Abstract
Abstract. This study presents a new ion–neutral chemical model coupled into the Whole Atmosphere Community Climate Model (WACCM). The ionospheric D-region (altitudes ∼ 50–90 km) chemistry is based on the Sodankylä Ion Chemistry (SIC) model, a one-dimensional model containing 307 ion–neutral and ion recombination, 16 photodissociation and 7 photoionization reactions of neutral species, positive and negative ions, and electrons. The SIC mechanism was reduced using the simulation error minimization connectivity method (SEM-CM) to produce a reaction scheme of 181 ion–molecule reactions of 181 ion–molecule reactions of 27 positive and 18 negative ions. This scheme describes the concentration profiles at altitudes between 20 km and 120 km of a set of major neutral species (HNO3, O3, H2O2, NO, NO2, HO2, OH, N2O5) and ions (O2+, O4+, NO+, NO+(H2O), O2+(H2O), H+(H2O), H+(H2O)2, H+(H2O)3, H+(H2O)4, O3−, NO2−, O−, O2, OH−, O2−(H2O), O2−(H2O)2, O4−, CO3−, CO3−(H2O), CO4−, HCO3−, NO2−, NO3−, NO3−(H2O), NO3−(H2O)2, NO3−(HNO3), NO3−(HNO3)2, Cl−, ClO−), which agree with the full SIC mechanism within a 5 % tolerance. Four 3-D model simulations were then performed, using the impact of the January 2005 solar proton event (SPE) on D-region HOx and NOx chemistry as a test case of four different model versions: the standard WACCM (no negative ions and a very limited set of positive ions); WACCM-SIC (standard WACCM with the full SIC chemistry of positive and negative ions); WACCM-D (standard WACCM with a heuristic reduction of the SIC chemistry, recently used to examine HNO3 formation following an SPE); and WACCM-rSIC (standard WACCM with a reduction of SIC chemistry using the SEM-CM method). The standard WACCM misses the HNO3 enhancement during the SPE, while the full and reduced model versions predict significant NOx, HOx and HNO3 enhancements in the mesosphere during solar proton events. The SEM-CM reduction also identifies the important ion–molecule reactions that affect the partitioning of odd nitrogen (NOx), odd hydrogen (HOx) and O3 in the stratosphere and mesosphere.
Highlights
Energetic charged particles that impact on the Earth’s atmosphere come from several different sources: in the case of protons, directly from the Sun during solar proton events (SPEs) and from outside the Solar System in the form of high-energy Galactic cosmic rays; in the case of electrons, from the radiation belts around the Published by Copernicus Publications on behalf of the European Geosciences Union.T
A sudden decrease in the rms error is observed after a long stagnation when those species are added which close the pathways; their early inclusion would not give a smaller reduced mechanism, as other species would be missing from the pathway
Bold-face-font species indicate the initial set of important species, while the others are the necessary species selected through the reduction process
Summary
Energetic charged particles that impact on the Earth’s atmosphere come from several different sources: in the case of protons (and some heavier ions), directly from the Sun during solar proton events (SPEs) and from outside the Solar System in the form of high-energy Galactic cosmic rays; in the case of electrons, from the radiation belts around the Published by Copernicus Publications on behalf of the European Geosciences Union.T. Protons with energy above 1 MeV can penetrate down to the mesosphere and the upper stratosphere, at high geomagnetic latitudes. EPP causes ion-pair formation, and the subsequent neutralization produces odd nitrogen (NOx = N + NO + NO2) and odd hydrogen (HOx = H + OH + HO2) species. The NOx and HOx species destroy mesospheric ozone via catalytic cycles (Crutzen, 1970; McElroy et al, 1992), which can have a significant impact on the radiative balance of the middle atmosphere and on climate (Sinnhuber et al, 2012)
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