The NOx-HNO3 system in the lower stratosphere: Insights from in situ measurements and implications of the J(HNO3)-[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 ra dicals, their precursors, and the radiation field were obtained in the lowe r stratosphere. We use these observations to evaluate the primary mechanism s that control NOchi- HNO3 exchange and to understand their control over th e partitioning between NO2 and HNO3 in regions of continuous sunlight. We c alculate NOchi production (P-NO chi) and loss (L-NO chi) in a manner direct ly constrained by the in situ measurements and current rate constant recomm endations, using approaches for representing albedo, overhead O-3 and [OH] that reduce model uncertainty. We find a consistent discrepancy of 18% betw een modeled rates of NOchi production and loss (L-NO chi = 1.18P(NO chi)) w hich is within the measurement uncertainty of 127%. The partitioning betwee n NOchi production processes is [HNO3 + OH (41 +/- 2)%; HNO3 + hv (59 +/- 2 )%] and between NOchi loss processes is [NO2 + OH, 90% to > 97%; BrONO2 + H 2O, 10% to <3%]. The steady-state description of NOchi-HNO3 exchange reveal s the significant influence of the tight correlation between the photolysis rate of HNO3 and [OH] established by in situ measurements throughout the l ower stratosphere. Parametrizing this relationship, we find (I)the steady-s tate value of [NO2](24h-avg)/[HNO3] in the continuously sunlit, lower strat osphere is a function only of temperature acid number density, and (2) the partitioning of NO, production between HNO3 + OH and HNO3 + hv is nearly co nstant throughout most of the lower stratosphere. We describe a methodology (functions of latitude, day, temperature, and pressure) for accurately pre dicting the steady-state value of [NO2](24h-avg)/[HNO3] and the partitionin g of NOchi production within these regions. The results establish a metric to compare observations of [NO2](24h-avg)/[HNO3] within the continuously su nlit region and provide a simple diagnostic for evaluating the accuracy of models that attempt to describe the coupled NOchi-HOchi photochemistry in t he lower stratosphere.