D-region ion–neutral coupled chemistry (Sodankylä Ion Chemistry, SIC) within the Whole Atmosphere Community Climate Model (WACCM 4) – WACCM-SIC and WACCM-rSIC

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 HO_x and NO_x 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 NO_x, HO_x 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 (NO_x), odd hydrogen (HO_x) and O₃ in the stratosphere and mesosphere.