Ammonium Nitrate Chemistry

Fake Plastic Trees

Air pollution is a big deal - it kills millions annually - but we aren't going to make progress in dealing with it by planting fake plastic trees in the middle of the world's cities.

When you attempt to reduce air pollution it is much easier to do it at the concentrated source - and easier yet to stop these compounds being formed all together - than it is to do once they've been released and diluted to one part per million or billion.  But yet again we see another story / press release about a tech startup trying to do just this.

The Crown Estate claim to have planted a small number of their CityTrees in several first world cities - cities with air quality issues, yes, but not on the level of their counterparts in the developing world.  These devices, like many others out there, are designed to suck in dirty air and purify it. They then release clean air to mix back in with the urban plume, where it can modestly dilute the remaining pollutants.  When you try and picture the vast amount of air that occupies a city's atmospheric boundary layer, and the machinery that would be required to process a significant fraction of this on a daily basis, you quickly see that you are on a fool's errand.  Just visualise having a clean counterpart to the exhaust pipe of every car on the road, and you get some idea of the scale of the problem.  Moreover, blowing hot air isn't free, it requires electricity.  Even if these devices work they are just going to export pollution back to the electricity source.  Furthermore, devices such as the CityTree which claim to capture carbon from air, will need to produce many tonnes of carbon in some form or another, and this needs to be handled.

Another bugbear I have with these stories is that they tend to misrepresent how trees actually interact with our atmosphere.  Trees, we're all taught, draw in carbon dioxide and release it as oxygen for us to breathe.  But thermodynamics tells us that, chemically, we need energy to make this happen. This is why trees undergo photosynthesis: to capture solar energy.  If you aren't consuming energy then it isn't going to work.  If you are generating solar electricity to cleanse the air of pollution generated by fossil fuels, you can never be as efficient as just using that electricity to mitigate the pollution directly.

It wears me out.