2. Angel in the sky, devil on the ground?

Ozone not only exists in the stratosphere but is also present at ground level. As oxygen molecules decrease and oxygen atoms increase with higher altitude, the high concentration of the ozone layer is found in the stratosphere while its lower concentration rests at the ground or higher above the atmosphere. That is to say, its concentration peaks at around 10 kilometers above the ground level and then drops in even higher altitude.

In the troposphere near the earth's surface, the natural concentration of ozone is about 0.02 to 0.06 ppm, which is harmless to the human being. As the concentration level grows, it can cause discomforts to human body and may even be harmful to eyes and the respiratory system. The FDA's maximum allowed ozone concentration in the air for residential areas is 0.05 ppm ozone by volume; the Japan Society for Occupational Health (JSOH) recommends the Occupational Exposure Limits (OELs) for ozone concentration is 0.1 ppm; while the China National Health Commission has set the safe ozone threshold as 0.1 ppm.

What really made ozone "notoriously famous" is the photochemical smog, which refers to a mixture of pollutants, including primary pollutants like nitrogen oxides (NOx) and volatile organic compounds (VOC), together with secondary pollutants ozone produced in the chemical reaction of UV ray. Although NOx and VOC are the primary source of photochemical smog, the share of ozone in the smog could reach as high as 90%. So people usually equate photochemical smog pollution with ozone pollution.

Photochemical smog not only stimulates mucosa tissues like eyes and respiratory system, it could also cause sore eyes, headaches, coughing and asthma. It could also inhibit plant growth that leads to crop failure, and even cause more hazards like acid rain and visibility reduction.

Since the industrial revolution, mass emission of NOx has led to an increase of ozone in the troposphere by 300% in the past 100 years. The rapid industrialization and urbanization in East Asia, led by China, has seen growing photochemical smog pollutants like Nox, which also accelerated increase of ozone concentration in the troposphere.

Although the concentration in the troposphere is merely a tenth of that in stratosphere, ozone is still the third largest contributor to global warming among all greenhouse gases, following carbon dioxide and methane.

All the factors above has led to a common belief that ozone is a harmful pollutant in the troposphere, and some even compare it "angel in the sky, devil on the ground." Several developed countries including Japan has made the observation and prevention of ozone cross-border pollution in the troposphere an important research topic.

However, it should be justified that the ozone in photochemical smog is at an unnaturally high concentration due to man-made pollution, much higher than the normal concentration of ozone in the troposphere. Moreover, unlike pure ozone in nature, photochemical smog consists of a large amount of hazardous pollutants like NOx and VOC. Concentration of ozone in nature varies by season and geography, but generally does not reach levels that can harm the human health. For example, one way ozone is naturally produced is through electrical excitation of oxygen molecules in lightening. Due to ozone's purification effect, the air is usually more refreshing after thunder and lightning. Another example would be the refreshing air in the coastlines and forest because of high ozone concentration.

Therefore, naturally produced ozone is anything but hazardous. We must recognize the difference between the naturally produced ozone and ozone in the photochemical smog, and should not blame it as a cause for environmental pollution.

Because of insufficient research and studies at this front, we often fail to recognize ozone as a protective shield in the troposphere.

Though harmless to big living creatures, ozone could pose serious threats to microorganisms. As a strong oxidizing agent, ozone has always been inhibiting microbe reproductions, while also acting as a balancing power to ecological equilibrium. Unfortunately, little attention has been given to its role in sanitation.

One reason is that low concentration of ozone was not believed to have sanitation values. According to a Japanese study, however, low concentration of ozone is still able to kill bacteria, viruses and molds, if given enough exposure. It is fair to say that ozone has balanced and inhibited the overgrowth and reproduction of microbes on earth.

Furthermore, naturally produced ozone can also decompose hazardous organics, stimulate the human's immune system, etc. Some researches even believe that it plays a vital role in reflecting to seasonal changes and controlling body function cycles. All in all, without ozone in the troposphere, the earth would be in a entirely different state, unfit for humanity to survive.

In fact, ozone is beneficial for human and nature in both troposphere and stratosphere. It is the man-made pollution that "demonized" ozone by breaking the natural balance on earth.

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