Observations of large reductions in the NONOy ratio near the midlatitude tropopause and the role of heterogeneous chemistry

During the 1993 NASA Stratospheric Photochemistry, Aerosols and Dynamics Expedition (SPADE), anomalously low nitric oxide (NO) was found in a distinct sunlit layer located above the mid-latitude tropopause. The presence of a significant amount of reactive nitrogen (NOy) in the layer implies the systematic removal of NO, which is without precedent in stratospheric in situ observations. Large increases in measured chlorine monoxide (C10) and the hyd•'operoxyl radical (HO2) also were observed in the layer. Heterogeneous reaction rate constants of chlorine nitrate (C1ONO2) with hydrogen chloride (HC1) and H20 to form nitric acid (HNO3) on sulfate aerosol are enhanced in the NO removal layer by local increases in H20 and aerosol surface area. The associated conversion of NOx (= NO + NO2) to HNO 3 is the most likely cause of the observed low NO and NOx/NOy values and high C10 values.