Inevitably, Quora also has this: “Why does New York City’s subway smell like an open sewer?”

Scent diffusion first caught on with hotels and casinos trying to mask cigarette smoke. “The quickest way to change mood or behavior is with smell,” said Dr. Alan R. Hirsch, the neurological director of the Smell and Taste Treatment and Research Foundation in Chicago. “You’ll smell something you like and will immediately be in a happier mood state.”

The opposite is also true, he said. If it’s something an ex-spouse wore, “it may induce a more negative or hostile mood.”

Research has shown, he said, that a mixed floral scent increases the speed of learning and baked goods can make people nostalgic for childhood. Other research on mood and behavior has indicated that scents can prompt people to spend more money, or at least to spend more time in a store — the “linger-longer factor,” marketers call it.

Officials for the company that developed the observatory’s scent , which also came up with a scent for the Samsung store in the Time Warner Center in Manhattan, said scents can make customers spend 40 percent more time strolling down store aisles, looking at products.

Scents drift through airports, sports stadiums and, more recently, tourist attractions like museums. The New Museum, on the Bowery, sells two scents created to smell like the galleries. Each bottle, which contains little more than an ounce, sells for $157.

Last month the Louvre Museum in Paris introduced eight scents at its gift shop, not in the galleries. They were inspired by works of art in the museum “to allow every visitor to take away with them a little piece of the Louvre,” a spokeswoman for the Louvre, Jeanne Scanvic, said in an email. She added that the Louvre was “a palace built by ancient French kings, so you have a sort of ‘natural’ scent” from the wooden floors and thick, old walls.

As for the observatory, Leslie Vosshall, a professor at Rockefeller University and an investigator at the Howard Hughes Medical Institute who oversaw one of the largest human smell studies, wondered if some visitors would have an emotional reaction about Sept. 11. She said that she had not gone to the trade center site since the attacks on the twin towers.