The NOx-HNO3 system in the lower stratosphere:: Insights from in situ measurements and implications of the JHNO3-[OH] relationship

During the 1997 Photochemistry of Ozone Loss in the Arctic Region in Summer (POLARIS) mission, simultaneous in situ observations of NOchi and HOchi radicals, their precursors, and the radiation field were obtained in the lower stratosphere. We use these observations to evaluate the primary mechanisms that control NOchi- HNO3 exchange and to understand their control over the partitioning between NO2 and HNO3 in regions of continuous sunlight. We calculate NOchi production (P-NO chi) and loss (L-NO chi) in a manner directly constrained by the in situ measurements and current rate constant recommendations, using approaches for representing albedo, overhead O-3 and [OH] that reduce model uncertainty. We find a consistent discrepancy of 18% between modeled rates of NOchi production and loss (L-NO chi = 1.18P(NO chi)) which is within the measurement uncertainty of 127%. The partitioning between NOchi production processes is [HNO3 + OH (41 +/- 2)%; HNO3 + hv (59 +/- 2)%] and between NOchi loss processes is [NO2 + OH, 90% to > 97%; BrONO2 + H2O, 10% to <3%]. The steady-state description of NOchi-HNO3 exchange reveals the significant influence of the tight correlation between the photolysis rate of HNO3 and [OH] established by in situ measurements throughout the lower stratosphere. Parametrizing this relationship, we find (I)the steady-state value of [NO2](24h-avg)/[HNO3] in the continuously sunlit, lower stratosphere is a function only of temperature acid number density, and (2) the partitioning of NO, production between HNO3 + OH and HNO3 + hv is nearly constant throughout most of the lower stratosphere. We describe a methodology (functions of latitude, day, temperature, and pressure) for accurately predicting the steady-state value of [NO2](24h-avg)/[HNO3] and the partitioning of NOchi production within these regions. The results establish a metric to compare observations of [NO2](24h-avg)/[HNO3] within the continuously sunlit region and provide a simple diagnostic for evaluating the accuracy of models that attempt to describe the coupled NOchi-HOchi photochemistry in the lower stratosphere.