Ozone is ubiquitous in the earth's atmosphere from the ground to 50 kilometers, most of which are concentrated in the stratosphere 10-30 kilometers from the ground, and only about 10% exist in the troposphere closer to the ground. Ozone has diametrically opposite effects on humans in the stratosphere and troposphere. The ozone in the stratosphere can absorb solar ultraviolet radiation and protect the earth's biosphere. The ozone in the troposphere is not only a greenhouse gas, but also the main component of photochemical smog, which can participate in the photochemical smog triggered by atmospheric photochemical reactions. Therefore, ozone has the title of being a Buddha in the sky and a demon in the earth.

When will ozone pollution occur?

Ozone pollution is not caused by direct emissions. It is caused by the atmospheric photochemical reaction of nitrogen oxides (NOx) and volatile organic compounds (VOCs) entering the atmosphere under sunlight, which is a secondary air pollutant. The level of ozone concentration is closely related to meteorological elements such as radiation, temperature, humidity, and boundary layer height.

Ozone pollution often occurs in late spring and summer and autumn when there are sunny and little clouds, because sufficient light (strong radiation), high temperature, and low humidity conditions help the occurrence of photochemical reactions and cause ozone pollution. This leads to obvious seasonal and regional characteristics of ozone pollution. In China, ozone pollution is mainly concentrated in the late spring and summer when the sun is strong, the temperature is high, and the cloud cover is low. In the south, it mostly occurs in the late spring, early summer and autumn.

In addition, because the generation of ozone depends on sunlight, the ozone concentration has obvious diurnal changes in the day, generally high during the day and low at night. In order to accurately analyze the changes in ozone, experts have reached a conclusion by using ozone sensors. The ozone concentration is low in the early morning. As the sunlight strengthens, the reaction of precursors such as NOx and VOCs accelerates, and the ozone concentration gradually rises, generally reaching the highest value of the day from 14:00 to 17:00 in the afternoon. Afterwards, it gradually decreases until the solar radiation weakens in the evening, and the ozone concentration returns to a low state. At night, ozone will decompose by itself or react with the reducing substances in the air to be eliminated, and the concentration will be reduced to a minimum.

Finally, for health considerations, it is recommended that sensitive people try to avoid outdoor activities in the afternoon when the sun is strong, the temperature is high, and the humidity is low.