Constraining the atmospheric N2O budget from intramolecular site preference in N2O isotopomers

Abstract

Nitrous oxide (N2O) is an important trace gas in the atmosphere. It is an active greenhouse gas in the troposphere and it also controls ozone concentration in the stratosphere through nitric oxide production. One way to trace the geochemical cycle of N2O is by measuring the natural abundance of stable isotopes, namely 15N and 18O (refs 2-15). Here we report the intramolecular distribution of 15N within the linear NNO molecule, determined by measuring molecular and fragment ions of N2O on a modified mass spectrometer. This revealed a preference for 15N at the central N position, or alpha-site, within N2O isotopomers (isotope-containing molecules). Moreover, this preference varied significantly throughout the atmosphere. In the troposphere, low alpha-site preference indicates local emission of N2O from soils and fossil-fuel combustion, each with distinct isotopomer signatures, which then mixes with background N2O. In the stratosphere, on the other hand, loss of N2O is observed as enhanced alpha-site preference for 15N, due to fractionation during ultraviolet photolysis of N2O. We have constructed an atmospheric mass balance of N2O, incorporating isotopomer abundance, which shows that the intramolecular distribution of 15N is a parameter that has the potential to increase significantly the resolution with which sources and sinks of N2O can be identified and quantified in the atmosphere.

Continuous measurements of nitrous oxide isotopomers during incubation experiments

“…The positions in the N 2 O molecule have been named N α and N β or short α and β (Yoshida and Toyoda, 2000). N 2 O molecules with 15 N substitution in the central and terminal positions are named 15 N α for 14 N 15 N 16 O and 15 N β for 15 N 14 N 16 O, respectively.…”

Isotopic signatures of N2O produced by ammonia-oxidizing archaea from soils

“…In addition to d 15 N bulk -N 2 O and d 18 O-N 2 O, Yoshida and Toyoda (2000) suggested that analyses of the intramolecular distributions of 15 N in N 2 O (N b ¼ N a ¼ O) (isotopomers), which are often expressed as the site preference (SP) (d 15 N a -d 15 N b ), may provide critical information that could help identify the precise sources and sinks of this greenhouse gas. Site preference has emerged as a potential conservative tracer for microbial N 2 O production because 1) it is independent of the isotopologue composition of the substrates and 2) it does not exhibit changes during the course of production.…”

Supersaturated N2O in a perennially ice‐covered Antarctic lake: Molecular and stable isotopic evidence for a biogeochemical relict

“…These observations suggest that an additional, reductive source of N 2 O exists at the base of the chemocline at about 19-20 m. Above this zone the isotopic signatures of N 2 O increase toward the ice cover (Fig. 1C), suggesting an influence of atmospheric N 2 O, which is characterized by d 15 N < 7%, d 18 O < 44%, and SP < 19% (Kim and Craig 1993;Yoshida and Toyoda 2000). An atmospheric source of N 2 O most likely results from the seasonal advection immediately beneath the ice cover of low-density atmospherically equilibrated glacial-melt water into the lake (Matsubaya et al 1979;Spigel and Priscu 1998).…”

“…Large kinetic nitrogen isotope effects are associated with NH (Yoshida 1988;Casciotti 2002;Casciotti et al 2003). These isotope effects would be expected to produce N 2 O with a d 15 N value that is significantly lower than that of the source NH to NH 2 OH and NO followed by reduction of the NO intermediate (Naqvi 1991), rather than from reduction of NO { 2 , but the kinetic isotope effects associated with NO oxidation and reduction are not established .…”

“…The extent of this depletion is greater than anticipated for either nitrification or denitrification. However, no known nitrogen cycle process results in a more extreme 15 N depletion than does nitrification (Yoshida 1988), and kinetic isotope effects associated with nitrification are known to vary between strains of AOB and possibly with environmental conditions. Importantly, within the N 2 O maximum from 25-27 m, the SP of N 2 O is 31.1% 6 2.7% (mean 6 SD).…”