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Abstract:
Nitrous acid (HONO) is an important precursor of the hydroxyl radical (OH), the atmosphere ́s primary
oxidant. An unknown strong daytime source of HONO is required to explain measurements in ambient
air. Emissions from soils are one of the potential sources. Ammonia-oxidizing bacteria (AOB) have
been identified as possible producers of these HONO soil emissions. However, the mechanisms for
production and release of HONO in soils are not fully understood. In this study, we used a dynamic soil-
chamber system to provide direct evidence that gaseous emissions from nitrifying pure cultures contain
hydroxylamine (NH
2
OH), which is subsequently converted to HONO in a heterogeneous reaction with
water vapor on glass bead surfaces. In addition to different AOB species, we found release of HONO
also in ammonia-oxidizing archaea (AOA), suggesting that these globally abundant microbes may also
contribute to the formation of atmospheric HONO and consequently OH. Since biogenic NH
2
OH is
formed by diverse organisms, such as AOB, AOA, methane-oxidizing bacteria, heterotrophic nitrifiers,
and fungi, we argue that HONO emission from soil is not restricted to the nitrifying bacteria, but is also
promoted by nitrifying members of the domains
Archaea
and
Eukarya
.
