The NO_x-HNO_3 System in the Lower Stratosphere: Insights from In Situ Measurements and Implications of the J_(HNO_3)-[OH] Relationship

Abstract

During the 1997 Photochemistry of Ozone Loss in the Arctic Region in Summer (POLARIS) mission, simultaneous in situ observations of NO_x and HO_x radicals, their precursors, and the radiation field were obtained in the lower stratosphere. We use these observations to evaluate the primary mechanisms that control NO_x−HNO_3 exchange and to understand their control over the partitioning between NO_2 and HNO_3 in regions of continuous sunlight. We calculate NO_x production (P_(NO)_x) and loss (L_(NO)_x) 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 NO_x production and loss (L_(NO)_x = 1.18P_(NO)_x) which is within the measurement uncertainty of ±27%. The partitioning between NO_x production processes is [HNO_3 + OH (41 ± 2)%; HNO+3 + hν (59 ± 2)%] and between NO_x loss processes is [NO_2 + OH, 90% to >97%; BrONO_2 + H_2O, 10% to <3%]. The steady-state description of NO_x−HNO_3 exchange reveals the significant influence of the tight correlation between the photolysis rate of HNO_3 and [OH] established by in situ measurements throughout the lower stratosphere. Parametrizing this relationship, we find (1) the steady-state value of [NO_2]_(24h-avg)/[HNO_3] in the continuously sunlit, lower stratosphere is a function only of temperature and number density, and (2) the partitioning of NO_x production between HNO_3 + OH and HNO_3 + hν 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 [NO_2]_(24h-avg)/[HNO_3] and the partitioning of NO_x production within these regions. The results establish a metric to compare observations of [NO_2]_(24h-avg)/[HNO_3] within the continuously sunlit region and provide a simple diagnostic for evaluating the accuracy of models that attempt to describe the coupled NO_x−HO_x photochemistry in the lower stratosphere.

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