO2) is converted to gaseous H2SO4 is determined by the OH concentration,” the authors state. “Here we show that there is another important source of gaseous H2SO4 that is not directly related to OH.”

Mauldin and colleagues used the technique known as chemical ionization mass spectroscopy (CIMS) to measure OH. When measuring OH concentrations using this technique, OH is first converted to into H2SO4 using sulfur dioxide containing an isotope of sulfur. By adding 34SO2 to the ambient sample flow, all the OH present is converted to a form of sulfuric acid whose weight differs slightly from other, naturally occurring H2SO4 molocules. The resulting H234SO4 can then measured using CIMS. The results are shown below.

Data from a Finnish boreal forest site, summer 2010.

In their field experiments, the authors were able to measure atmospheric concentrations of ambient sulfuric acid at the same time as they detected OH. They then calculated the H2SO4 concentration that would have been produced by the oxidation of sulfur dioxide by OH alone, based on the measured concentrations of atmospheric OH and sulfur dioxide. As they observed the difference at all times of the day, it was noted that the difference scaled in proportion to the concentration of X. The authors' report their observations in their own words:

We started our investigation using field observations performed at the SMEAR II station in the Finnish boreal forest region (Supplementary Information). Figure 1a and b shows the concentration time series of [OH], [X] and [H2SO4] measured over one week during the summer of 2010. The OH concentration shows a typical diurnal cycle, with maximum concentrations around noon and much lower ones during the night. The value of [X] does not show a clear diurnal cycle, but it typically exceeds [OH]. During several evenings and nights, we identify instances when [OH] is close to 105 molecules cm–3, [X] simultaneously exceeds 106 molecules cm–3, and [H2SO4] is remarkably high, up to about 106 molecules cm–3. This observation indicates the presence of a non-OH source for H2SO4 production, and further suggests that there might be a connection between this source and the oxidant X.

They then went back to the laboratory to try and identify just what this oxidant X compound is. The field and lab measurements gave strong evidence of the existence of a previously unknown oxidant, but its identity could not be ascertained. Its mysterious identity not withstanding, Mauldin et al. are certain that X comes from the trees:

To confirm vegetation as a source of the alkenes responsible for X formation in the boreal forest environment, we performed an additional experiment where branches of different trees were cut and placed in the immediate vicinity of the CIMS inlet. The production of OH from ozonolysis of branch emissions during this experiment was minor in comparison to production of X. This experiment indisputably substantiates our conclusion, demonstrating the role of trees in producing compound X and, consequently, affecting gaseous sulphuric acid production.

The scientists' best guess as to what X is is that it is a form of stabilized Criegee Intermediate (sCI), a family of free radicals that form from the reaction of ozone with alkenes. This flies in the face of common wisdom that the rates of the reaction of Criegee intermediates with sulphur dioxide are too slow to have any atmospheric relevance to the formation of sulfuric acid. The authors' provide the following diagram, showing how the mysterious oxidant X might be formed.

Proposed mechanism for the formation of oxidant X.

Regardless of the actual chemical composition of X, its discovery has once again highlighted the fact that nature still holds many surprises for science when it comes to the mechanisms and contributing factors for climate change. Claims of “settled science” aside, the IPCC boffins need to scuttle back to their computer code and update their atmospheric chemistry models before churning out their next, undoubtedly erroneous, set of climate predictions. In a news & views article in the same issue of Nature, “The X Factor,” chemist Dwayne Heard of the University of Leeds summed things up this way:

In calculations predicting regional and global temperature rises caused by human activities, the largest uncertainties are associated with aerosols and clouds. Until now, OH has been assumed to be the only oxidizer that converts sulphur dioxide to sulphuric acid. Mauldin and colleagues' findings will therefore help to reduce the uncertainties in climate predictions that aim to take into account future changes in man-made sulphur dioxide emissions and in natural hydrocarbon emissions from plants.

This is why I am a practicing skeptic—science has demonstrated over and over just how little we really know about Earth's climate and the processes that affect it. When the climate change alarmists tried to sell the rest of mankind their dumbed down, implausible anthropogenic global warming “theory” they should have set off alarm bells on every scientifically literate person's bullshit-o-meter. “It's all because of human CO2 emissions,” they say. Yeah, right.

Take a look a the diagram above that tries to explain a possible mechanism behind a newly discovered unknown factor that may have a major impact on cloud creation—clouds being one of the less understood regulators of climate. Science does not know everything that affects climate change and most of what it does know is not fully understood. Our ignorance aside, we are expected to implement radical economic and sociological changes to our civilization to avoid a postulated climate “catastrophe.” Evidently scientific consensus means “let's all be stupid together.”

Be safe, enjoy the interglacial and stay skeptical.

[ Editor's note: you may notice the use of the UK spelling of “sulphur” and “sulphuric” in this piece mixed in with the US spellings “sulfur” and “sulfuric.” This is because Dr. Hoffman is an American (with an American spell checker) and Nature is a UK journal. Hopefully, no one finds this too confusing. ]

Sierran forests smell sooo good.

This article's reference to chemistry in the air above Finnish forests and the word 'alkene' set my brain off recalling the typical odor of a pine forest in the Sierra foothills on a hot day. The next thought was 'isoprene' and in general 'aromatic terpenes.' A little searching turned up a most pertinent article, "Atmospheric chemistry: The man who smells forests" by Erik Vance, published online 27 May 2009 | Nature 459, 498-499 (2009) |.

The chemistry in your post is above my pay grade, but this old geezer Chem Engineer can still recall those Ponderosa - Cedar odors and root out a reference. [ ]

Hunter Paalman
Walnut Creek CA

missing superscripts

105/106 should be 10^5/10^6?


Thanks. It is often hard to catch all of the super/subscript stuff in a quotation from a scientific journal. Good to know that we can depend on the sharp eyes of our readers to keep things on the up and up.