Air pollution is a global issue; therefore, it is important to have some metrics and standards to help governments and scientists to know how bad the pollution is and by how much. More detailed measurements of the air also help identify the major pollutants, which can then tell us something about which sources need to be under better control. By having a standard metric, countries can also tell if their pollution regulations are actually working. The air quality index (AQI), usually reported by region or city, is used let the public know how polluted the air is and will be.

Each country has its own way of calculating an AQI, but generally, most are based on some combination of ozone (O₃), particulate matter (PM), nitrogen oxides (NO_x), sulfur dioxide (SO₂), carbon monoxide (CO) and volatile organic compounds (VOCs). Not every country, or city, has enough air quality monitoring stations to measure the air quality equally across all neighborhoods. This is really clear when we look at this map showing the real-time air quality for more than 10,000 stations in the world, which also shows which countries are monitored more than others.

Tropospheric Ozone Chemistry
The main ingredient needed to form ozone in the troposphere, where we live, is sunlight, on top of hydrocarbons and nitrogen oxides, or in this case, NO_x (sum of NO and NO₂). However, the balance between all these ingredients is not a simple relationship. Having [X] amounts of hydrocarbons and [Y] amounts of NO_x doesn’t automatically mean that [X+Y] amounts of O₃ will be formed. There are limits to how much a compound can influence a reaction because there could be so much of that compound it no longer matters how much more is there. Therefore, in this complex system, there are situations in which less NO_x in the system will actually lead to more O₃. This was observed in big cities like Rome, where the stay-at-home orders during the Covid pandemic suddenly reduced the amount of traffic pollution (NO_x) which caused an increase in the amount of O₃ in the atmosphere. Understanding this complicated relationship is important when policy makers try to make regulations that help reduce O₃ pollution.

There are many interconnected factors that drive complex chemistry in the atmosphere. Check out this Ted-Ed Lesson to learn more about the science of smog.