Daniel Huguenin

The possible detection of OBrO in the stratosphere

The analysis of spectral residua recorded at night by the balloon-borne AMON (Absorption par Minoritaires Ozone et Nox) UV-visible spectrometer during five stratospheric flights at middle and high latitudes shows that some absorption features remain in the 475–550 nm range, while the Rayleigh, aerosol, ozone, and NO2 contributions are subtracted. The data reduction relating to these spectral lines is presented for the flight of February 26, 1997, at Kiruna (Sweden) inside the polar vortex. A good agreement exists between these unknown absorption features and those attributed to OBrO during recent laboratory measurements. The results of measurements from the other AMON flights are also presented. Assuming a OBrO cross section maximum similar to that of OClO, an upper limit for the OBrO mixing ratio is found to be around 20 pptv at midlatitude, implying that OBrO would be the principal bromine species at night in the middle stratosphere. At high latitude the OBrO mixing ratio decreases, particularly in the presence of OClO (also measured by AMON). The results are contradictory to current knowledge and, if confirmed, could argue for major revision of the assumed bromine chemistry in the stratosphere.

Vertical distribution of nighttime stratospheric NO2 from balloon measurements: Comparison with models

Vertical distributions of NO2 and O3 mixing ratios and the aerosol extinction coefficient were measured by night between 18 and 32 km altitude (9–80 hPa) at mid latitudes using the balloon-borne instrument AMON on March 24, 1994. The NO2 profile is compared with results of simulations involving the REPROBUS 3D Chemical-Transport Model and a Lagrangian model in which the ozone mixing ratio and the aerosol surface area are initialized using AMON measurements (other mixing ratios are initialized with REPROBUS). As confirmed by Lidar observations, the surface areas were larger than the monthly and zonally averaged SAGE 2 data available for March 1994 at 45°N. The Lagrangian model shows relatively good agreement with AMON results for pressure higher than 40 hPa and smaller than 15 hPa, but the computed NO2 value is too high between 15 and 40 hPa. This seems to indicate that heterogeneous reactions involving the NOY species in the aerosol layer are still incompletely understood.