U. Frieß

Multi axis differential optical absorption spectroscopy (MAX-DOAS) of gas and aerosol distributions

We present and demonstrate a relatively simple algorithm, which converts a set of slant column density measurements of oxygen dimers (O4) and NO2 at several different elevation angles to determine the atmospheric aerosol extinction and the absolute concentration and mixing ratio of NO2 within the atmospheric boundary layer. In addition the height of the atmospheric boundary layer can usually be derived, also the technique can be readily extended to determine the concentration of several other trace gases including SO2, CH2O, or glyoxal. The algorithm is based on precise radiation transport modelling determination, taking into account the actual aerosol scenario as determined from the O4 measurements. The required hardware is simple encompassing essentially a miniature spectrometer, a small telescope, a pointing mechanism, and a Personal Computer (PC). Effectively the technique combines the simplicity of a passive MAX-DOAS observation with the capability of a much more complex active DOAS instrument to determine path-averaged, absolutely calibrated mixing ratios of atmospheric trace gases at relatively high accuracy.

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.

Intercomparison of measured and modelled BrO slant column amounts for the Arctic winter and spring 1994/95

Diurnal variations of the slant column amount of BrO (BrO-SCD) measured at Kiruna/Esrange (67.9°N, 21.1°E) in winter/spring 1994/95 are compared with photochemical calculations using the SLIMCAT model. SLIMCAT assumes a total stratospheric bromine of 20 ppt, JPL-97 kinetics but no tropospheric inorganic bromine. Observed local noon BrO-SCDs are underestimated by the model by 20% to 40%. Sensitivity test of existing uncertainties in the stratospheric chemistry of bromine can not resolve the discrepancy. A tropospheric contribution to the total atmospheric BrO, corresponding to a vertical BrO column of 1 to 2×1013/cm², (or mixing ratios of 0.75 ppt to 2 ppt assuming an uniform tropospheric height profile) is suggested.

Glyoxal observations in the global marine boundary layer

Glyoxal is an important intermediate species formed by the oxidation of common biogenic and anthropogenic volatile organic compounds such as isoprene, toluene, and acetylene. Although glyoxal has been shown to play an important role in urban and forested environments, its role in the open ocean environment is still not well understood, with only a few observations showing evidence for its presence in the open ocean marine boundary layer (MBL). In this study, we report observations of glyoxal from 10 field campaigns in different parts of the worlds oceans. These observations together represent the largest database of glyoxal in the MBL. The measurements are made with similar instruments that have been used in the past, although the open ocean values reported here, average of about 25 parts per trillion by volume (pptv) with an upper limit of 40 pptv, are much lower than previously reported observations that were consistently higher than 40 pptv and had an upper limit of 140 pptv, highlighting the uncertainties in the differential optical absorption spectroscopy method for the retrieval of glyoxal. Despite retrieval uncertainties, the results reported in this work support previous suggestions that the currently known sources of glyoxal are insufficient to explain the average MBL concentrations. This suggests that there is an additional missing source, more than a magnitude larger than currently known sources, which is necessary to account for the observed atmospheric levels of glyoxal. Therefore, it could play a more important role in the MBL than previously considered.

Ground-Based DOAS Measurements of Stratospheric Trace Gases at Two Antarctic Stations during the 2002 Ozone Hole Period

Abstract Compared to recent years, the development of the Antarctic ozone hole in 2002 showed very unusual dynamical features. The midwinter polar vortex was one of the smallest observed during the past decade. Driven by planetary waves, the vortex showed a strong asymmetry in early spring. A large air mass separated in late September, leaving what was previously a small vortex even smaller. Furthermore, stratospheric temperatures exceeded the polar stratospheric cloud (PSC) threshold earlier than in previous years, leading to a decrease in halogen activation by heterogeneous surface reactions. Ground-based observation of stratospheric trace gases in austral spring of 2001 and 2002 using passive Differential Optical Absorption Spectroscopy (DOAS) observations of zenith-scattered sunlight in the UV and visible wavelength region (320–650 nm) are presented. Using DOAS measurements of ozone, NO2, BrO, and OClO at two different Antarctic sites, Neumayer Station (70°S, 8°W) and Arrival Heights (78°S, 167°E), th...

Vertical distribution of BrO in the boundary layer at the Dead Sea

Environmental context Reactive halogen species affect chemical processes in the troposphere in many ways. The reactive bromine species bromine monoxide (BrO) is found in high concentrations at the Dead Sea, but processes for its formation and its spatial distribution are largely unknown. Information on the vertical distribution of BrO at the Dead Sea obtained in this work may give insight into the processes leading to BrO release and its consequences. Abstract We present results of multi-axis differential optical absorption spectroscopy (MAX‐DOAS) and long‐path DOAS (LP‐DOAS) measurements from two measurement campaigns at the Dead Sea in 2002 and 2012. The special patterns of its dynamics and topography in combination with the high salt and especially bromide content of its water lead to the particular large atmospheric abundances of more than 100 ppt BrO close to the ground and in several hundred meters above ground level. We conclude that vertical transport barriers induced by the special dynamics in the Dead Sea Valley lead to an accumulation of aerosol and reactive bromine species. This occurs in situations of weak synoptic winds and of mountain induced thermal circulations. Thus BrO release strongly depends on the topography and local and meso-scale meteorology. In case of strong zonal winds, the Dead Sea valley is flushed and high BrO levels cannot accumulate. NO2 levels below 1–2 ppb seem to be a prerequisite for a high BrO production. We assume that at least a part of the missing NO2 might be converted to BrONO2 leading to a deposition of nitrate within the aerosol and acting as a reservoir for reactive bromine. From these measurements, it was possible for the first time to simultaneously retrieve vertical profiles of aerosols, BrO and NO2 and gain also information on the distribution at the Dead Sea, allowing for a thorough characterization of the chemical processes leading to halogen release in the context of the special atmospheric dynamics in the Dead Sea Valley.

Polar nighttime chemistry produces intense reactive bromine events

By examining the origin of air masses that arrive at Utqiaġvik (formerly Barrow) Alaska soon after polar sunrise (late January/early February), we identified periods when air arriving at Utqiaġvik had previously resided primarily at higher latitudes in near total darkness. Upon illumination, these air masses produced high concentrations of reactive bromine, which was detected by differential optical absorption spectroscopy (DOAS) as bromine monoxide (BrO). These observations are consistent with nighttime production of a photolabile reactive bromine precursor (e.g. Br2 or BrCl). A large polar‐night source of photolabile reactive bromine precursors would contribute seed reactive bromine to daytime reactive bromine events and could export reactive halogens to lower latitudes and the free troposphere.