Articles | Volume 16, issue 13
https://doi.org/10.5194/acp-16-8667-2016
https://doi.org/10.5194/acp-16-8667-2016
Research article
 | 
15 Jul 2016
Research article |  | 15 Jul 2016

A semi-empirical model for mesospheric and stratospheric NOy produced by energetic particle precipitation

Bernd Funke, Manuel López-Puertas, Gabriele P. Stiller, Stefan Versick, and Thomas von Clarmann

Abstract. The MIPAS Fourier transform spectrometer on board Envisat has measured global distributions of the six principal reactive nitrogen (NOy) compounds (HNO3, NO2, NO, N2O5, ClONO2, and HNO4) during 2002–2012. These observations were used previously to detect regular polar winter descent of reactive nitrogen produced by energetic particle precipitation (EPP) down to the lower stratosphere, often called the EPP indirect effect. It has further been shown that the observed fraction of NOy produced by EPP (EPP-NOy) has a nearly linear relationship with the geomagnetic Ap index when taking into account the time lag introduced by transport. Here we exploit these results in a semi-empirical model for computation of EPP-modulated NOy densities and wintertime downward fluxes through stratospheric and mesospheric pressure levels. Since the Ap dependence of EPP-NOy is distorted during episodes of strong descent in Arctic winters associated with elevated stratopause events, a specific parameterization has been developed for these episodes. This model accurately reproduces the observations from MIPAS and is also consistent with estimates from other satellite instruments. Since stratospheric EPP-NOy depositions lead to changes in stratospheric ozone with possible implications for climate, the model presented here can be utilized in climate simulations without the need to incorporate many thermospheric and upper mesospheric processes. By employing historical geomagnetic indices, the model also allows for reconstruction of the EPP indirect effect since 1850. We found secular variations of solar cycle-averaged stratospheric EPP-NOy depositions on the order of 1 GM. In particular, we model a reduction of the EPP-NOy deposition rate during the last 3 decades, related to the coincident decline of geomagnetic activity that corresponds to 1.8 % of the NOy production rate by N2O oxidation. As the decline of the geomagnetic activity level is expected to continue in the coming decades, this is likely to affect the long-term NOy trend by counteracting the expected increase caused by growing N2O emissions.

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Short summary
We present a semi-empirical model for the reconstruction of polar winter descent of reactive nitrogen (NOy) produced by energetic particle precipitation (EPP) into the stratosphere. It can be used to prescribe NOy in chemistry climate models with an upper lid below the EPP source region. We also found a significant reduction of the EPP-generated NOy during the last 30 years, likely affecting the long-term NOy trend by counteracting the expected increase caused by growing N2O emission.
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