| A new instrument is developed and used to make the first in situ measurements of ClONO2 in the lower stratosphere. With this instrument, NASA's ER-2 aircraft platform provides observations of all the major inorganic chlorine species in the gas-phase lower stratosphere as well as an independent estimate of their sum (Cly). Inorganic chlorine partitioning is examined using observations made during the Photochemistry of Ozone Loss in the Arctic Region in Summer (POLARIS) campaign. Improved steady-state and numerical modeling methodology, tightly constrained by observations and validated by a comprehensive photochemical model, allows us to test a wide range of model inputs and assumptions. We find that currently accepted inorganic chlorine photochemistry overestimates observed [ClONO 2]/[HCl] by approximately 65% at mid- and high latitudes (note that comprehensive models currently simulate [ClO] in good agreement with observations because model-measurement discrepancies in [HOx] and [NOx ] compensate for discrepancies in Cly partitioning). Based on POLARIS studies of the inorganic chlorine budget, [ClO]/[ClONO2], and an inter-comparison with balloon observations, the most direct explanation for the model-measurement discrepancy in Cly partitioning is an error in the reactions, rate coefficients, and measured species concentrations linking HCl and ClO (simulated [ClO]/[HCl] too high) in combination with a possible systematic error in the ER-2 ClONO2 measurement (too low). The precision of our simulation (±15% 1σ relative to individual [ClONO2]/[HCl] observations) is far better than that of comprehensive models, greatly increasing confidence in the observations, photolysis calculations, and laboratory rate coefficients. These results, along with other findings, should lead to significant improvements in the accuracy and precision of stratospheric photochemical models. |
