MarHal
Research topics of MarHal


Our research focuses on the following topics:
These research areas are motivated by an increasing number of measurements and model studies that find reactive halogen chemistry in the troposphere to be of potentially major importance, both in the marine boundary layer and in the free troposphere. The main source of halogens over the oceans is release from seasalt aerosol and organic halides that both originate from the ocean. The international IGAC/SOLAS task "Halogens in the Troposphere" (of which Roland von Glasow is co-chairman) aims at elucidating halogen chemistry by combining field, laboratory and modelling studies.

The other group of topics is within the scope of the international Surface Ocean Lower Atmosphere Study (SOLAS) project "To achieve quantitative understanding of the key biogeochemical-physical interactions and feedbacks between the ocean and the atmosphere, and how this coupled system affects and is affected by climate and environmental change" (quote from the SOLAS science plan).

Our research topics are shown schematically in the following figure:



The main tools of this research group are numerical modelling (box, one-dimensional, three-dimensional) and model-measurement comparisons. Below we list some of the current/previous projects in more detail.


Influence of halogen chemistry in the marine boundary layer
The importance of halogens for the chemistry of the stratosphere has been proven by numerous field, laboratory and modelling studies. In the stratosphere the most important source for halogens is decomposition of man-made CFC. In the troposphere, however, the main source is the emission of sea salt aerosol from the ocean surface. Sea salt aerosol contains chloride and bromide which can get released from the aerosol by a sequence of gas phase - aqueous phase reactions. Once in the gas phase the halogen radicals (especially Br) have the potential to destroy ozone and to oxidize dimethyl sulphide (DMS), which is a main precursor for small cloud condensation nuclei in the marine boundary layer (MBL). Another halogen source in the MBL is the release of organic precursors that contain iodine, bromine and/or chlorine. Photolysis rapidly releases halogen radicals.
To study these topics we use numerical models like the one-dimensional model for the MBL MISTRA. We discussed the following chemical effects in these publications:
Chemistry in the plumes of passively degasing volcanoes
From an atmospheric point of view, the chemistry of passively degasing erupting volcanoes, i.e. those non-explosive eruptions where the volcanic plume remains in the troposphere, has received only little attention until recently. The detection of the largest concentrations of BrO found so far anywhere in the atmosphere by Bobrowski et al. (2003) triggered many follow-up studies. We've have been involved in comparisons of measurements in plumes with numerical model calculations to gain a better understanding of the complex multi-phase chemical processes occuring in volcanic plumes. The focus in these studies is on the chemistry of halogens (Bobrowski et al., 2007) and sulphur species (Aiuppa et al., 2007). We were able to show that a process very similar to the polar "bromine explosion" is occuring in volcanic plumes leading to the build-up of very large concentrations of BrO with distance from the crater edge. The review von Glasow et al (2009) gives an overview of the relevance of volcanic emissions on atmospheric chemistry with a focus on the troposphere. In von Glasow (2010) an updated version of the model used in Bobrowski et al. (2007) is employed which I used to study more details of reactive chemistry in volcanic plumes and the interaction of bromine and mercury chemistry in volcanic plumes. This paper discusses teh widespread impacts on atmospheric chemistry including strong ozone destruction in the plume and rapid oxidation of elementary gaseous mercury. Further studies are currently going on. We are developing an innovative measurement platform for the in situ measurements of volcanic plumes, a remote controlled blimp. The first test flights were successful and we are currently finalising the payload.

Influence of halogen chemistry in the free troposphere
From the combination of ground-based, balloon-borne, and satellite observations of BrO in the combination with a model of stratospheric chemistry the widespread presence of BrO in the free troposphere was postulated in the literature. We used the global 3D CTM MATCH to investigate the influence of BrO on the oxidation power of the atmosphere, its influence on the NOx, HOx, and sulphur cycles and found the presence of 0.5 - 2 pmol/mol (as shown in the observations) to be very important for chemistry of the free troposphere (von Glasow et al., 2004).

Polar atmospheric chemistry
For more than two decades it is known that bromine is involved in the so-called Ozone Depletion Events (ODE) during "bromine explosions" in polar regions. Yet the details of the "birth" of ODEs are still not fully understood (e.g., Morin et al., 2008). The international project AICI has published a series of review papers that discuss the involved chemistry in detail and stress the importance of snow photochemistry; we have co-led and contributed to two of these reviews (Simpson et al., 2007 and Grannas et al., 2007, respectively). Our work with a box and a 1D model helped in defining time scales involved and guiding future field and model studies (Piot and von Glasow 2008, 2010). In collaboration with US investigators (mainly Jochen Stutz, UCLA) during the GSHOX campaigns at Summit, Greenland we expanded our existing 1D atmospheric model by coupling it to a 1D snow photochemistry model (Thomas et al., 2010). We could reproduce observations of BrO and NOx at Summit and showed quantitatively the importance of snow photochemistry for atmospheric chemistry. This new coupled model is currently being used in a collaboration with the British Antarctic Survey to help analyse data from the Halley research station in Antarctica. We are planning to develop a simplified snow photochemistry model for inclusion into regional 3D models.

Chemistry - cloud - climate links
Dimethyl sulphide (DMS) is the main natural source of sulphur in the marine boundary layer. It is being produced by phytoplankton in the oceans and released to the atmosphere. Many studies in the past 20 years have been trying to elucidate links between DMS emissions, cloud properties and climate forcing but a proof of a climate regulating feedback as proposed by Charleson et al (1987) remains elusive. Our work focusses on a detailed understanding of the breakdown of DMS in the remote MBL and the role of halogens in this (von Glasow et al., 2002b, von Glasow and Crutzen, 2004). There are still significant uncertainties in the chemical pathways of DMS oxidation but what is obvious is, that the assumption of fixed sulphate yields from DMS and a related straight-forward increase of cloud droplet number and cloud albedo is too simplistic. Our research suggests that the presence of halogens (esp. BrO) and sea salt aerosols leads to important shifts in DMS oxidation pathways leading to the growth of pre-existing aerosol particles rather than nucleation of new particles. The uptake of DMS and its oxidation products into short-lived sea salt aerosol effectively shortcuts the marine sulphur cycle. Details can be found in von Glasow and Crutzen (2004) and von Glasow (2007).

Effects of ship emissions on the marine boundary layer
It has been known for a long time that particle emissions of ocean-going ships can distinctively change the albedo of marine clouds. Only recently the importance of ship emissions for the chemistry was realised and emission inventories became available. We investigated the impact on the chemistry of the clean atmosphere in another model study (von Glasow et al., 2003) and found that it is very important to account for plume dilution. In a clean marine environment it takes about 2 days before air in the plume is essentially indistinguishable from unpolluted air ("chemical lifetime of the plume"). The lifetime of nitrogen oxides - which are important for the formation of ozone - is shorter in the ship plume, nevertheless significant amounts of ozone are formed in the plume. We also participated in the analysis and discussion of satellite observations of NOx emmissions from ships in the Bay of Bengal (Beirle et al., 2004). Satellite studies have recently found the wide-spread presence of HCHO in ship plumes. We collaborated in a model investigation of the most likely sources (Song et al., 2010), showing that the oxidation of CH4 by elevated levels of in-plume OH radicals is the most likely source for the elevated HCHO.

Interactions between radiation fog and tall vegetation
Vegetation has a strong influence on heat and moisture fluxes to the atmosphere. It can also impede the formation of radiation fog and lead to the interception of cloud droplets. We investigated this using a one-dimensional model with coupled microphysics, radiation and vegetation processes (von Glasow and Bott,1999).