[1] The analysis for BrO using the technique of differential optical absorption spectroscopy as applied to spectra of light scattered from the zenith sky has historically presented something of a challenge, leading to uncertainty about the accuracy of measurements. This has largely been due to the large sensitivity of the measurement to many analysis parameters and due to the small size of the absorption features being measured. BrO differential slant columns have been measured by six different groups taking part in an intercomparison exercise at Observatoire de Haute-Provence in France from 23 to 27 June 1996. The data are analyzed in a collaborative attempt to improve the overall analysis for BrO through investigation of a series of sources of errors in the instrumentation, calibration, input to the analysis, and the spectral analysis itself. The study included comprehensive sensitivity tests performed using both actual measurements and synthetic data. The latter proved invaluable for assessing several aspects of the spectral analysis without the limitations of spectral quality and instrument variability. The most significant sources of error are identified as the wavelength calibration of several of the absorption cross sections fitted and of the measured spectra themselves, the wavelength region of the fitting, the temperature dependence of the O 3 absorption cross sections, failure to adequately account for the so-called I 0 effect, inadequate offset correction, and inadequate measurement of the individual instrument slit functions. Recommendations for optimal analysis settings are presented, and comparing the results from the analysis of the campaign data shows BrO differential slant column observations from the various groups to be in agreement to within 4% on average between 87°and 90°s olar zenith angle, with a scatter of 16%.

Abstract. Pollutants are longer-lived in the free troposphere than the boundary layer, hence the transport of pollutants from the boundary layer to the free troposphere has significant implications for long-range transport and global warming. It is important to quantify the transport of air between the boundary layer and the free troposphere and to understand the role different meteorological mechanisms play. Idealised passive tracer experiments, with tracer initially only in the boundary layer, are performed in a numerical model for three case study days with different synoptic conditions. After 24 hours, more than 50% of the tracer resides in the free troposphere for the two frontal cases, and 40% resides there for the high-pressure case. The tracer was transported to maximum heights of 8 km. To elucidate the role of different mechanisms for each case, the tracer amount transported by advection only, advection and turbulent mixing, and advection and convection was calculated. Advection is found to be the most important mechanism in transporting the tracer to the free troposphere; however, the addition of upright convection and turbulent mixing increases the amount by up to 24% with convection transporting the tracer to heights of 5 km. The inclusion of convection and turbulent mixing to the advection are not linearly additive processes. This study shows the possibility of a large proportion of the pollutant emitted in the boundary layer being transported to the free troposphere in a short time and the importance of representing all the meteorological processes.

No abstract

The diurnal variation of BrO through sunrise and sunset has been measured above Cambridge, England, (52°N) during March and April 1994 using zenith sky spectroscopy. The measured BrO slant column at 90° solar zenith angle (SZA), relative to that at 80° SZA, is typically 1.5×1014 cm−2 but varies significantly from day to day. The average variation of BrO slant column with SZA through sunrise and sunset is in good agreement with predictions from a photochemical model, coupled to a radiative transfer model. The zenith sky measurements are consistent with in situ measurements of BrO concentrations and indicate that the inorganic bromine content of the stratosphere is 18–24 parts per trillion by volume (pptv).

Abstract.A recently published analysis of slant columns of NO2 observed at twilight at 45øS has identified trends of about 5% /decade between 1980 and 1998. This is twice the trend in tropospheric N20, which is the source of stratospheric NO2. By means of a column photochemical model, we explore the sensitivity of NO2 to the observed trends in stratospheric temperature, 0 3 and H20. The resulting calculated trends in NO 2 are smaller than observed, and we cannot force agreement by varying the ozone or temperature trends. The calculated sensitivity of NO2 to stratospheric aerosol is large, and a 20% per decade decrease in aerosol surface area creates agreement. We conclude that a small residual in the statistical fit of aerosol to the NO2 measurements may remain, and is a likely cause of the trends found in the NO 2 measurements.