Nitrous oxide in the atmosphere: First measurements of a lower thermospheric source

Abstract

Nitrous oxide (N2O) is an important anthropogenic greenhouse gas, as well as one of the most significant anthropogenic ozone‐depleting substances in the stratosphere. The satellite‐based instrument Atmospheric Chemistry Experiment‐Fourier Transform Spectrometer has been observing the Earth's limb since 2004 and derives profiles of N2O volume mixing ratios in the upper troposphere to the lower thermosphere. The resulting climatology shows that N2O is continuously produced in the lower thermosphere via energetic particle precipitation and enhanced N2O is present at all latitudes, during all seasons. The results are consistent with an N2O production source peaking near or above 94 km via low‐energy particles, as well as a polar wintertime source near 70 km via medium energy particles. N2O produced in the polar upper atmosphere descends each winter to as far down as ~40 km.

An Explanation for the Nitrous Oxide Layer Observed in the Mesopause Region

“…Longer simulations with such models could also provide quantification of the impact of upper atmospheric N 2 O production on total stratospheric ozone destruction. Based on typical conversion efficiencies of N 2 O to NO y around an SSW event, Sheese et al (2016) estimate the upper limit of this to be around 2%.…”

“…Sheese et al () provided first measurements of what appears to be this previously overlooked source, using v3.5 of the ACE‐FTS data. Compared to the data available to Semeniuk et al (), the altitude limit for N 2 O was increased from 60 to 94.5 km by employing less conservative microwindow sets.…”

“…However, they acknowledged the potential for a significant contribution to enhancements from the lower thermosphere via the mechanism described by Zipf and Prasad (1982). Sheese et al (2016) provided first measurements of what appears to be this previously overlooked source, using v3.5 of the ACE-FTS data. Compared to the data available to Semeniuk et al (2008), the altitude limit for N 2 O was increased from 60 to 94.5 km by employing less conservative microwindow sets.…”

Assessing stratospheric transport in the CMAM30 simulations using ACE-FTS measurements

“…For example, a source of N 2 O in the lower thermosphere has recently been identified in ACE-FTS measurements by Sheese et al (2016). The N 2 O source descends into the mesosphere and stratosphere, thereby influencing air that is circulated in the BDC (Sheese et al, 2016). The 5 transport of enhanced N 2 O downwards from the upper atmosphere has also been detected by Funke et al (2008a, b).…”

“…However, this propagation doesn't occur to the same extent in the N 2 O comparisons, which may be a reflection of the differences in the chemistry of the two tracers. For example, a source of N 2 O in the lower thermosphere has recently been identified in ACE-FTS measurements by Sheese et al (2016). The N 2 O source descends into the mesosphere and stratosphere, thereby influencing air that is circulated in the BDC (Sheese et al, 2016).…”