Of all the questions we get, this one might be the most common.
It’s counter intuitive – how can NO₂ levels be low during rush hour and peak hours afterwards?
The answer is in atmospheric chemistry.
Atmospheric chemistry of NO₂
It’s not unusual for NO₂ measurements on warm and sunny days to be very low. When NO and NO₂ are present in sunlight, ozone formation occurs because of the effect of solar radiation on NO₂. The high-energy rays of the sun rip away an oxygen atom from the NO₂ molecule, forming NO, and this oxygen atom then joins an O₂ molecule to form ozone. Warm and sunny days often show very low NO₂ levels while NO and ozone have higher concentrations.
At night, there’s no light to split the NO₂, and as a result, the atmospheric chemistry of this pollutant is entirely different. Any NO present at night will react rapidly with ozone boosting the levels of NO₂ after nightfall.
Seasonality
These effects are most common in spring and summer but can occur all year round, especially when the weather is clear and warm. As the days grow shorter through the seasons, the emissions peak due to traffic will come closer to nightfall until it’s completely at dark during the winter holidays. The combination of nighttime chemistry with peak emissions is why we see peak concentrations of NO₂ during the winter.
For more info, we’d recommend Atmospheric Chemistry and Physics by Seinfeld and Pandis, which was used to inform this article.