Lightning: Surprisingly Important
Both N2 (N≡N) and O2 (O=O) tie up all their otherwise solitary electrons, meaning they are unavailable for reactions. This means no reaction would naturally occur between N≡N and O=O. This lack of reactivity is an essential property of our stable atmosphere. But we do need NO, and then NO2, to act on Earth’s soil as the key ingredients for nitrates (NO3-) and nitrites (NO2-). And this N2 consumption had to happen while still preserving the atmosphere, so recycling N2 and O2 cycles had to be promptly in place. How to solve this chemical paradox of needing both chemical stability and eventual reactivity? The solution: lightning. Lightning is a spectacular pyrotechnic show of light and sound and has always fascinated men. It is so awe-inspiring that humans throughout history have associated lightning with the anger of gods. Lightning is caused by sudden flows of electric charge between charged clouds or between the ground and a charged cloud. Lightning strikes on Earth happen an estimated fifty to a hundred times per second on average, well over a billion a year. Scientists still debate the exact mechanisms of lightning, but generally believe that water freezes in clouds at temperatures ranging from 5o F to −13o F, forming ice crystals that collide with water droplets. In the process the ice crystals become positively charged, and the slushy mix of ice and supercooled water becomes negatively charged. The lighter, positively charged ice crystals tend to accumulate near the top of clouds, and the heavier, negatively charged ice-water mix, near the bottom. When the charged cloud passes over the Earth, it induces an opposite charge in the Earth below, and a natural capacitor is formed. Eventually, a cloud to- cloud or cloud-to-ground discharge occurs through the N2 plus O2 mixture that forms our atmosphere. In this way, lightning provides enough energy to break the triply bonded (and thus hard to break) N≡N molecule to form single, highly reactive nitrogen atoms. The nearly inert nitrogen molecules are thus turned into reactive nitrogen atoms in our troposphere. The nascent nitrogen atoms react with O=O to form NO + O, and NO is in turn rapidly oxidized to NO2. It has been estimated that a flash of lightning produces about 4 × 10^26 molecules of NOx (NO plus NO2), or about forty kilograms. Next, all of the major atmospheric ions—N+, N2 +, O+, O2 +, and NO2 +—rapidly transfer charge to NO to produce NO+, so lightning’s final product is NO+. Thunder clouds hold enormous amounts of electric energy—enough to overcome the high activation energy for the N2 + O2 reactions that makes NO and then NO2 and on to NO2 - and NO3 - anions in the soil. This cycle that lightning helps drive isn’t just helpful. As David Fowler and his colleagues explain, “The global nitrogen cycle is central to the biogeochemistry of the Earth, with large natural flows of nitrogen from the atmosphere into terrestrial and marine ecosystems through biological nitrogen fixation,” and back to the atmosphere. Biological nitrogen fixation (BNF) and lightning, which reduce unreactive molecular N2 into NH3, NO2 -, and NO3 - and then to N-containing chemicals, provide fixed-nitrogen forms that, again are “subsequently transformed into a wide range of amino acids and oxidized compounds by micro-organisms, and finally returned to the atmosphere as molecular nitrogen through microbial denitrification in soils, fresh and marine waters and sediments.” And emission of N2O in the wake of denitrification “plays a key role in the radiative balance of the Earth and in the chemistry of the ozone layer in the stratosphere, where N2O is destroyed by photolysis,” a chemical process that breaks molecules into smaller units via light absorption. Biological nitrogen fixation and the production of NOx by lightning are the solution to sources of new reactive nitrogen in our biosphere. A steady supply of reactive nitrogen is crucial not just for agriculture but for all life forms. Although the quantity of reactive nitrogen from lightning is believed to be more than an order of magnitude smaller than that from biological nitrogen fixation today, lightning is a key player in the nitrogen cycle and is important for forming ozone and maintaining the oxidation capacity of the atmosphere. Without lightning to make NO from the reaction of N2 with O2, there would be no life. And keep in mind that clouds, the intricate properties of changing phases, and the charge separation for ice crystals all result from the strong chemical forces that hold H2O molecules together, namely their polarity and unique H-bonding properties. In other words, we need both lightning and charged aqueous clouds with billions of kilowatts in electrical power, or again, no life.