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E-book Agricultural Meteorology and Climatology
The Earth, our home planet, was “born” some 4.5 billion years ago. Vol-canic and hot, the planet was rotating and cooling. In volcanic eruptions, gases such as CO2, CO, NOx and water vapour were released. Additionally, some atmospheric components including water (vapour) came from the Earth’s bombardment by meteors and comets. After a solid surface was formed whose temperature cooled down far below 100 oC, liquid water remained on Earth and initiated the formation of the world’s oceans. The first ocean was the only place on the young Earth capable of harbouring life. On its bottom, some 3.5 (but certainly no less than 2.7!) billion years ago, emerged Cyanobacteria or blue–green algae, the first microbes which produce oxygen by photosynthesis. As a result of dif-ferent geological and chemical processes, life–giving oxygen released to the first atmosphere additionally reduced the (ultraviolet) UV radiation that reached the ocean’s surface. It helped the first life forms to survive in shallower water and, finally, to reach the surface. After more than 2 billion years of very slowly rising through the water, the first land plants evolved on Earth 700 million years ago. Today, the Earth’s atmosphere consists of essentially the same gases as in remote times but in proportions that have changed throughout the ages. The atmosphere is a mixture of permanent gases, aerosols and trace gases, and solid and liquid particles. Permanent constituents of the atmosphere with constant concentration are: nitrogen (N), oxygen (O2) and argon (Ar) (Tab. 1.1), while water vapour (H2O), carbon dioxide (CO2) and ozone (O3) are gases with variable concentrations (Tab. 1.2). Let us point out that water vapour is an atmospheric component whose content changes the most over time and space. Another gas, whose changes are not so great but which troubles us even more because of its specific and very important role, is ozone. Ozone (O3) effects intensive absorption of biologically harmful UV radiation in the stratosphere, thereby shield-ing living organisms On the other hand, because of its strong oxidizing properties, direct contact with O3 is harmful both to plants and humans. his is the dual nature of O3, usually described as “good” and “bad” O3, with the latter represented by tropospheric O3. Near the surface, O3 is created in complex photochemical reactions involving NO2 and hydrocar-bons,most of which are volatile organic compounds (VOCs). Tropospheric O3 has the strongest impact on plants. In highly polluted air, O3 will cause more damage to plant tissue than all other pollutants together. Together with other gases, ozone enters leaves through open stomata, where it at-tacks cells as a strong oxidant. Exposure symptoms include visible lesions on the leaf surface, reduced growth and a hypothesized reduction in the allocation of carbohydrates to roots.
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