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High Ozone Days in USA

This map shows how stratospheric contribution to U.S. surface ozone peaks in the western Rockies during late spring -- from May to June.
The Earth’s protective ozone layer, which exists 9.3 to 18.6 miles above the surface and shields us from getting too much of the Sun’s UV radiation, is a really good thing, which is why we’re glad that the hole caused by human-generated chlorofluorocarbons is finally closing up
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But when too much ozone is present at ground level where we can breathe it in, it’s a really, really bad thing. The chemical can cause all sorts of respiratory woes, ranging from difficulty in drawing deep breaths to inflamed airways, and it can aggravate problems such as asthma and emphysema. It’s particularly tough on kids and older people.

Normally, ground-level ozone gets high when sunlight alters chemical compounds in the exhaust from cars and trucks. But in a cruel paradox of nature, certain weather patterns also can cause the good ozone from the stratospheric ozone layer to drift down to the Earth’s surface in some places, causing hazardous levels.
That’s the bad news. The good news is that scientists have figured out a way to predict that sort of high-ozone day, and to do it several months in advance. That’s because they’ve discovered a connection between high ozone levels in the western United States and La Niña, an ocean-atmosphere temperature phenomena that affects global weather patterns.
In a newly-published study in Nature Communications, a team led  by Meiyun Lin of NOAA’s Geophysical Fluid Dynamics Laboratory and NOAA’s cooperative institute at Princeton University report that after the three most recent La Niña events — 1998-1999, 2007-2008 and 2010-2011 — they saw spikes in ground level ozone for periods of two to three days at a time during late spring in high altitude locations of the U.S. West.

“Ozone in the stratosphere, located 6 to 30 miles (10 to 48 kilometers) above the ground, typically stays in the stratosphere,” . “But not on some days in late spring following a strong La Niña winter. That’s when the polar jet stream meanders southward over the western U.S. and facilitates intrusions of stratospheric ozone to ground level where people live.”
That’s huge, because it would enable public health authorities to warn vulnerable people and provide them with information on how to protect themselves.

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