Understanding the evolution of atmospheric nitrous oxide over the last century from the stable isotopes of the firn air at Styx Glacier, East Antarctica

Abstract

<p>The increase in mixing ratio of greenhouse gas (GHG) has been believed to be the primary driver for the ongoing global warming. Among the GHGs, the mixing ratio of nitrous oxide (N₂O) has increased by 23% since 1750 CE. N₂O has a long residence time of ca. 120 years, and a potential to destruct the ozone layer. The Global Warming Potential of N₂O is about 300 times greater than that of CO₂ over 100 years. However, the temporal changes in magnitude and geographic distribution of different N₂O sources are uncertain, hence, understanding the dynamics of atmospheric N₂O has been a challenge to the researcher during the last few decades. Here, we present new stable isotope data of N₂O from the firn air at Styx Glacier, East Antarctica to comprehend the atmospheric evolution for the last 100 years. Our results show that the N₂O mixing ratio has increased, whereas the &#948;¹⁵N<em>^bulk</em> (&#8240;, AIR) and &#948;¹⁸O (&#8240;, VSMOW) values decreased during the last 100 years, consistent with the existing firn air records. The progressive increase in the N₂O mixing ratio and the decrease in the isotope ratios suggest a higher contribution from the anthropogenic sources assuming that the N₂O flux from the natural sources is constant. Our box model analysis using the stable isotopes and mixing ratio data of N₂O of Styx firn air suggests that anthropogenic N₂O emission at 2014 CE was ca. 37.5% higher than 1919 CE. The box model calculation with Styx and other firn air and ice core data suggests that in comparison to the pre-industrial era, the total N₂O emission is ca. 61% higher at present (2014 CE), where ca. 62% and 38% contributions are from natural and anthropogenic sources, respectively to the total N₂O emission. The isotope-based mass-balance calculation indicates that continental emission was ca. 45% higher in 2014 CE than in 1919 CE. Although there is a large scatter in existing data, the site preference of ¹⁵N in N₂O molecules (&#948;¹⁵N<em>^SP</em> &#8240;, AIR) shows an increasing trend during the post-industrial era, which is consistent with the idea that enhanced fertilization increased soil N₂O emissions by activating nitrification processes.</p>