Thursday, October 25, 2007

Ozone in the Troposphere

…well yes you did get some kind of award for “Mostest Detailed Information on an Obscure Topic”. –JP Stormcrow

[D]on’t tempt me to go all photochemical on your ass. If you want detailed information on really obscure topics, I can bury you. –James Killus

Ozone is the key ingredient in photochemical smog. Air quality standards for smog are designed to limit ozone on the assumption that, if ozone is reduced, other photochemical smog constituents will also be reduced. While other air pollutants like carbon monoxide and fine particulates are, by and large, directly emitted, ozone is a “secondary air pollutant,” meaning that it is formed by chemical processes in the atmosphere.

There is, however, a natural background of ozone in the troposphere, the layer of air that contains 90% of the atmosphere, and the part of the atmosphere where we breathe, where weather happens, etc. Some of the tropospheric ozone is from the stratosphere; it slowly leaks down from above, through the tropopause, a very strong thermal inversion that exists, in fact, because the stratosphere contains so much ozone. Ozone absorbs infra-red radiation, so the stratosphere heats up and forms a thermal inversion “cap” that suppresses convective mixing. (The stratosphere is hot only in the relative sense. It’s still very cold; it’s just not as cold as the air immediately below it).

Most ozone in the stratosphere is formed by the direct photo-dissociation of oxygen by very short wave UV light, i.e., a different production pathway that can exist in the troposphere, because the stratosphere absorbs all the UV in those wavelengths before it reaches the troposphere. But near the bottom of the stratosphere, some of the ozone that is produced is formed by the “smog reactions,” from traces hydrocarbons (mostly methane) that manage to get through the tropopause, plus some nitrogen oxides that are formed from solar proton events, emitted from high altitude aircraft, and produced from the photolysis of nitric acid.

The smog reactions also work in the troposphere, of course, and not just in urban areas, although that’s where they are most obvious and where they were first studied. But there are natural sources of both reactive hydrocarbons (from trees, oil seepage, etc.) and nitrogen oxides (forest fires, lightning, bacterial action), and it’s pretty obvious (and inevitable) that some ozone will form that way.

So how much of the troposphere’s background of ozone begins in the stratosphere, and how much is formed in situ? There have been a lot of studies of this, from different directions. Leakage from the stratosphere, for example, can be estimated via tracers, including radioisotopes that were left over from nuclear bomb tests as they slowly leaked out of the stratosphere, and CFCs, as they slowly leaked into the stratosphere from below.

Estimating the strength of the tropospheric source is a little more difficult. I made a stab at it in the early 1980s, after an earlier stab at estimating the natural sources of nitrogen oxides.

The key to the analysis is a sort of “pseudo-stoichiometry” that exists in the smog reactions. Essentially, the process that forms ozone also destroys nitrogen oxides, although at a rate that depends on a lot of factors. One example of this destruction is in the case of the hydroxyl radical.

The hydroxyl radical (HO) is necessary for the oxidation of hydrocarbons that drives photo-oxidation, but HO also reacts with nitrogen dioxide, which is also necessary to form smog, to give nitric acid, the termination species (except in the stratosphere). This is only one of the radical species in the smog reactions that produces a sink for nitrogen oxides, so, as smog is formed, nitrogen dioxide is destroyed.

The ratio between ozone produced and nitrogen oxides being destroyed varies substantially, from less than 1 at high concentrations of precursors (such as in NOx rich plumes from power plants), to as high as 5 or even 10 to 1 at low concentrations under ideal conditions. I made various plausible estimates of the production ratios under various conditions for various NOx sources, and estimated the additional tropospheric ozone background that could be achieved from each source.

My estimates wound up being in the same ball park as prior estimates of the stratospheric ozone sources, around 1-4 ppb ozone, give or take. Given the uncertainties in just about every type of data going into the process, that was pretty good. It also had some bearing on a number of other questions, like the general photochemical background of “clean air,” and the fact that, over the past few decades, tropospheric ozone has been increasing. Since ozone is a (minor) greenhouse gas, the matter even has some application to climate change and global warming.

I presented these results in a poster session at a conference in the early 1980s, and one fellow who came by told my boss that he thought I was just the sort of smart fellow the field needed. That fellow was Paul Crutzen, who later got the Nobel Prize in Chemistry for his work on stratospheric ozone depletion. Having a future Noble laureate call me smart is one of those things that I don’t seem to be able to work into conversation nearly often enough.

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