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

Abstract

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.

Interhemispheric Differences in Polar Stratospheric HNO 3 , H 2 O, ClO, and O 3

“…The MLS instrument does not detect HNO, or H,O in condensed phase, but the significant decrease observed in gas-phase HNO, without a corresponding decrease observed in gas-phase H 2 0 is a necessary (albeit not sufficient) condition for the decoupling of these processes.PSCs over most of the Antarctic continent, and gas-phase HNO, values are extremely low ( 1 2 ppbv) throughout this region. A "collar" of high HNO, (r 10 ppbv) is seen just outside the low-temperature region, consistent with previous aircraft observations(23,26) and colocated CLAES measurements(16). The outer boundary of enhanced C10 roughly corresponds to the insist, with mixing ra'tio values less than 6 ppbv and less t h m 3 ppmv, respectively, throughout most of the vortex.…”

“…Because temperatures remain above the frost point at this time, a comparable decrease in H 2 0 vapor is not expected and indeed is not present in the MLS data. Although in situ observations of correlated low concentrations of water vapor and total reactive nitrogen over Antarctica (23) have been taken to imply that the processes of denitrification and dehydration are coupled, other observations (24) have been interpreted to suggest that denitrification can occur without significant dehydration. Theoretical studies (7, 2.5) have found that the sedimentation rate of HNO, particles can be sufficiently rapid-given low cooling rates or cooling to just below the NAT condensation point-for substantial HNO, loss to take place over short time scales, independent of the formation of water ice clouds.…”

The uptake of HNO3 on meteoric smoke analogues

“…Uptake of NO x (NO and NO 2 ) onto MSPs may also have an influence on the atmospheric nitrogen balance, which is dominated by NO in the mesosphere and NO 2 in the stratosphere (Fahey et al, 1989). The coating of MSP surfaces with any of these species, as well as H 2 O, may also alter the chemical reactivity of the surface.…”

Denitrification through PSC formation: A 1-D model incorporating temperature oscillations

“…Measured reductions in HNO 3 (Coffey et al, 1989;Fahey et aL, 1989b;Gandrud et al, 1989;Toon et al, 1989a) and H20 (Fahey et al, 1989b concentrations inside the antarctic polar vortex compared to those outside point directly to denitrification and dehydration, respectively. Furthermore, the conditions under which PSCs occur over Antarctica suggest that the amounts of HNO 3 and H20 fall from 5-7 ppbv and 4-5 ppmv, respectively, in June to 1-2 ppbv and 2 ppmv in October (see, for example, Watterson and Tuck, 1989).…”