Dance of the Air Pollutants

I wrote this blog for ClimaCell on how ozone and NO₂ change within a day. Reposted from ‘ClimaCell Blog’

The most romantic part of the air pollution mechanism (i.e., atmospheric chemistry) is the dance between ozone (O₃) and nitrogen dioxide (NO₂) on the surface of Earth. During this late night air molecule tango one pollutant takes a step forward, the other takes a step back. Following the rhythm set by the sun, they take turns leading the dance. As the sun rises, ozone plays the leading role. When dusk falls, O₃ yields to NO₂.

We video-captured this dance in Los Angeles on October 29 2019, between midnight and 11:00 pm. The top two panels depict the concentrations of O₃ and NO₂ in parts per billion (ppb). Background colors are archives of real-time pollutant concentrations from ClimaCell, and the colored circles are the data from ‘AirNow’ (a governmental air pollution data platform).

Roughly, when O₃ exceeds 70 ppb or NO₂ exceeds 100 ppb, it means bad air quality – based on ‘National Ambient Air Quality Standards’. The lower panel is the diurnal cycle of the two pollutants. The x-axis ranges from 00:00 to 23:00 local time, and y-axis represents the pollutant concentrations in parts per billion.

During the wee small hours, O₃ gracefully drops to below 5 ppb and regains its spirit at around 9:00 am, peaking at over 40 ppb around 14:00. Meanwhile, NO₂ drops after 9:00 am and reaches its nadir around 14:00.

Reads like a great partnership, doesn’t it?

So what’s the trick? Stratospheric O₃ (good ozone protecting us from UV) high above us is produced from oxygen (O₂) directly. But in the lower troposphere where we live, ozone is bad and can be produced by robbing one oxygen atom from NO₂.

That’s why NO₂ stays low during the daytime. The chemical reaction is:

O₂ (oxygen) + NO₂ + hv (sun light) → O₃+ NO (nitrogen monoxide)

When the sunlight’s gone and night comes, O₃ proves to be most generous and gives the oxygen atom back to NO₂. Fresh NO is still produced during the nighttime, via fossil fuel combustion. NO grasps the oxygen, and sometimes leaves a little O₃ behind. Hereby, we see very low levels of O₃ during the nighttime.

The take-away formula is:

O₃ + NO → O₂ + NO₂

One good turn deserves another…