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, NO y , 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, NO 2 , NO 3 , N 2 O 5 , HNO 3 , and ClONO 2 . The technique utilized the conversion of NO y components to NO on a gold catalyst and the subsequent detection of NO by the chemiluminescent reaction of NO with O 3 . 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, NO y 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 NO y and NO were found with values decreasing poleward. Total NO y 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 NO y observed preclude a chemical loss of ozone due to reaction with NO. The NO y 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 NO y values indicate denitrification, defined as the removal of NO y from an air parcel. Low H 2 O levels, which indicate dehydration, are observed to coincide with denitrification, suggesting that the respective processes are coupled. The partitioning of the remaining NO y inside the CPR likely favors HNO 3 and ClONO 2 . Outside the CPR, the concurrent measurements of ClO, NO, NO y , and O 3 along with photochemical steady state relations indicate that NO 2 and ClONO 2 are minor NO y components. Near the boundary, the variation of NO with ClO is shown to be consistent with heterogeneous reactions of HCl and ClONO 2 producing reactive chlorine.