What is the source of nitrous acid in the atmosphere? Mainz scientists study the exchange of gases between soil samples and the air in this reaction chamber. Photo: Hang Su, Max Planck Institute for Chemistry. |
Eutrophication
harms the environment in many ways. Unexpectedly, nitrogen fertilizer
may also be positive for the environment. And even acidic soils,
promoting the destruction of forests, can have a positive effect.
Researchers from the Biogeochemistry Department at the Max Planck
Institute for Chemistry in Mainz found out that nitrogen fertilizer
indirectly strengthens the self-cleaning capacity of the atmosphere.
Their
study shows that nitrous acid is formed in fertilized soil and released
to the atmosphere, whereby the amount increases with increasing soil
acidity. In the air, nitrous acid leads to the formation of hydroxyl
radicals, oxidizing pollutants that then can be washed out. Previously,
this nitrogen-effect has not been taken into account by geoscientists.
The gap has now been closed by the Max Planck researchers.
Our
air partly cleans itself as pollutants are being oxidized by hydroxyl
radicals and washed out by rain. Now, researchers at the Max Planck
Institute in Mainz and colleagues in Beijing have discovered the origin
of a bulk part of the nitrous acid that is acting beside ozone as a
source of hydroxyl radicals.
According
to their studies, large quantities of the acid are released into the
atmosphere from soil. In nitrogen-rich soils the acid is formed from
nitrite ions produced through microbiological transformations of
ammonium and nitrate ions. The more acidic the soil is and the more
nitrite it contains, the more nitrous acid is released. Through this
pathway some of the nitrogen in fertilized soil escapes into the air.
In
the latest issue of the journal Science, the Mainz researchers describe
how they demonstrated the existence of this previously unnoticed
pathway in the nitrogen cycle. They measured the concentration of HONO—a
chemical term for gaseous nitrous acid—that escaped from a defined
volume of arable soil. They added nitrite to a soil sample and varied
its water content. The quantity of released HONO closely matched the
researchers’ estimates based on acid/base and solubility equilibria.
Based on these findings they can also explain why previous studies had
measured high levels of HONO in the air above fertilized agricultural
soil.
In the future, soil-based HONO emissions may even increase on a world-wide basis.
The source of the high concentrations of HONO observed in the lower atmosphere had long been a mystery.
“Soil
is a complex system involving interactions between countless chemicals
and biological organisms,” says Hang Su, the lead author of the paper.
“Before us, no one seems to have investigated the soil-atmosphere
exchange of nitrous acid.”
The fact that soil emits HONO is not just locally, but also globally significant for air quality and the nitrogen cycle.
“Next,
we plan to work across disciplines with soil and climate researchers to
quantify the effect in different types of soil and under different
environmental conditions,” adds research group leader Ulrich Pöschl. The
findings will then be incorporated into a global model.
The
Max Planck researchers suspect that soil-based HONO emissions could
strongly increase especially in developing countries due to more
extensive fertilization, soil acidification, and climate-related rise in
temperature. This is expected to produce more hydroxyl radicals, which
increase the oxidizing power of the air.
Soil Nitrite as a Source of Atmospheric HONO and OH Radicals
