Seasonal Patterns and Trends of N2O Isotopes Observed in the Unpolluted Atmosphere
Abstract
Atmospheric nitrous oxide (N2O) levels have been steadily growing at almost 1 ppb per year for the past few decades. Increasing levels of N2O have raised worldwide concerns, due to its strong greenhouse gas effect and stratospheric ozone-depleting potential. Although the rapid growth of global atmospheric N2O has been mainly attributed to anthropogenic sources such as agricultural practices, only limited information is available from past studies on geographic distribution and temporal variability of N2O sources.
Recent developments in isotope analytics have provided important constraints for distinguishing N2O sources and studying their variability. However, limitations still exist due to the insufficient precision of isotopic measurements for the background atmosphere and the coarse temporal resolution obtained with available air samples. In this study, applying a recently developed laser spectroscopy system coupled to an upstream preconcentration unit, we present a unique dataset of atmospheric N2O isotope samples collected weekly to biweekly at the high-altitude research station Jungfraujoch, Switzerland. Over a period of five years, off-line isotopic measurements were accompanied by on-site continuous measurements of N2O mixing ratios. We observed a distinct seasonal pattern showing a minimum in N2O mixing ratios during late summer, associated with a maximum in δ15Nbulk and a minimum in intramolecular 15N site preference (SP) of N2O. Supported by atmospheric transport modelling to analyse the recent surface influences on sampled air and regional mole fraction simulations, we propose that the observed seasonal pattern of N2O mixing ratio and δ15Nbulk mainly reflects stratosphere-troposphere exchange, which transports N2O-depleted but isotopically enriched air into the troposphere. On the contrary, seasonal variation of SP seems to be driven by biogeochemical processes responsible for N2O emissions in late summer. Global box model simulations and deseasonalized trends in N2O isotopes both confirm that isotopically light anthropogenic sources are responsible for current N2O growth in the atmosphere. Ongoing work will include further isotopic measurements of archived air from Cape Grim (Tasmania) and firn air samples from Eastern Greenland to constrain global trends in atmospheric N2O.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.B13L2482Y
- Keywords:
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- 0414 Biogeochemical cycles;
- processes;
- and modeling;
- BIOGEOSCIENCES;
- 0428 Carbon cycling;
- BIOGEOSCIENCES;
- 0469 Nitrogen cycling;
- BIOGEOSCIENCES;
- 0490 Trace gases;
- BIOGEOSCIENCES