Ground-level ozone is highly reactive, oxidizing organic tissues and contributing to photochemical smog. Nitrogen oxides (NOx) and volatile organic compounds (VOCs) are key ingredients for its formation. NOx is derived from combustion sources like car engines, power plants, and fires. VOCs come from various sources, including anthropogenic and biogenic emissions. When UV, NOx, and VOCs combine, ozone and other oxidants are generated. Daily variations in these ingredients lead to fluctuating ozone levels in urban areas. The chemical processes involve photolysis and reactions between NO and NO2. This reactive cycle can lead to the rapid buildup of ground-level ozone and the formation of additional radicals. Millions of Americans are living in areas with ozone levels exceeding safety standards.

We all know that ozone is very important in our atmosphere - it protects us from the dangers of UV light from the sun. 

But at ground level, it is very different and very problematic. It is very reactive and attacks the double-bonds that we find in biological molecules, oxidizing organic tissues, such as in the lining of human or animal lungs, plant walls, and also in non-organic materials too.

It has a short lifespan, because it is so reactive, and so it is entirely uncoupled from that which we find in the upper troposphere and stratosphere - the long vertical mixing times in our atmosphere mean they never connect.

Ozone is a major component of what we know as photochemical smog. The key ingredients are UV light, nitrogen oxides, known as NOx, volatile organic compounds - simply small organic molecules in their gas phase.

So where do these ingredients come from?

We derive NOx mainly through combustion at high temperatures - N2 and O2 are both widely present in the atmosphere; during combustion they combine to give us 2NO. 

Car engines are a major contributor - the internal combustion engine draws in air and so generates NOx at high temperatures - diesel engines are hotter, so they produce more. Much is also produced from powerplants, any fires - wildfires or domestic, even lightning. Inevitably, NOx in the US tracks population centers - the Bay Area in the west and and the northeastern conurbations being the worst.

VOCs come from many different sources. Anything organic in a  gas phase (which normally corresponds to anything you can smell) is likely to be a source. We have many anthropogenic VOCs - solvents, gasoline, benzine, small hydrocarbons, such as propane. We also have many industrial sources, such as gas tanks, dry-cleaning chemicals, and biogenic sources, such as isoprenes produced by trees and plants (deciduous trees are the biggest emitters of biogenic VOCs). 

So, together, when we bring together UV, NOx and VOCs we generate ozone and other oxidants. We see daily changes of the ingredients in typical urban areas. 

Diurnally, hydrocarbons increase rapidly and then diminish as they react with oxidants. NOx tends to peak in the morning - associated with the morning rush hour - and then a slightly less pronounced peak in the afternoon. The resulting oxidants predictably peak slightly later than their ingredients.

So what is the chemical process at work?

Well, the null cycle is when NO2 is emitted, then the molecule is broken down in sunlight - photolyzed. This splits the O from NO2 giving you NO and O. This O is extremely reactive and will combine with O2 to form ozone. The ozone can then react again with NO to form NO2. 

What breaks this null cycle at ground level is that we have organic compounds around that also oxidize NO into NO2 - allowing NO2 to be reformed which can then photolyze and give you additional ozone. In other words, you can transition NO into NO2 without following the ozone pathway - allowing for rapid buildup on ozone concentration at ground level. 

What is worse, ozone can also get photolyzed, react with water vapor in the air. You then get hydroxyl radicals which can keep reacting with VOCs, making the compounds into new radicals.

The R’s in this formula do not necessarily relate to one specific compound, but to thousands or organic compounds, both biogenic and anthropogenic. Ozone is just one of these many that contribute to photochemical smog. 

The point here is reactivity. All these radicals are extremely reactive - a hydrocarbon radical will combine with O2 very readily (which constitutes 20% of the air). We are describing a daily chain reaction with potentially catastrophic consequences. 

In 2021, more than 90 million Americans lived in areas where ozone levels exceeded the national air quality standards threshold safety level of 70 parts per billion over an 8-hour period.