Mention ozone as an environmental issue, and most people think of the ozone hole: aerosol spray cans and styrofoam, skin cancer, and so on. Its discovery warranted a Nobel Prize for chemistry and also the Montreal Protocol, a highly successful forerunner to modern climate talks. While important, in a sense the ozone hole is nevertheless a faraway problem, and not just because most of us live in the Northern Hemisphere (and last year’s Arctic "ozone hole" remains up to semantic debate). The ozone layer, holed or otherwise, exists in the lower stratosphere. This is the part of the atmosphere where weather balloons give out, too high for commercial aircraft other than the Concorde, where the barometric pressure is less than 10% what it is at the surface. In other words, humans don’t breathe it.
This is just as well, because ozone is toxic stuff. Its chemical configuration, three oxygen atoms bonded together Mickey Mouse-fashion, is not especially stable. Ozone will readily react with organic molecules to reach a more stable arrangement, which makes it an oxidizer so effective it can kill living cells even at low concentrations. This makes ozone great for water treatment, where it is sometimes used in place of chlorine. Like chlorine, it’s not so great for your lungs.
Or for potato plants.
High ozone in the surface air is linked to childhood asthma, and it exacerbates heart and lung disease in those who already have it. Very high levels cause headaches, sore throats and sometimes permanent damage even in healthy people. While the threshold of danger varies greatly from person to person, this is more than enough reason for the EPA to include ozone in its air quality standards. US cities are in violation of the Clean Air Act if they allow surface ozone concentrations to rise too high.
This is where it gets tricky. To keep most pollutants in check, we track down whoever is emitting the offending substance and get them to stop. The EPA has a most wanted list full of freon smugglers. But hardly anyone emits ozone directly. Its chemical instability means that any given ozone molecule in the atmosphere is extremely short-lived. Virtually all of the freon that has ever been released to the atmosphere is still there today, but ozone lasts a matter of hours at most. If ozone were coming out of smokestacks, it wouldn’t have time to accumulate.
Instead, ozone comes from the reactions between chemicals we do emit: NOx and VOCs. NOx refers to the compounds NO and NO2 taken as a group; they’re emitted together when the nitrogen and oxygen in air react with each other at high temperatures (say, in a car engine). VOCs are volatile organic compounds, a group of compounds with many, many members. For the purposes of ozone chemistry, the important features VOCs share are that they exist in the atmosphere as gases, and they have at least one C-H bond that can be broken by reaction with OH.
In the reaction series below, VOCs are collectively written RH; the R is a placeholder for everything connected to the carbon end of the C-H bond. VOCs can be very large molecules, so this saves a lot of space. M is another placeholder, standing for either N2 or O2—it doesn’t matter which—acting to quench the extra energy without being part of the reaction. Those familiar with quantum mechanics will remember that hν denotes the energy of a photon. In this case, all it means is that sunlight is an essential part of the reaction.
RH + OH → R + H2O
R + O2 + M → RO2 + M
RO2 + NO → RO + NO2
NO2 + hν → NO + O
O + O2 + M → O3 + M
Because of that hν in the second-to-last step, ozone is only produced when the sun is up. Therefore the highest ozone concentrations occur in the heat of the day. On days with poor air quality, afternoon is when it’s best to stay indoors. The exception is when high levels of NOx and VOCs become trapped above the surface overnight due to boundary layer dynamics. Then there can be a sudden ozone spike in the mid-morning, as soon as the surface warms up enough to mix yesterday’s pollution back to the surface.
There’s also an ideal ratio between NOx and VOC concentrations, outside of which ozone will not form as efficiently. The relationship is complicated, to say the least. In the plot below, the diagonal line marks the ideal ratio; the curves are contours showing how much ozone will form given different combinations of NOx and VOC content.
Madness. Source: Finlayson-Pitts and Pitts 1993.
Look at that. If you wanted to control the ozone concentrations for a city in the upper left corner—high NOx relative to VOC content—you might sensibly make NOx your priority and reduce its emissions before trying to reduce VOCs. If you did, however, the ozone concentration would increase. You’d have moved vertically down the graph, across ozone contours and out of the VOC-limited regime.
Lastly, ozone formation reactions are temperature-dependent, and will take place faster in hot weather than in cooler weather. This is one more reason for heat waves to be compounded by poor air quality, and it’s especially unsettling in the face of climate change: we will have to reduce NOx and VOC concentrations simply to keep ozone levels constant as the subtropical climate zone expands into the midlatitudes (Seidel et al. 2007).
This is not to say we should give up on keeping ozone out of our breathing air. After all, ozone kills people. But it does show why the task is an ongoing challenge.
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