The city grime that builds up on urban sidewalks may be polluting the air we breathe

City dwellers, take note. New research suggests that the grime and muck coating our statues, sidewalks, and windows isn't just harmlessly sitting there.

When urban grime is zapped by sunlight, it may activate and release harmful components back into the air, making it more polluted than we originally thought.

James Donaldson, a physical chemist from the University of Toronto, analyzed how sunlight affects "urban grime" both in and out of the lab. He presented his work at the American Chemical Society meeting in Boston on August 17.

The muck he's looking at is everywhere in big cities. "Urban grime" is a blend of thousands of chemical compounds spewed from car exhaust and factories. Those compounds include nitrogen oxides, highly active gases that can react with other air pollutants and produce ozone - a lung-and-airway-damaging gas. Ozone also makes up the primary component of smog.

Scientists used to think that once nitrous oxides get caked and locked onto surfaces as grime, they no longer contribute to air pollution. But Donaldson's work has suggested otherwise.

His team previously showed that after exposing grime to artificial sunlight in a lab, the nitrates that were previously packed into the grime "disappeared" from the grime about 10,000 times faster than they did in a water-based solution that was also exposed to artificial sunlight. This told Donaldson that something about the combination of grime and sunlight made the nitrates hop out of the grime quickly.

In another study, they showed that grime exposed to artificial sunlight lost more nitrates than the grime left in the dark, suggesting that it's the light that somehow excites nitrogen in grime and converts it back into its active form that can float back into the atmosphere and react with other ozone-forming compounds.

"Most think of pollutants as being lost from the atmosphere onto surfaces ... and therefore they get removed from air pollution events," Donaldson said in a press conference. But his work seems to suggest that sunlight "recycles these compounds" and brings them back into active play in the atmosphere where they can go on to pollute another day.

To test this further, Donaldson collaborated with researchers in Germany and set up a 6-week field study in Leipzig. They set containers of grime-collecting glass beads on surfaces of buildings throughout the city, some in direct sunlight, some in the shade. All containers had adequate air flow to create maximal griminess.

After analyzing the composition of the grime, Donaldson's team found that the beads exposed to sunlight contained about 10% fewer nitrates than those in the shade, suggesting that the sunlight zapped the nitrates out of the grime and back into the atmosphere.

"If our suspicions are correct, it means that the current understanding of urban air pollution is missing a big chunk of information," Donaldson said in a press release.

The team is currently analyzing data from a similar year-long study in Toronto, and are working on setting up a study in Shanghai. They hope to eventually expand their studies from glass to more chemically-reactive surfaces, such as concrete, asphalt, and brick, to include a total city surface, Donaldson said in a press conference.

As this is the first study of its kind, the team also doesn't understand how the influence of relative humidity and local pollution will change nitrate activity levels in different regions, or how this changes our current estimates of total pollution levels in cities Donaldson said in a press conference.

All of this goes to say that we have a lot more to learn about how pollution works on a mechanistic level. But one thing is fairly certain: living in a city isn't great for your health or the environment, and this is just another study showing that.