Stefan Versick

NO y production, ozone loss and changes in net radiative heating due to energetic particle precipitation in 2002–2010

We analyze the impact of energetic particle precipitation on the stratospheric nitrogen budget, ozone abundances and net radiative heating using results from three global chemistry-climate models considering solar protons and geomagnetic forcing due to auroral or radiation belt electrons. Two of the models cover the atmosphere up to the lower thermosphere, the source region of auroral NO production. Geomagnetic forcing in these models is included by prescribed ionization rates. One model 5 reaches up to about 80 km, and geomagnetic forcing is included by applying an upper boundary condition of auroral NO mixing ratios parameterized as a function of geomagnetic activity. Despite the differences in the implementation of the particle effect, the resulting modeled NOy in the upper mesosphere agrees well between all three models, demonstrating that geomagnetic forcing is represented in a consistent way either by prescribing ionization rates or by prescribing NOy at the model top. Compared with observations of stratospheric and mesospheric NOy from the MIPAS instrument for the years 2002–2010, 10 the model simulations reproduce the spatial pattern and temporal evolution well. However, after strong sudden stratospheric warmings, particle induced NOy is underestimated by both high-top models, and after the solar proton event in October 2003, NOy is overestimated by all three models. Model results indicate that the large solar proton event in October 2003 contributed about 1–2 Gmol (10 mol) NOy per hemisphere to the stratospheric NOy budget, while downwelling of auroral NOx from the upper mesosphere and lower thermosphere contributes up to 4 Gmol NOy. Accumulation over time leads to a constant particle15 induced background of about 0.5–1 Gmol per hemisphere during solar minimum, and up to 2 Gmol per hemisphere during solar maximum. Related negative anomalies of ozone are predicted by the models nearly in every polar winter, ranging from 10– 50% during solar maximum to 2–10% during solar minimum. Ozone loss continues throughout polar summer after strong solar proton events in the Southern hemisphere and after large sudden stratospheric warmings in the Northern hemisphere. During mid-winter, the ozone loss causes a reduction of the infrared radiative cooling, i.e., a positive change of the net radiative heating 20 (effective warming), in agreement with analyses of geomagnetic forcing in stratospheric temperatures which show a warming in the late winter upper stratosphere. In late winter and spring, the sign of the net radiative heating change turns to negative (effective cooling). This spring-time cooling lasts well into summer and continues until the following autumn after large solar proton events in the Southern hemisphere, after sudden stratospheric warmings in the Northern hemisphere. 1 Atmos. Chem. Phys. Discuss., https://doi.org/10.5194/acp-2017-514 Manuscript under review for journal Atmos. Chem. Phys. Discussion started: 22 June 2017 c

The Influence of Energetic Particles on the Chemistry of the Middle Atmosphere

Energetic particle precipitation (EPP) during solar and geomagnetic active periods causes chemical disturbances in the lower thermosphere and in the middle atmosphere. Additional HOx (H, OH, HO2) and NOx (N, NO, NO2) are produced in the middle atmosphere, and enhancements of NOx produced in these events can be transported to the winter stratosphere. These trace species take part in ozone chemistry and, by chemical-radiative coupling, the dynamical state in the middle atmosphere can be altered. There is evidence both from observations and from chemistry-climate models that the EPP induced signal in the middle atmosphere may then propagate into the troposphere. Thus particle precipitation could connect to possible climate effects. The first step in this functional chain is the impact of EPP on the chemical composition in the middle atmosphere and lower thermosphere, and the downward transport in the polar winter middle atmosphere. The general objective of this project was to assess quantitatively the chemical composition change in the middle atmosphere by combining model simulations and observations. The study relays mainly on the observations of the MIPAS instrument on the ENVISAT satellite, whose data set has been expanded in the context of this project by a newly developed retrieval of the gas H2O2, a reservoir for the members of the HOx family. Simulations have been carried out with the two chemical transport models CLaMS and KASIMA, which cover chemistry and transport effects in the stratosphere up to the mesosphere/lower thermosphere region. The impact on the global NOy budget and (the resulting) total ozone change are assessed in these studies. In addition, the ion reaction mechanism for the conversion of N2O5 to HNO3 based on positive ion chemistry was refined. The detailed comparison of model results and observation for the SPE 2003 showed that models can simulate the impact of EPP on ozone chemistry but deficiencies exist for some minor species.