Showing papers by "Folkard Wittrock published in 2005"

Global observations of stratospheric bromine monoxide from SCIAMACHY

Abstract: [1] The Scanning Imaging Absorption Spectrometer for Atmospheric Chartography (SCIAMACHY) onboard the ENVISAT satellite provides for the first time a global view on stratospheric bromine monoxide (BrO). Here we focus on a 10 day period in September 2002. The BrO retrievals are compared with modeled BrO profiles, based on estimated inorganic bromine (Bry) from CFC-11 retrievals by the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) on ENVISAT and the calculated BrO/Bry ratio from a photochemical model constrained by SCIAMACHY NO2 retrievals. The BrO observations are broadly consistent with our current understanding of stratospheric bromine chemistry and a total stratospheric bromine loading of 18 ± 3 pptv. Comparisons between the measured stratospheric BrO column and the simultaneously measured total BrO column from SCIAMACHY nadir observations suggest an average global background tropospheric BrO mixing ratio of 1.0 ± 0.5 pptv.

An intercomparison campaign of ground-based UV-visible measurements of NO2, BrO, and OClO slant columns: Methods of analysis and results for NO2

Abstract: [1] Within the framework of the Network for the Detection of Stratospheric Change (NDSC), an intercomparison campaign of ground-based zenith-sky viewing UV-visible spectrometers was held at the Andoya Rocket Range (69°N, 16°E) at Andenes, Norway, from February 12 to March 8, 2003. The chosen site is classified as a complementary NDSC site. Eight groups from seven countries participated in the campaign which focused on the measurements of slant columns of NO 2 , BrO, and OClO. This first campaign publication concentrates on measurements of the NO 2 slant columns. Different analysis criteria were investigated during the campaign. These included the use of fitting parameters as chosen by each group to provide what they considered to be optimized retrievals. Additional sets of parameters, imposed for all the groups, were also used, including the wavelength interval, absorption cross sections, and species fitted. Each instrument's results were compared to the measurements of selected reference instruments, whose choice was based on a technique combining regression analysis and examination of the residuals with solar zenith angle. Considering the data obtained during the whole campaign for solar zenith angles between 75° and 95°, all instruments agreed within 5% in the case of NO 2 with imposed analysis parameters in the 425-450 nm region. Measurements agree less well when retrieving the NO 2 slant columns in the 400-418 nm region or when using parameters optimized by each investigator for their instrument.

Intercomparison exercise between different radiative transfer models used for the interpretation of ground-based zenith-sky and multi-axis DOAS observations

Abstract: We present the results of an intercomparison exer- cise between six different radiative transfer (RT) models car- ried out in the framework of QUILT, an EU funded project based on the exploitation of the Network for the Detection of Stratospheric Change (NDSC). RT modelling is an important step in the interpretation of Differential Optical Absorption Spectroscopy (DOAS) observations. It allows the conversion of slant column densities (SCDs) into vertical column densi- ties (VCDs) using calculated air mass factors (AMFs). The originality of our study resides in comparing SCD simula- tions in multi-axis (MAX) geometry (trace gases: NO2 and HCHO) and in taking into account photochemical enhance- ment for calculating SCDs of rapidly photolysing species (BrO, NO2, and OClO) in zenith-sky geometry. Concerning the zenith-sky simulations, the different models agree gener- ally well, especially below 90 SZA. At higher SZA, larger discrepancies are obtained with relative differences ranging between 2% and 14% in some cases. In MAX geometry, good agreement is found between the models with the calcu- lated NO2 and HCHO SCDs differing by no more than 5% in the elevation and solar zenith angle (SZA) ranges inves- tigated (5 -20 and 35 -85 , respectively). The impacts of aerosol scattering, ground albedo, and relative azimuth on MAX simulations have also been tested. Significant discrep- ancies appear for the aerosol effect, suggesting differences between models in the treatment of aerosol scattering. A bet-

GOME Observations of Stratospheric Trace Gas Distributions during the Splitting Vortex Event in the Antarctic Winter of 2002. Part I: Measurements

Abstract: Measurements from the Global Ozone Monitoring Experiment (GOME) are used to study the chemical evolution of the stratosphere during the unusual 2002 winter in the Southern Hemisphere. The results show that chlorine activation as indicated by OClO columns was similar to previous years in the vortex until the major warming on 26 September 2002 after which it decreased rapidly. Similarly, NO2 columns were only slightly larger than in previous years before the warming, indicating strong denoxification and probably also denitrification. After the warming, very large NO2 columns were observed for a few days, which then decreased again as the vortex reestablished itself until the final warming. Ozone columns were much larger than in any previous year from September onward, mainly as a result of the unusual dynamical situation. Analysis of the global long-term time series of GOME measurements since 1996 provides a unique opportunity to set the austral winter 2002 into perspective. The GOME data reveal th...

A study of the trace gas columns of O3, NO2 and HCHO over Africa in September 1997

Abstract: Retrievals of trace gas columns from the measurements of backscattered radiation by GOME (Global Ozone Monitoring Experiment) show that enhanced tropospheric columns of ozone (O3), nitrogen dioxide (NO2) and formaldehyde (HCHO), over the African continent occur frequently. This study focuses on the behaviour of trace gases over Africa in September 1997, a period impacted by the strongest known El Nino phase of the ENSO. It investigates our qualitative and quantitative understanding of the retrieved tropospheric trace gas column densities. The emissions of NOx and volatile organic compounds (VOC) from biomass burning, biogenic sources and lightning and their photochemical transformation have been investigated. By performing a trajectory analysis, the transport of air masses from the different emission regions was analysed and the potential atmospheric spatial distribution determined. BRemen’s Atmospheric PHOtochemical model (BRAPHO) was applied to compute the chemistry along a large number of trajectories. From these results, tropospheric column amounts of O3, NO2 and HCHO were derived. Tropospheric trace gas columns retrieved from GOME measurements and those calculated are in reasonable agreement. Their general spatial extent was similar in the lower troposphere but the modeled trace gas columns in the upper troposphere were located south of the retrieved columns. We attribute this behaviour to uncertainties in the ERA-40 meteorological data in the upper troposphere. The significance of biomass burning and of biogenic emissions with respect to HCHO columns over Africa was investigated. The analysis reveals that the total amounts of HCHO generated over Africa during September 1997 as a result of biomass burning and biogenic emissions are similar. However the HCHO from biogenic sources has the highest specific columns and these are located close to their source. In comparison the HCHO from biomass burning is predicted to be produced and transported over a much wider area. Overall all the emission processes mix together to produce the plume of O3.

Tropospheric O 3 over Indonesia during biomass burning events measured with GOME (Global Ozone Monitoring Experiment) and compared with trajectory analysis

Abstract: . Tropospheric ozone columns of up to 50 DU were observed by GOME (Global Ozone Monitoring Experiment) above Indonesia in September 1997, while only background amounts were measured in September 1998. The Traj.x trajectory model along with BRemen's Atmospheric PHOtochemical model (BRAPHO) were used to investigate the higher than average ozone columns above Indonesia. The transport analysis reveals that biomass burning over central Africa and northern Australia does not significantly influence ozone columns over Indonesia in September 1997. El Nino conditions, leading to extreme dryness and uncontrolled fires in Indonesia, produced ozone precursors, which are initially only slowly advected westwards to the central Indian Ocean. Joint transport and chemistry modelling was able to reproduce the spatial distribution and amounts of ozone, NO 2 and formaldehyde columns over Indonesia. The chemistry modelling shows a net production of 3.1 Tg of ozone produced by biomass burning in Indonesia in September 1997. Transport analysis further reveals that ozone columns over the Indian Ocean, between 10 and 20° S can be accounted for by the mixing of air masses containing NO x from lightning over the Congo Basin with air masses containing volatile organic compounds from biomass burning.