Alan Stahler: Oxygen is not always good
Oxygen atoms don’t like to hang out by themselves — they like to stick together.
Press two snowballs together, gently, so they stick. Each snowball represents an atom of oxygen. Stuck together, the two atoms form a dumbbell-shaped oxygen molecule — ordinary, garden-variety oxygen, the oxygen we love to breathe.
As much as they like to bond to each other, oxygen atoms prefer to bond to other types of atoms. You can find atoms of carbon and hydrogen by the gazillions in wood, gasoline, diesel, natural gas, all sorts of fuels. Slam an oxygen dumbbell against an atom of carbon, or an atom of hydrogen, and the dumbbell falls apart. The loose oxygen atoms now stick to atoms of carbon and atoms of hydrogen. As they link up, the atoms release heat and light — fire.
On rare occasions, instead of sticking together in twos, oxygen atoms stick together in threes. Three snowballs, stuck together in a “V” shape, comprise a molecule of ozone.
A v-shaped ozone falls apart much more easily than the dumbbell. These loose oxygen atoms, too, stick to atoms of carbon and hydrogen. The carbons and hydrogens may come from whatever is exposed to the air. Rubber, perhaps, or plastic … or lung.
Ozone is rare — it takes special conditions to make ozone.
We’re all familiar with nitrogen gas — the air we breathe is almost all (four-fifths) nitrogen.
Nitrogen, like oxygen, is also a dumbbell-shaped molecule, two nitrogen atoms stuck together. But nitrogen snowballs stick together much tighter than oxygen — prising them apart is very, very hard.
Unless you get them very, very hot.
Nearly two centuries ago, an engineer showed that an engine would use its fuel most efficiently if it ran very, very hot.
Gasoline engines run hot; diesels run hotter yet. Inside a hot engine, nitrogen and oxygen dumbbells collide, and break apart. Loose oxygen atoms stick to loose nitrogen, forming oxides of nitrogen (NOx — rhymes with “knocks” — the “x” implies there could be a variable number of oxygen atoms attached to nitrogen atoms).
No engine burns a 100 percent of its fuel; some unburned fuel escapes out the tailpipe, as volatile organic compounds (VOCs; volatile = evaporates easily; organic = carbon-rich).
Mix NOx — oxides of nitrogen — with carbon-rich VOCs; zap the mixture with ultraviolet from the sun and you get one of several recipes for ozone.
Ozone forms slowly in cold air, but quickly when the air is hot.
To control ozone, we’ve been reducing the VOCs and NOx coming out of tailpipes and smokestacks. This has reduced ozone in many parts of California. But the foothills have a source of VOCs that cannot be reduced: Trees. VOCs from trees smell great, but when the air is hot and the sun is bright, they mix with NOx, wafting up from the valley, to create some of the worst ozone pollution in California.
Ironically, once the sun goes down, NOx destroys ozone, causing ozone levels in the valley to drop. But here in the foothills, we don’t produce enough NOx to scavenge the ozone, and ozone levels hardly drop overnight.
Ozone conditions representative of the county are measured on the outskirts of Grass Valley, and can be obtained, in almost real time, at http://www.sparetheair.com. Click on current conditions, then click on Grass Valley, then scroll down. The numbers on the graph are crowded together, so give yourself a moment to read them.
Alan Stahler enjoys sharing nature with students of all ages. His science stories can be heard on radio station KVMR (89.5 FM), and he may be reached at email@example.com.
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