Measurements of nitric oxide and total reactive nitrogen in the Antarctic stratosphere: Observations and chemical implications

Measurements of nitric oxide, NO, and the sum of reactive nitrogen species, NOy, were made as part of the Airborne Antarctic Ozone Experiment (AAOE) on flights of the NASA ER-2 aircraft over the Antarctic continent. Reactive nitrogen species include NO, NO2, NO3, N2O5, HNO3, and ClONO2. The technique utilized the conversion of NOy components to NO on a gold catalyst and the subsequent detection of NO by the chemiluminescent reaction of NO with O3. NO was measured on two of the flights by removing the catalyst from the sample line. The flights ranged from 53°S to 72°S latitude in the lower stratosphere, with the majority of flights following the 425 K (±10 K) or 450 K (±10 K) potential temperature surfaces. The boundary of a chemically perturbed region (CPR) above the continent occurred on average near 66°S as indicated by a sharp increase in the level of ClO. Outside or equatorward of the CPR, NOy mixing ratios ranged between 6 and 12 parts per billion by volume (ppbv), with values increasing with latitude. At the edge of the CPR, large latitude gradients of NOy and NO were found with values decreasing poleward. Total NOy levels dropped to 4 ppbv or less within 5° poleward of the boundary. NO values were 0.1–0.2 ppbv outside and below the detection limit of 0.03 ppbv inside the CPR. The levels of NO and NOy observed preclude a chemical loss of ozone due to reaction with NO. The NOy values outside the CPR are in accord with the results of two-dimensional photochemical models that incorporate only homogeneous chemistry when allowance is made for enhanced diabatic descent of air parcels. NO is somewhat lower than the model predictions. Inside the CPR, low NOy values indicate denitrification, defined as the removal of NOy from an air parcel. Low H2O levels, which indicate dehydration, are observed to coincide with denitrification, suggesting that the respective processes are coupled. The partitioning of the remaining NOy inside the CPR likely favors HNO3 and ClONO2. Outside the CPR, the concurrent measurements of ClO, NO, NOy, and O3 along with photochemical steady state relations indicate that NO2 and ClONO2 are minor NOy components. Near the boundary, the variation of NO with ClO is shown to be consistent with heterogeneous reactions of HCl and ClONO2 producing reactive chlorine.

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