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NOxious gas flushed down from Heaven…

February 1st, 2007 by seok · 2 Comments

Here is a great paper written by Randall et al. published in GRL in late 2006 entitled “Enhanced NOx in 2006 linked to strong upper stratospheric Arctic vortex”. (And I am not just saying that this paper is great because one of the authors happens to be an instructor for one of my classes…) The authors showed that changes in polar vortex strength influences the amount of ozone-destroying gases (in this case NOx) in the polar stratosphere. This implies that not only pollution from the surface is an issue for ozone destruction; even global climate change can indirectly affect the ozone concentration in the stratosphere.

An exciting fact I learned from this paper is that stratospheric NOx is not only produced in the stratosphere and come from reduced tropospheric N2O, it can also come from the mesosphere and far above.

“It is now well documented that NO produced in the mesosphere or thermosphere can descend to the stratosphere where it participates in catalytic O3 destruction…” says the authors.

In the winter of 2003-2004, the Randall et al. documented this event. However, “[t]he … winter of 2003–2004 was unusual in two ways. First, large solar storms occurred during late Oct and Nov of 2003, leading to substantial EPP [...]. Second, the upper stratospheric polar vortex in Feb–Mar 2004 was the strongest on record at that time [...]. One or both of these characteristics led to observations of unprecedented stratospheric NOx enhancements and O3 reductions due to the EPP IE, but distinguishing the relative importance of the high particle activity and strong vortex was not possible given the available measurements [...].”

Fortunately, in 2006 with low energetic particle activity, but with strong upper stratospheric vortex, they were able to see the effect of the polar vortex have on NOx enhancements in the stratosphere. “Thus even though energetic particle activity was not enhanced, observations show enhanced transport of EPP-NOx to the stratosphere in Feb–Mar 2006, illuminating the important role that meteorology plays in determining the EPP IE.”

How did the authors come to the conclusion that the polar vortex was the primary driver in enhancing NOx in the stratosphere?

First, they used the correlation between CH4 and NOx to show that the source of the NOx was from EPP IE by noting that “[h]igh NOx correlating with low CH4 is indicative of the EPP IE, since this is a signature of NOx-rich mesospheric air descending to the stratosphere [...],” which they observed. This methane and nitrogen oxides correlation is new to me. So to learn more about it, I visited the paper that was cited, Siskind et al.’s 2000 paper published in GRL, which did not help me much; because it cited another paper that talked about this methane and nitrogen oxides relationship. Eventually, I ended up with Siskind and Russell III’s 1996 paper published in GRL, which shows how it was determined that CH4 and NOx were anti-correlated, etc.

Next, they used the Ap index to show that there were no significant planetary-scale geomagnetic activity in 2006. Finally, “using United Kingdom Meteorological Office (MetO) analyses since 1992 for the 2000 K (~50 km) potential temperature level,” they determined that “the upper stratospheric vortex was exceptionally strong, and suggest that the large EPP IE in 2006 was a direct result of unusual meteorological conditions that led to confined descent of EPP-NOx.”

While this maybe the case, would not a strong polar vortex in the upper stratosphere “push” all the air mass below it down? Effectively creating a dip in the boundary layer. And knowing that the stratosphere is a stably stratified layer in the atmosphere, there would be very little mixing. Or would I have to look at the upper stratospheric polar vortex as a force that creates a hole that allow lower mesosphere air to descend into the stratosphere and mix with it?

From a dynamics point of view, I might want global warming to occur so only a limited amount of EPP-NOx would enter the upper stratosphere and interact with the ozone. Assuming that the polar vortex covers a fixed area over the poles, with warming temperature, it would raise the air column over the poles, which would decrease the strength of the vortex. This would, in turn, limit stratospheric NOx enhancement from the EPP IE.

Tags: climate · stratosphere

2 responses so far ↓

  • 1 seand // Feb 1, 2007 at 11:48 pm

    Brian, this is probably a dumb question, but …

    EPP = Energetic Particle Precipitation
    IE = ?

  • 2 Cora Randall // Feb 2, 2007 at 10:12 am

    “(And I am not just saying that this paper is great because one of the authors happens to be an instructor for one of my classes…)” Of course, it can’t hurt….

    Seriously — with regard to your question about the polar vortex. Descent in the polar vortex generally slows down as you go lower in altitude, then air is mixed out horizontally at the bottom of the vortex. But especially in the northern hemisphere, it is often the case that the vortex is strong in one altitude region and weak in another. In 2004 and in 2006, the two years when we saw such large transport of NOx into the stratosphere, the vortex was unusually strong in the upper stratosphere, but not in the lower stratosphere. One of the main effects that this had with regard to NOx descent was that it led to the NOx descending from above to be confined to the polar region as it descended, where its lifetime is long. If the NOx had been transported to lower latitudes while it was at high altitudes (60-90 km), it would have been dissociated, with no recombination reaction. That is, it would have been lost forever. So the quick answer to your question is that you absolutely have to look at the properties of the upper stratospheric vortex.

    Now, I just wrote something that might at first seem to make no sense: why would the upper stratospheric vortex have anything to do with the altitude region from 60-90 km? There are a couple answers to this. First, we don’t know very much about the “mesospheric” vortex, because there are so few measurements of winds and temperature, so we just assume that conditions in the upper stratosphere are similar to those in the mesosphere.

    Second, though, is that it just so happens that the stratopause, which is typically around 50 km, was in fact displaced to around 80 km in the winter of 2006 (as far as I know, we have never before observed a stratopause this high, although that may be due to a lack of observations). This we only found out after we wrote the paper that came out in GRL in September(so we’ve submitted another paper that’s now under review at GRL — Dave Siskind is the first author). Our belief is that this displacement was the result of gravity wave filtering in the lower stratosphere — the disturbed vortex in the lower stratosphere did not allow the gravity waves to propagate up to the mesosphere. This led (by mechanisms that I’m frankly not familiar with) to faster descent in the 60-90 km region than normal.

    So two things were compounded to cause the effects that we saw in 2006: Air was confined in the polar region as it descended because of the strong vortex, isolating it from lower, sunlit latitudes where the NOx would have been dissociated, and the descent from the production region (probably 100-120 km) was faster than normal.

    The natural question is why the vortex conditions were so unusual (in both 2004 and 2006). Did it have anything to do with global warming? Some calculations indicate that these are the types of effects that one would expect from global warming, but not all agree by any means. Indeed, some calculations would say just the opposite. So for now, the jury is still out.

    Sean: IE is the “Indirect Effect” of EPP on the stratosphere. It refers to the process by which NO is created in situ in the thermosphere (or upper mesosphere) but then descends into the stratosphere. It would be contrasted with a “Direct Effect” whereby very high energy electrons or protons (which occur only sporadically) would create NO in situ in the stratosphere.

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