Ah, breathe in that alpha pinene and bornyal acetate. You can just tell that Christmas is right around the corner.

Those are the two chemicals that give pine its distinctive odor, and because people are just as likely these days to get their pine scent from an aerosol or a candle as from the real thing, they can thank chemists in the fragrance industry for figuring out how to synthesize those molecules.

The role of those chemists has become even more important as an increasing number of Americans switch to artificial Christmas trees. And if they want a pine scent to go with the tree, they are unlikely to get it from the tree itself, even though the technology exists to make scented plastic, said Craig Warren, former research director for International Flavors and Fragrances.

"It's very, very difficult to sell a scented product like that," Dr. Warren said, "because you'll find a subset of people who hate the odor, and you'll find another subset who don't want any odor in their house."

For those who like the aroma, keeping it strong past the first year is a challenge, he said.

The big artificial scent companies obviously are ready to capitalize on that.

Febreze, a Procter & Gamble line, is running ads now for its three holiday air sprays, including Fresh Evergreen and Snow. Not to be outdone, SC Johnson Co.'s Glade is offering a piney air freshener and candle fragrance called Glistening Snow.

Like apple, cranberry and cinnamon, pine is definitely a cold-weather, seasonal fragrance, said Ray Rodriguez, a technical director at The Waterbury Cos., which makes aerosol air fresheners for restrooms.

Thanks largely to Harry Cole, pine also is associated with cleanliness.

Mr. Cole was the Jackson, Miss., chemist who invented the formula for Pine-Sol, the cleaner that's still going strong 77 years later.

The use of pine oil in a detergent is not just a marketing gimmick, though, said Jim Turner, a consultant and editor of the newsletter Forest Chemicals Review.

Natural pine oil has germ-killing properties, Mr. Turner said, and, in the tree itself, the chemicals in the oil help control insect and fungus infestations.

In fact, when he worked at Hercules Chemical Co. Inc., many of its employees swore by pine oil as an ointment to relieve aching joints.

Pine oil was also the original ingredient in most pine-scented products, said Dr. Warren, although today, synthetic production of its signature chemicals makes the use of real oil more limited.

The most expensive version of pine oil, Siberian pine needle oil, at $50 a pound, is limited to high-end men's cologne, which uses it for the "fresh notes," he said.

Synthetic pine scent, which can be made for less than $10 a pound, is what goes into many cleaning products, Dr. Warren said. Synthetic pine chemicals, he said, are used more often to create tiny fractions of other scents than to make a product that has a predominant pine smell.

The ability to mimic pine or any other natural scent with synthetic chemicals comes from a technique known as "headspace analysis," said Mark Crames, CEO of Demeter Fragrance Library in Long Island.

It involves putting a natural substance into a vacuum chamber and analyzing the chemicals it emits with gas chromatography and mass spectrometry.

That's how his company came up with its two pine scents, Christmas Tree and Fraser Fir, Mr. Crames said.

The Christmas Tree aroma is a little more "piney" and "greeney" than the Fraser Fir scent, he said.

The vagueness of his terms points to one of the major challenges in the fragrance industry.

Most natural aromas are so complex chemically that it's impossible to duplicate them exactly with synthetics, Mr. Crames said. But with scents such as pine and fir, "if I find the main ingredients and I get close enough, your nose won't be able to tell the difference."

That approach has allowed Demeter Fragrances to create several specialty colognes that mimic both natural scents, such as grass, tomato and potting soil, and artificial ones, such as vinyl, baby powder and Play-Doh.

In the pine arena, Dr. Warren added, a big breakthrough for fragrance chemists was discovering that a touch of synthetic ambergris, the natural version of which comes from the intestines of sperm whales, could create the darker scent of balsam firs.

The associations we make with those scents, "Christmas" and "Clean," come almost entirely from our culture, not our genes, according to Rachel Herz, a Brown University psychology researcher who specializes in olfactory cognition.

With few exceptions, aromas mean only what we were taught, she said.

That means, if someone grew up in a household where Christmas always meant fighting and disappointment, the aroma of pine could be painful.

Fortunately for the fragrance industry and our holiday moods, pine scent, for most people, evokes "the forest and that very ionized air that energizes us," Mr. Crames said.