Aromatic ads.

You're standing at a bus stop, sniffing the city air. Suddenly, you take a whiff of lemon-lime. What's going on?

Executives at Young & Rubicam, an ad agency, are tickling your senses with a sweet-smelling ad, hoping you'll get a craving for a fruit drink called "Citrus Twist Batik." They've placed the ads at 10 busy bus stops in London, England.

But the fruit-filled posters don't smell; it's the air around the bus stop, says Leigh O'Connell, an account manager at Y & R. When you enter the bus shelter, you pass through an infrared sensor that triggers a scent-releasing machine atop the bus shelter, she says.

The sensor is actually a harmless beam of invisible "light." When you step in front of it, your body blocks the path of the beam. That action turns on small, powerful fans in a metal box overhead. These blow air over a lemon-lime-soaked pad and a fruity mist wafts through a vent.

"It's not a strong smell that will knock you down," says O'Connell. "It's very light. You can almost taste it." And that's the idea. The ad agency hopes that scent will send people racing out to buy Citrus Twist Batik.

One glitch: Past smell studies in stores show that when a company sprays a pleasant scent into the air, consumers linger longer to inhale the aroma--but they don't necessarily spend more money.

Another drawback: location, location, location. "I don't think launching the ad at a bus stop is the best idea," says Rachel Herz, a smell researcher at Monell Chemical Senses Center in Philadelphia. For one thing, how long do you linger at a bus stop?

In addition, says Herz, smells evoke strong associations. For example, the smell of homemade cookies may remind you of happy times at your Grandma's house. If commuters associate the lemon-lime scent with pushy passengers, long waits, and bus fumes, chances are they won't buy Citrus Twist Batik.

What do London commuters have to say about the ad? At least one had a strong reaction: "It smells like my husband's armpits," " she said.