『Abstract
Nitrogen fixation has been reported at numerous active volcanoes.
Where it occurs, it should enhance nitrogen bioavailability in
the local environmental and reactive nitrogen chemistry in the
troposphere. Measurements of reactive nitrogen species in volcanic
emissions are highly variable, in part due to an incomplete assessment
of a diverse speciation (e.g., NO, NO2, HNO3, NO3- and NO2HO2), but suggest typical
molar ratios of NOy/H2O〜10-5.
These high levels of reactive nitrogen species may be explained
by equilibrium in high temperature (＞1000℃) mixtures of magmatic
and atmospheric gases. However, using a kinetic model for a simplified
magmatic and atmospheric gas mixture (i.e., the C-O-N-H-Ar system),
we show that the rate of thermal nitrogen fixation is too slow
for equilibrium to be attained in transient mixtures forming at
volcanic vents. Other mechanisms for high NOy/H2O should therefore be considered including the
catalysis of nitrogen fixation (e.g., by metal salts or silicate
particles). Our results also suggest that H2
and CO should oxidise extremely rapidly (＜10-3 s at
1100℃) at volcanic vents, conflicting with recent measurements
indicating the stability of these reduced species. We therefore
propose that high temperature oxidation at volcanic vents is a
sporadic process due to variability in the mixture lifetime (with
time and across the mixing front between magmatic and atmospheric
gases). This variability would result in minimal losses of H2 and CO while still allowing oxidised trace species
(e.g., NO, OH, Br, Cl) to form in significant concentrations and
thus influence the chemical evolution of the volcanic plume at
ambient temperatures.』
1. Introduction
2. Methodology
2.1. Previous measurements of reactive nitrogen in volcanic
emissions
2.2. Chemical modelling
3. Results and discussion
2.1. Reactive nitrogen species in volcanic emissions
3.2. Nitrogen fixation at volcanic vents
3.2.1. Thermodynamic model results for O-N-H-Ar chemistry
3.2.2. Kinetic model results for O-N-H-Ar chemistry
3.3. On the reactivity of H2, CO and H2S at volcanic vents
4. Conclusions
Acknowledgments
Appendix A
References