Interesting fact: Air pollution can look faint or hazy, but over a large city the total mass of particles and gases suspended in the air can add up to many tons.
The answer: Pollution in the air can range from trace amounts to many tons over a city, depending on what you measure and over what area. Fine particles such as PM2.5 are often measured in micrograms per cubic meter, but when summed across an urban air column the total can become surprisingly large.
Pollution is difficult to weigh because it is spread unevenly, constantly moving, and made of many different substances. Scientists estimate its mass from concentration measurements, wind profiles, and modeled air volumes. The weight matters because it helps turn invisible health risk into something concrete for policymakers, engineers, and the public.
