Why is there an ozone hole in the atmosphere when there is too much ozone at ground level?

—H. Cox, San Antonio, Tex.

Ross J. Salawitch, a senior research scientist at NASA's Jet Propulsion Laboratory in Pasadena, Calif., explains:

The abundance of atmospheric ozone (O3) is relative—levels that are dangerously high in the atmosphere's lowest layer, the troposphere, would be dangerously low in the stratosphere, one layer above. As such, ground-level ozone is not plentiful enough to fill the so-called ozone hole. In addition, ozone is regulated primarily by local chemical processes, and a temperature barrier at the troposphere-stratosphere border prevents a large-scale mixing of ozone across atmospheric levels.

Stratospheric ozone provides a shield from harmful ultraviolet solar radiation. Conversely, elevated levels of tropospheric ozone can lead to human health problems and damage to crops and forests.

Natural processes continuously produce and remove ozone throughout the atmosphere. In the stratosphere, ultraviolet sunlight breaks down some molecular oxygen (O2); the separate oxygen atoms then recombine with another oxygen molecule to form ozone.

Some industrial pollutants, such as chlorofluorocarbons (CFCs), are able to reach the stratosphere because they are nonreactive in the troposphere. Eventually they break down into molecules such as chlorine monoxide (ClO), which depletes stratospheric ozone by transforming it back into molecular oxygen.

Stratospheric ozone levels are typically about 400 Dobson units (DU), the standard unit of ozone concentration. Every spring over Antarctica, extremely cold conditions enable chemical reactions that produce very high levels of ozone-destroying ClO. Within the Antarctic ozone hole, levels can drop to 85 DU.

Typical tropospheric ozone levels are only about 25 DU but depend greatly on local conditions. Natural ozone production in the troposphere is inefficient because the intensity of ultraviolet sunlight is very low. Human activities such as fossil-fuel combustion and biomass burning lead to elevated levels of carbon monoxide, hydrocarbons and nitrogen oxides—gases that participate in a series of chemical reactions that produce tropospheric ozone.

The Montreal Protocol has banned production of CFCs throughout the world, and the stratospheric ozone layer is expected to recover over the next 50 to 100 years. Efforts are being undertaken to implement emission-control strategies that will limit tropospheric ozone to less than prescribed levels. These initiatives are challenged by global industrialization and the fact that tropospheric ozone is affected by pollutants emitted both locally and from distant upwind sources, even other countries or continents.

How does catnip work its magic on cats?

Ramona Turner, a longtime veterinarian who owns two Fresno, Calif.–based animal hospitals, replies:

From our small domestic companions to the largest lions, cats are exquisitely susceptible to nepetalactone, a compound in the stems and leaves of the catnip plant that essentially acts as an artificial feline pheromone.

Nepetalactone in the catnip enters the animal's nasal tissue, where it is believed to bind to protein receptors that stimulate sensory neurons. These cells trigger a cascade of responses that eventually reach the amygdala (two neuronal clusters in the midbrain that mediate emotional responses to stimuli) and the hypothalamus (the brain's “master gland” that plays a role in regulating everything from hunger to emotions). The amygdala integrates the information flow, and its neurons project to areas governing behavioral responses. The hypothalamus regulates neuroendocrine responses through the pituitary gland, creating a sexual response to the artificial pheromone.

When cats smell catnip, they exhibit several telltale behaviors: they may rub their heads and body on the herb or jump, roll around, vocalize and salivate. This response lasts for about 10 minutes. About 70 to 80 percent of cats demonstrate this hereditary reaction to the plant's presence but not before they are a few months old.

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