Model studies on the sensitivity of upper tropospheric chemistry to heterogeneous uptake of HNO₃ on cirrus ice particles

Abstract

A chemistry box model is applied to investigate potential chemical perturbations in the midlatitude upper troposphere caused by the reactive and nonreactive uptake of trace gases on cirrus ice particles. Chemical implications of denoxification caused by heterogeneous reactions with HNO 3 as a product and adsorption of gas phase HNO 3 on ice surfaces are the special focus. The role of denitrification due to gravitational settling of ice particles is investigated. The simulations suggest that cirrus cloud chemistry has the potential to induce strong local reductions in upper tropospheric HNO 3 and NO x . Because of NO reduction the OH/HO 2 ratio and the OH concentration decrease. As a consequence of these effects a significant reduction in the net ozone production rate is modeled. Sensitivity studies were performed varying the HNO 3 adsorption efficiency, the particle sedimentation efficiency, and the ambient NO x concentration. The sensitivity experiments reveal that the modeled cirrus cloud impacts are mainly driven by the nonreactive uptake of HNO 3 on ice particles and the subsequent particle sedimentation. The effects strongly depend on the efficiency of HNO 3 adsorption. If a very efficient uptake of HNO 3 is assumed, decreases in the ozone mixing ratio up to 14% are modeled. The simulations also suggest that the cirrus cloud impact on ozone is sensitive to the ambient NO x concentration. The heterogeneous reactions on ice with HNO 3 as a product appear to be of secondary importance in the upper troposphere. Chlorine activation due to heterogeneous reactions on cirrus ice particles has a minor effect on model ozone chemistry under the conditions regarded. The model calculations further suggest that chemical perturbations caused by the uptake of OH, HO 2 , and H 2 O 2 on ice are potentially significant only in periods of high cirrus cloud activity. Since observational data on cirrus chemical impacts in the upper troposphere are currently too sparse to perform a detailed model validation, it remains unclear whether the potentially large chemical perturbations caused by HNO 3 adsorption on cirrus ice particles are of relevance for the real atmosphere.