24 March 2006

Ozone - The Pollutant That Got Away

By Rusty Rockets

Like kids in a candy store, we humans love to exploit the resources that nature has on offer, but this, as we are becoming painfully aware, comes at an immense cost. As we pump out pollutants and clear-fell vast tracks of wilderness in the name of progress, the pressure that we put on the Earth's ecosystems threatens every species continued existence on this planet. Late last century, scientists realized that while everyone smelled wonderful and had their hair perfectly coiffed, the chlorofluorocarbons (CFCs) found in many aerosols were contributing to an expanding hole in the ozone layer. The ozone layer is a good example of just how precariously balanced the Earth's systems really are, as ozone is both essential and detrimental to life. While people may justly agonize over the problems associated with the hole in the ozone layer positioned high in the stratosphere, most people don't realize that too much ozone at ground-level is just as deadly. One study found that even extremely low levels of ozone, the principal ingredient in smog, can have serious health implications.

The ozone found in the high-altitude regions of the stratosphere filters out the ultraviolet (UV) radiation that we know is responsible for sunburn and skin cancer. It was British physicist Sidney Chapman, who in 1930 discovered that when UV light hits an oxygen molecule (O2), the rays separated the two oxygen atoms, producing what is known as atomic oxygen. Ozone (O3) is formed when atomic oxygen goes on to bond with other oxygen molecules. As an allotrope of oxygen, ozone's 3-atom structure is not as stable as its 2-atom oxygen counterpart. As a result, free radical catalysts, such as CFCs (an organohalogen compound) produced can break down ozone molecules causing ozone depletion, which in turn allows an increase of UV rays hitting the Earth.

While ozone at higher altitudes is clearly beneficial, the ozone found within the Earth's troposphere (the lowest region of the atmosphere that comprises 75 percent of its mass) is a dangerous pollutant and greenhouse gas. At normal levels, ozone is one component of many that produce ongoing chemical reactions and processes that contribute to a functioning atmosphere. While naturally occurring background levels of ozone are acceptable, higher levels, caused by fossil fuel combustion, result in it becoming a pollutant. As with stratospheric ozone, tropospheric ozone pollutants are the result of photochemical processes. The culprits this time are nitrogen oxides (NOx), carbon monoxide (CO) and other unstable organic compounds that produce ozone when exposed to sunlight. The source of all these chemicals can be traced back to what are called ozone precursors, such as exhaust fumes, industrial emissions and other man-made chemical compounds. Worryingly, it's possible that a depleted stratospheric ozone layer will allow greater UV penetration that in turn, may exacerbate the tropospheric ozone pollutant problem. What's more, the effects of these highly concentrated levels of ozone are not localized, as the pollution can be carried many hundreds of miles away from its original source. The prognosis for human health in both scenarios is decidedly grim.

The added concern, only recently realized, is that because of ozone's high volatility, scientists cannot assume the presence of an evenly distributed level of ozone in the troposphere. Studies released by Harvard scientists in early 2004, confirmed fears that the US's Environmental Protection Agency (EPA) were overestimating background levels of ozone. In the Journal of Geophysical Research - Atmospheres, atmospheric chemist Arlene Fiore argued that: "Our results actually indicate that the EPA is overestimating the background level, and as a result is underestimating the health risk associated with ozone pollution." Fiore's team also claimed that: "results from our modeling study also indicate that frequent springtime high-ozone events, which were previously attributed by some researchers to a natural, stratospheric source, are driven largely by pollution." The team showed that ozone levels are highly variable depending on what area was being tested, so when asked if a sliding scale should be implemented to measure background ozone levels, the team's response was a resounding "yes!" "Our modeling study shows that background ozone concentrations in surface air are highly variable, and this variability in background ozone - and its associated risk level - should be taken into account," said Fiore.

As a more recent study funded by the EPA shows, there is little margin for error when measuring ozone levels, as only a tiny rise above background levels is enough to impact health. "Any anthropogenic contribution to ambient O3, however slight, still presents an increased risk for premature mortality," said co-author of the study, Francesca Dominici. The study found that even a 10 part-per-billion increase in the average of the two previous days' ozone levels is associated with a 0.30 percent increase in mortality. The authors of the study claim that: "tropospheric ozone is a common urban area pollutant linked to numerous harmful health effects, including reduced lung function, increased frequency of respiratory symptoms and development of asthma." Even short-term ozone exposure, say the authors, is linked to health problems, but they add that the: "exposure-response curve for ozone remains unknown." Dominici explains that: "more than 100 million people in the US live in areas that exceed the current health-based US National Ambient Air Quality Standard (NAAQS) for ozone." But even as number crunchers within the US add up these less-than-cheery figures, Dominici adds that developing nations are pumping out ozone precursors by the ton, as their transportation networks rapidly grow.

All of this has wider implications for global climate change. Recently, scientists at the NASA Goddard Institute for Space Studies (GISS) evaluated how ozone in the troposphere has contributed to warming in specific regions of the world over the past 100 years. They found that ozone was responsible for a staggering one-third to one-half of observed warming in the Arctic region during winter and spring. The team say that ozone is carried from industrialized regions in the Northern Hemisphere to the Arctic most efficiently during these particular seasons.

These latest findings, say the GISS team, add some extra incentive for authorities to tackle urban air pollution more seriously. "We now see that reducing ozone pollution can not only improve air quality, but also have the added benefit of easing climate warming, especially in the Arctic." Ozone is now one of several air pollutants regulated in the United States by the EPA but whether it's too-little, too-late, remains to be seen.