K. Pfeilsticker

Spatial and temporal distribution of enhanced boundary layer BrO concentrations measured by the GOME instrument aboard ERS-2

The temporal and spatial distribution of enhanced boundary layer BrO concentrations in both hemispheres during 1997 is presented using observations of the Global Ozone Monitoring Experiment (GOME) on board the European research satellite ERS-2. BrO concentrations (up to 50 ppt) are the major cause for catalytic boundary layer ozone destruction typically observed during polar spring in both hemispheres. While autocatalytic mechanisms are most probably responsible for the release of the observed high concentrations of reactive bromine compounds, uncertainties still remain with respect to the primary release mechanisms and whether the autocatalytic reactions are taking place on sea-salt aerosol or the surface of sea ice. We find that enhanced boundary layer BrO concentrations correlate very well with ozone depletion events. Enhanced BrO concentrations are always found over or near to areas of frozen salt water (above sea ice or also above the frozen surface of the Caspian Sea) consistent with the assumption that such conditions are a prerequisite for the autocatalytic release of high BrO concentrations to the troposphere.

Lower stratospheric organic and inorganic bromine budget for the Arctic winter 1998/99

In the Arctic winter 1998/99, two balloon payloads were launched in a co-ordinated study of stratospheric bromine. Vertical profiles (9-28 km) of all known major organic Br species (CH 3 Br, C 2 H 5 Br, CH 2 BrCl, CHBrCl 2 , CH 2 Br 2 , CHBr 2 Cl, CHBr 3 , H1301, H1211, H2402, and H1202) were measured, and total organic Br (henceforth called Br org y ) originating from these organic precursors was inferred as a function of altitude. This was compared with total inorganic reactive Br (henceforth called Br in y ) derived from spectroscopic BrO observations, after accounting for modeled stratospheric Br y partitioning. Within the studied altitude range the two profiles differed by less than the estimated accumulated uncertainties. This good agreement suggests that the lower stratospheric budget and chemistry of Br is well understood for the specified conditions. For early 1999 our data suggest 2 Br in y mixing ratio of 1.5 ppt in air just above the local Arctic tropopause (∼ 9.5 km), whilst at 25 km in air of 5.6 yr mean age it was estimated to be 18.4(+1.8/-1.5) ppt from organic precursor measurements, and (21.5±3.0) ppt from BrO measurements and photochemical modelling, respectively. This suggests a Br in y influx of 3.1(-2.9/+3.5) ppt from the troposphere to the stratosphere.

On the influence of tropospheric clouds on zenith‐scattered‐light measurements of stratospheric species

Examples for the influence of tropospheric clouds on the ground-based measurement of stratospheric species using the DOAS-technique (Differential Optical Absorption Spectroscopy) are reported. At Camborne/Great Britain (50.216°N, 5.316°W) on Sept. 11–15, 1994, episodic enhancement of absorption lines of O4, H2O, O3 and NO2 were observed in coincidence with tropospheric clouds being in the instrumental field of view (1.1° full angle). At a solar zenith angle (SZA) of 88°, absorption enhancements up to roughly a factor of 3 were detected for the tropospheric species O4 and H2O and the tropospheric fractions of the total column of O3 and NO2. The additional absorptions in the visible spectral range are probably caused by multiple Mie-scattering in tropospheric clouds. For our conditions, a tropospheric light path enhancement (TLPE) of 135±40 km can be inferred, being largely independent of SZA. This observation has several important implications for the atmospheric radiative transport, which are briefly discussed.

First profile measurements of tropospheric BrO

Tropospheric BrO profiles (about 0.6+0.2 ppt, and 2.0-t-0.8 ppt at profile maximum) were mea- sured for the first time. Our measurements add new information to recent speculations - based on indirect evidence- of BrO possibly being ubiquitous in the free troposphere (Harder.et al., 1998; Friefi et al., 1999; Van Roozendael et al., 1999; Pundt et al., 2000). Our study relies on a detailed comparison of BrO slant column densities (BrO-SCD) measured in the tropos- phere from the LPMA/DOAS (Laboratoire de Physi- que Mol6culaire et Applications and Differential Op- tical Absorption Spectroscopy) gondola by direct Sun absorption, and BrO-SCD values subsequently measu- red in the lowermost stratosphere during balloon ascent. The difference in total atmospheric BrO-SCDs measu- red in the troposphere and lowermost stratosphere- af- ter a suitable correction for the change in BrO due to photochemistry and the observation geometry- is then attributed to tropospheric BrO.

Intercomparison of BrO measurements from ERS-2 GOME, ground-based and balloon platforms

The consistency of BrO column amounts derived from GOME spectra and from correlative ground-based and balloon measurements performed in 1998-1999 during the Third European Stratospheric Experiment on Ozone (THESEO) has been investigated. The study relies on W-visible observations at several mid- and high latitude ground-based stations in both hemispheres, complemented by balloon-borne solar occultation profile measurements and 3D chemical transport model simulations. Previous investigations have reported GOME BrO columns systematically larger than those deduced from balloon, suggesting BrO being present, possibly ubiquitously, in the free troposphere. The robustness of this hypothesis has been further tested based on the presently available correlative data set. It is shown that when accounting for the BrO diurnal variation and the solar zenith angle dependency of the sensitivity of correlative data to the troposphere, measurements from all platforms are consistent with the presence of a tropospheric BrO background of 1-3 ~10’~ mole&m’ extending over mid- and high

Global tropospheric NO2 column distributions: Comparing three‐dimensional model calculations with GOME measurements

Tropospheric NO2 columns derived from the data products of the Global Ozone Monitoring Experiment (GOME), deployed on the ESA ERS-2 satellite, have been compared with model calculations from two global three-dimensional chemistry transport models, IMAGES and MOZART. The main objectives of the study are an analysis of the tropospheric NO2 data derived from satellite measurements, an interpretation of it and evaluation of its quality using global models, and an estimation the role of NO2 in radiative forcing. The measured and modeled NO2 columns show similar spatial and seasonal patterns, with large tropospheric column amounts over industrialized areas and small column amounts over remote areas. The comparison of the absolute values of the measured and modeled tropospheric column amounts are particularly dependent upon uncertainties in the derivation of the tropospheric NO2 columns from GOME and the difficulty of modeling the boundary layer in global models, both of which are discussed below. The measured tropospheric column amounts derived from GOME data are of the same order as those calculated by the MOZART model over the industrialized areas of the United States and Europe, but a factor of 2-3 larger for Asia. The modeled tropospheric NO2 columns from MOZART as well as the column amounts measured by GOME are in good agreement with NO2 columns derived from observed NO2 mixing ratios in the boundary layer in eastern North America. The comparison of the models to the GOME data illustrates the degree to which present models reproduce the hot spots seen in the GOME data. The radiative forcing of NO2 has been estimated from the calculated tropospheric NO2 columns. The local maxima in the radiative forcing of tropospheric NO2 for cloud-free conditions over the eastern United States and western Europe represent 0.1-0.15 W m -2, while values of 0.04-0.1 W m -2 are estimated on a continental scale in these regions, of the same order of magnitude as the forcing of N20 and somewhat smaller than the regional forcing of tropospheric ozone. The globally averaged radiative forcing of tropospheric NO2 is negligible, --0.005 W m -2.

Absorption of Solar Radiation by Atmospheric O4

Abstract Spectroscopic measurements of the atmospheric solar radiation attenuation reveal that the near ultraviolet–visible–near-infrared absorption of the oxygen collision complex (O2)2, thus far omitted from models, is important for the direct heating for both clear and cloudy skies. Atmospheric line-by-line radiative transfer calculations show that the absorption by (O2)2 leads to a globally averaged clear sky shortwave (SW) heating of about 0.53 W m−2. It is therefore proposed that the absorption by (O2)2 should be included in models designed to calculate the SW heating. It is shown that due to its weak absorption under clear sky, the SW heating by (O2)2 approximately increases linearly with increasing optical pathlengths for cloudy sky conditions. This is in contrast to SW heating by the molecular absorptions of H2O or O2, whose absorption lines are already partially saturated under clear sky, causing the SW heating (due to these gases) to increase rather as the square root of the optical path. From ...

Comparison of measurements and model calculations of stratospheric bromine monoxide

Ground-based zenith sky UV-visible measurements of stratospheric bromine monoxide (BrO) slant column densities are compared with simulations from the SLIMCAT three-dimensional chemical transport model. The observations have been obtained from a network of 11 sites, covering high and midlatitudes of both hemispheres. This data set gives for the first time a near-global picture of the distribution of stratospheric BrO from ground-based observations and is used to test our current understanding of stratospheric bromine chemistry. In order to allow a direct comparison between observations and model calculations, a radiative transfer model has been coupled to the chemical model to calculate simulated slant column densities. The model reproduces the observations in general very well. The absolute amount of the BrO slant columns is consistent with a total stratospheric bromine loading of 20 ± 4 ppt for the period 1998-2000, in agreement with previous estimates. The seasonal and latitudinal variations of BrO are well reproduced by the model. In particular, the good agreement between the observed and modeled diurnal variation provides strong evidence that the BrO-related bromine chemistry is correctly modeled. A discrepancy between observed and modeled BrO at high latitudes during events of chlorine activation can be resolved by increasing the rate constant for the reaction BrO + ClO → BrCl + O 2 to the upper limit of current recommendations. However, other possible causes of the discrepancy at high latitudes cannot be ruled out.

Stratospheric BrO profiles measured at different latitudes and seasons: Atmospheric observations

Stratospheric BrO profiles were measured at different latitudes and in different seasons in 1996/97 during three flights of the LPMA/DOAS balloon gondola (LPMA/Laboratoire Physique Moleculaire et Application and DOAS/Differential Optical Absorption Spectrometry). Using direct sunlight DOAS spectrometry the following BrO mixing ratios were measured; (1) 9 to 14 ppt in the height range from 20 to 30 km (at solar zenith angles, SZA < 88°) during ascent, (2) about (14±2) ppt for altitudes above the balloon float altitude at 30.6 km, 30.0 km, and 39.8 km, and (3) 5 to 10 ppt in the 20–30 km region during sunset. The lower BrO concentrations during sunset than those observed prior at daytime indicate a conversion of BrO into nighttime reservoir species (BrONO2, HOBr, and BrCl). The overall agreement of our UV spectroscopic BrO profiles with recent measurements using the chemical conversion/resonance fluorescence technique is good. Our BrO profiles are also in reasonable agreement with the present stratospheric Bry burden and chemistry. Conversily collocated ground-based and satellite column measurements, however, show significantly more total atmospheric BrO (50–100%) than the integrated stratospheric BrO balloon profiles can account for. This indicates a global tropospheric BrO background, estimated at 1–2 ppt.

Toward a better quantitative understanding of polar stratospheric ozone loss

Previous studies have shown that observed large O3 loss rates in cold Arctic Januaries cannot be explained with current understanding of the loss processes, recommended reaction kinetics, and standard assumptions about total stratospheric chlorine and bromine. Studies based on data collected during recent field campaigns suggest faster rates of photolysis and thermal decomposition of ClOOCl and higher stratospheric bromine concentrations than previously assumed. We show that a model accounting for these kinetic changes and higher levels of BrO can largely resolve the January Arctic O3 loss problem and closely reproduces observed Arctic O3 loss while being consistent with observed levels of ClO and ClOOCl. The model also suggests that bromine catalysed O3 loss is more important relative to chlorine catalysed loss than previously thought.