We have had several days of very warm, sunny, dry weather, but today there are a few dark clouds about.
Just now, as I let the cat out, the smell of ozone hit me full-on. I know the smell of ozone because my Master’s degree had me piping it into greenhouses, to test the effect on trees. It’s a pungent, fresh smell, slightly bleach-like, sharp and tangy on the air. And I often smell it before a storm comes.
This isn’t the ozone we have in the stratosphere, the ‘ozone layer’, which helps protect us from the sun’s UV radiation, and has a few, er, holes in it. What I can smell is ground-level – tropospheric – ozone, created in the air and wafted down to us on air currents.
Ground-level ozone is created by chemical reactions between nitrogen oxides and volatile organic compounds (mostly from fossil fuels), in the presence of sunlight. Or by lightening during a storm. At ground level it is considered a pollutant; it forms a major component of smog. It also has a detrimental effect on the photosynthetic abilities of plants (hence my studies), and can be detrimental to crop production. 100 – 125 parts per billion is generally considered a health hazard. We can smell it at 10 ppb.
So several days’ hot sun and the action of lightening created pots of ozone. Then the action of wind and rain pushed it downwards, where I can smell it. And the storm followed a little later.
The cat stayed in by the way, not at all impressed.
References & Further Reading