Denis Pöhler

Observation of halogen species in the Amundsen Gulf, Arctic, by active long-path differential optical absorption spectroscopy

In the polar tropospheric boundary layer, reactive halogen species (RHS) are responsible for ozone depletion as well as the oxidation of elemental mercury and dimethyl sulphide. After polar sunrise, air masses enriched in reactive bromine cover areas of several million square kilometers. Still, the source and release mechanisms of halogens are not completely understood. We report measurements of halogen oxides performed in the Amundsen Gulf, Arctic, during spring 2008. Active long-path differential optical absorption spectroscopy (LP-DOAS) measurements were set up offshore, several kilometers from the coast, directly on the sea ice, which was never done before. High bromine oxide concentrations were detected frequently during sunlight hours with a characteristic daily cycle showing morning and evening maxima and a minimum at noon. The, so far, highest observed average mixing ratio in the polar boundary layer of 41 pmol/mol (equal to pptv) was detected. Only short sea ice contact is required to release high amounts of bromine. An observed linear decrease of maximum bromine oxide levels with ambient temperature during sunlight, between -24 °C and -15 °C, provides indications on the conditions required for the emission of RHS. In addition, the data indicate the presence of reactive chlorine in the Arctic boundary layer. In contrast to Antarctica, iodine oxide was not detected above a detection limit of 0.3 pmol/mol.

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.

Applying light-emitting diodes with narrowband emission features in differential spectroscopy

LEDs are a promising new type of light source for differential optical absorption spectroscopy (DOAS). Varying differential structures in the emission spectrum of LEDs, however, display a potentially severe problem. We show that the structures, which originate from a Fabry-Pérot etalon, may be removed by tilting the emitter, which at the same time increases the radiant flux coupled into the subsequent optical system. The results of long-path DOAS measurements, where we apply our method on a blue LED for the suppression of periodic structures, are also presented.

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.

Observations of tropospheric aerosols and NO2 in Hong Kong over 5 years using ground based MAX-DOAS

In this paper, we present long term observations of atmospheric aerosols and nitrogen dioxide (NO2) in Hong Kong using a Multi-AXis Differential Optical Absorption Spectroscopy (MAX-DOAS) instrument. Ground based MAX-DOAS measurements were performed over 5years from December 2010 to November 2015. Vertical distribution profiles of aerosols and NO2 were derived from MAX-DOAS O4 and NO2 observations by applying the optimal estimation method. Retrieved MAX-DOAS measurements of aerosols and NO2 show good agreement with sun photometer observation of aerosol optical depths (AODs) and long path DOAS measurement of ground level NO2 mixing ratios. Tropospheric vertical column densities (VCDs) of NO2 derived from MAX-DOAS measurements are used to validate OMI satellite NO2 observations. Daily data show reasonably good agreement with each other with Pearson correlation coefficient R=0.7. However, MAX-DOAS NO2 VCDs are on average higher than OMI observations by a factor of 2. Introducing aerosols in the air mass factor calculation would enhance the OMI VCDs by 7-13%, the remaining discrepancy is mainly due to the differences in spatial coverage between the two instruments. Diurnal variation patterns of aerosols and NO2 indicated significant contributions from local anthropogenic emissions. Analysis of air mass transport shows that the enhancement of surface aerosols and NO2 concentrations mainly results from accumulation of local emissions under low wind speed conditions.

Luftverschmutzung in den Städten

Spatestens seit dem VW-Abgasskandal sind Stickoxid-Emissionen in den Fokus offentli- cher Debatten geruckt. Stickoxide sind sehr giftig und ihre Konzen- tration in den Stadten gesundheitsgefahrdend hoch – unter realen Bedingungen liesen sie sich bislang aber nur schwer messen. Heidel- berger Umweltphysiker haben eine innovative Losung fur das Problem.

Two- and Three Dimensional Observation of Trace Gas and Aerosol Distributions by DOAS Techniques

Spatially resolved measurements of trace gas abundances by satellite have revolutionised the field of large-scale tropospheric chemistry observation and modelling during recent years. Now a similar revolution is imminent on local and regional scales. A key role in these advances is played by spatially resolving spectroscopic techniques like active and passive – DOAS tomographic measurements of two-dimensional trace gas distributions, as well as ground based and airborne Imaging DOAS (I-DOAS) observation of 2D- and 3D- trace gas patterns. A particularly promising approach is the combination of tomographic techniques with imaging – DOAS on airborne platforms, which can provide three-dimensional trace gas distributions. While satellite-based 2D – mapping of trace gas distributions is now in widespread use for global and regional investigations aircraft based instruments allow complementary studies at much higher spatial resolution (tens of meters instead of tens of km). Since state of the art instruments can be employed rather than technology from the last decade (which is dictated by reliability requirements and long lead times of satellite experiments) novel approaches like tomographic techniques or Short-Wave Infra-Red (SW-IR) observations can be applied. Technological approaches and sample results are discussed.