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 (NH2OH), 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 NH2OH 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.
Highlights
The photolysis of nitrous acid (HONO) yields the hydroxyl radical (OH), the primary oxidizing agent in the atmosphere, and thereby contributes significantly to the total daily primary OH production, in the extratropics
The biochemical mechanism of ammonia oxidation in ammonia-oxidizing archaeon (AOA), which is distinct from the bacterial pathway[19], exhibits lower KM values than those of ammonia-oxidizing bacteria (AOB) and enables AOA to thrive at extremely low ammonia concentrations[17]
Our study reveals that NH2OH released by microorganisms, especially under the conditions of increased permeability of cell membranes or cell death during dry-out, is decomposed in soils and yields HONO by a heterogeneous reaction on soil particles involving NH2OH and H2O
Summary
The photolysis of nitrous acid (HONO) yields the hydroxyl radical (OH), the primary oxidizing agent in the atmosphere, and thereby contributes significantly to the total daily primary OH production (up to 56%1), in the extratropics. Soil was shown to be an important source of HONO due to the partitioning of nitrous acid between the aqueous phase of soil and the gas phase[4]. Favorable conditions for microbial production (neutral-high pH) appear to be more important than favorable conditions for release (low pH) Despite these recent advances, and the insights in microbial pathways such as NH2OH oxidation to NO2−, the underlying mechanisms that govern the release and the precursor species for non-enzymatic HONO soil emissions remain largely unknown. An essential and reactive intermediate of nitrification is hydroxylamine (NH2OH) This species was shown to decompose to N2O10,11, which has been considered as a product of heterogeneous decomposition of HONO/NO2− on soil surfaces[12,13]. The HONO formation from the reaction of gaseous NH2OH on glass beads was observed with a LOng Path Absorption Photometer (LOPAP)
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