Isotopomeric characterization of N₂O produced, consumed, and emitted by automobiles

Abstract

Fossil fuel combustion is the second largest anthropogenic source of nitrous oxide (N2O) after agriculture. The estimated global N2O flux from combustion sources, as well as from other sources, still has a large uncertainty. Herein, we characterize automobile sources using N2O isotopomer ratios (nitrogen and oxygen isotope ratios and intramolecular site preference of 15N, SP) to assess their contributions to total global sources and to deconvolute complex production/consumption processes during combustion and subsequent catalytic treatments of exhaust. Car exhaust gases were sampled under running and idling state, and N2O isotopomer ratios were measured by mass spectrometry. The N2O directly emitted from an engine of a vehicle running at constant velocity had almost constant isotopomer ratios (delta15Nbulk = -28.7 +/- 1.2 per thousand, delta18O = 28.6 +/- 3.3 per thousand, and SP = 4.2 +/- 0.8 per thousand) irrespective of the velocity. After passing through catalytic converters, the isotopomer ratios showed an increase which varied with the temperature and the aging of the catalysts. The increase suggests that both production and consumption of N2O occur on the catalyst and that their rates can be comparable. It was noticed that in the idling state, the N2O emitted from a brand new car has higher isotopomer ratios than that from used cars, which indicate that technical improvements in catalytic converters can reduce the N2O from mobile combustion sources. On average, the isotopomeric signatures of N2O finally emitted from automobiles are not sensitive to running/idling states or to aging of the catalysts. Characteristic average isotopomer ratios of N2O from automobile sources are estimated at -4.9 +/- 8.2 per thousand, 43.5 +/- 13.9 per thousand, and 12.2 +/- 9.1 per thousand for delta15Nbulk, delta18O, and SP, respectively.

Constraining N<sub>2</sub>O emissions since 1940 using firn air isotope measurements in both hemispheres

“…However, the latest IPCC Assessment Report (Ciais et al, 2013) for the first time separated oceanic emissions into a natural component and an anthropogenic component, e.g., due to atmospheric N deposition to rivers (Syakila and Kroeze, 2011;Duce et al, 2008;Kroeze et al, 2005). The oceanic fraction of the anthropogenic source was estimated as 1 Tg year −1 N. N 2 O emitted from agricultural soils and biomass burning is more depleted in δ 15 N av and δ 18 O than the tropospheric background (Park et al, 2011;Goldberg et al, 2010;Ostrom et al, 2010;Tilsner et al, 2003;Pérez et al, 2001Pérez et al, , 2000, while N 2 O emitted from other minor sources, such as automobiles, coal combustion, and industry, has values closer to tropospheric N 2 O values (Syakila and Kroeze, 2011;Toyoda et al, 2008;Ogawa and Yoshida, 2005a, b). An increase in strongly depleted agricultural emissions in the first part of our reconstruction, followed by a decreasing relative contribution from agriculture and increasing contributions from more enriched sources like industry, automobiles, and coal combustion, could qualitatively explain the reconstructed changes in isotope signatures of both the total source and the anthropogenic component.…”

Characterization and production and consumption processes of N2O emitted from temperate agricultural soils determined via isotopomer ratio analysis

“…Reported values for N 2 O added after the Industrial Revolution, which are estimated from archived air analyses, are also shown. Grey ovals represent the reported range of other anthropogenic N 2 O sources; BB, biomass burning [ Ogawa and Yoshida , 2005a]; CC, coal combustion [ Ogawa and Yoshida , 2005b]; CE, car exhaust [ Toyoda et al , 2008].…”

Molecular hydrogen (H<sub>2</sub>) emissions and their isotopic signatures (H/D) from a motor vehicle: implications on atmospheric H<sub>2</sub>

“…Combined pre-and post-TWC measurements were generally not conducted except for the N 2 O study by Toyoda et al (2008). In their study, both isotopic enrichment and depletion were observed across the TWC, depending on the settings of the experimental variables, but in their case, the situation is complicated by both destruction and production of N 2 O in the exhaust stream.…”

“…For vehicle exhaust, progress has recently been achieved through isotopic studies of e.g. carbon monoxide, CO (Tsunogai et al, 2003), methane, CH 4 (Chanton et al, 2000;Nakagawa et al, 2005), and nitrous oxide, N 2 O (Toyoda et al, 2008). Here we present results from a study on a passenger car engine for which we have measured pre-and post-catalytic H 2 concentrations and -to our knowledge for the first time -H/D signatures under variable engine and fuel settings.…”