The smell of emissions in London, with the worst scents of fuel, gasoline and dust in the darkest red. Daniele Quercia, Rossano Schifanella, Luca Maria Aiello, Kate McLean; designed by Rossano Schifanella

Data scientists at the University of Cambridge explore how urban odors can guide better city design.

What can we learn from pausing, deeply inhaling and smelling—yes, smelling—our cities? A new paper from University of Cambridge researchers argues that while urban planners and policymakers have a lot to say about road diets, the sharing economy, and housing policy, too little attention has been paid to urban smellscapes, those scents that emanate from and so influence urban life.

So the scientists, led by researchers Daniele Quercia and Kate McLean, got to work. First, they took local volunteers on a series of “smellwalks,” jaunts around seven global cities during which participants identified distinctive urban smells. From this exercise, the researchers created a comprehensive urban smell dictionary, including less pleasant odors (“exhaust,” “manure,” “trash,” “putrid,” and “vomit” among them) and downright lovely-sounding ones (“lavender,” “fruity,” “BBQ,” and “baked,” for example).

Then they turned to Big Data. Zooming in on just two cities—London and Barcelona—the researchers searched for their designated smell words (and their Spanish equivalents) in geotagged posts on Twitter, Flickr, and Instagram. In all, their sample included 17 million Flickr images, 436,000 Instagram posts, and 1.7 million tweets published and geotagged between 2010 and 2014.

Their results? Take a gander below. First up is Barcelona, with areas where social media users found the most emissions smells in red, and where they found the most nature smells in green:

(Quercia, Schifanella, Aiello and McLean)

Notice that the green “nature” words are concentrated around Barcelona’s three big parks: Parc de Montjuïc towards the south, Parc de la Ciutadella in the center of the city, and Park Güell in the north. The red “emissions” words, meanwhile, are found in social media postings centered around the city’s main thoroughfares: Avinguda Diagonal and Avinguda Meridiana.

Next up, London, where the same color key applies:

(Quercia, Schifanella, Aiello and McLean)

The pattern follows here, too: Nature words abound in postings from Regent’s Park, Hyde Park, and Green Park, while emissions words are found in postings from the Strand and Park Lane, to the north of the Thames.

So what exactly do these maps tell us? The researchers discovered that there is a high correlation between areas with poor air quality and the areas in which social media users detected emissions smells like “gasoline,” “dusty,” “exhaust,” and “car.” Conversely, air pollutants were less present in areas where social media users detected nature smells: “floral,” “lavender,” “grass,” and “sulphur.” The human nose is a powerful thing.

The researchers envision these maps being used in a variety of ways. Urban planners, they suggest, can use them to figure out which areas of the city smell the worst—and then consider using air-flow manipulation, green spaces, and pedestrian-friendly streets to change them. Maybe computer scientists will one day create a wayfinding app that gives users the most pleasant-smelling path to their destination. Or maybe city officials will be inspired to use social media data to more consistently monitor how their residents are being affected by smells—and by the pollution that creates it.

Earlier this year, my colleague Tanvi Misra wrote about how the 19th century’s foul stenches shaped modern cities. But the Cambridge researchers take pains to point out that not all of today’s urban smells are bad. “Cities are victims of a discipline’s negative perspective—only bad odors have been considered,” they write in a press release. “The researchers hope that their work will disrupt this negative view and, as such, we will all be able to celebrate the complex smells of our cities.”

Bring on the “BBQ” smells!

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