The smell of a place at a particular moment could be said to follow the same rules as any perfume, but with a far greater degree of complexity. Typically, a perfume is composed of some head notes (bright smells that dissipate quickly), heart notes (heavier molecules that define the overall smell), and a bottom note (a smell such as sandalwood, which doesn’t dominate a fragrance but may stay on your clothes for days). On a midsummer afternoon after a light rain, the smell of my block in Brooklyn might include petrichor as a heart note. But that would only be the simplest part: there would be head and heart notes of gasoline, rotting food, fallen leaves, pollen, flowers, car exhaust, a great variety of feces from organisms sharing the space, and perhaps bottom notes from the soil, grime, and soot ground into pavement, with an occasional contribution from the East River when the wind is right. My own Wythe Avenue smell-of-rain moment would compare to an isolated attempt at synthetic petrichor about as well as a Big Ten marching band to a tin whistle.