Biologia

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Bacteria responsible for missing 'atmospheric brooms' that sweepthe sky clean

19 August 2011

Hydroxyl radicals play a central role in cleaning pollutants from our atmosphere - but the ultimate source of Earth's 'atmospheric brooms' has proven difficult to track down. An international team of researchers have now found that the answer could lie not in the air above us, but in the ground beneath our feet. 
Previous research has established that up to onethird of the hydroxyl radicals formed in the lower atmosphere come from the photochemical breakdown of nitrous acid (HONO). But here the trail ran cold, the source of much of this HONO unknown. Yafang Cheng at Peking University in Beijing, China, Hang Su at the Max Planck Institute in Mainz, Germany, and their colleagues, have now shown that it can come from soil. 
The team looked to the soil afterprevious research showed that the ground could act as a surface on which the chemical conversion of nitrite into HONO can take place. 'We thought that this reaction was not strong enough to explain the missing source - but that there might be some other process to release HONO from soil directly,' says Cheng. 
To test out their idea, the team placed soil samples inside a Teflon chamber, andflushed it with synthetic air, measuring HONO concentrations in the air by the chamber exit. They found that the soil was indeed releasing HONO - and at a rate high enough for soil to be the missing source of the gas in the atmosphere, the team concluded. 

Soil bacteria produce chemicals that help to cleanse the atmosphere |
Although it hadn't been considered as a source of atmospheric HONObefore, the basic chemistry taking place in the soil is well known. Soil microbes produce nitrite ions, which are known to react reversibly with protons to give aqueous nitrous acid. This can then partition into the gas phase and enter the atmosphere. The more acidic the soil, and the more heavily fertilised it is, the more HONO enters the atmosphere, the researchers say. However, alkaline soils andnatural environments can also produce the gas. 
Importantly, the process can also account for the characteristic diurnal fluctuation of HONO levels in the air, says Cheng. Models of atmospheric chemistry processes show that HONO production peaks at midday - a fact which tallies with the temperature dependence of the equilibrium processes involved in HONO emission from the soil. 
Barbara...