Dr. Roland von Glasow
email: R.Von-Glasow@uea.ac.uk
School of Environmental Sciences
University of East Anglia
Norwich NR4 7TJ
U.K.

Prof. Ulrich Platt
email: Ulrich.Platt@iup.uni-heidelberg.de
Institut für Umweltphysik
Universität Heidelberg
Im Neuenheimer Feld 229
6920 Heidelberg
Germany

Reactive halogen compounds (X, XO, X₂, XY, OXO, HOX, XONO₂, XNO₂, where X,Y=Cl, Br, I) - in particular halogen oxides - are present in various domains throughout the troposphere. The primary objective of the project HitT - Halogens in the Troposphere - is to determine and quantify the importance of reactive halogen compounds (RHCs) in tropospheric chemistry and climate forcing. Key themes are the influence of RHC on the oxidative capacity of the atmosphere, the ozone budget, as well as in aerosol nucleation and growth.

HitT addresses several of the foci of IGAC (esp. 1.1, 1.2, 1.3, 2.1) as it helps constrain processes affecting tropospheric O₃, aerosol number and composition, as well as chemistry-climate feedbacks. Most - but not all - sources for RHCs are natural but these are subject to global change and we have to increase our knowledge about the feedbacks in order to assess the impacts of global change on RHCs and the resulting changes in the troposphere.

Historically, reactive halogens were first recognized to be of importance for stratospheric chemistry. In the early 1980s the discovery of "ozone depletion events" (ODEs), drastic ozone depletions in near-surface air during polar sunrise in the Arctic boundary layer (BL) were observed, and these have triggered us to rethink the usually held notion that inorganic halogens in the troposphere would mainly be in the form of hydrogen halides (HX, where X=F, Cl, Br, I) and would be rapidly be removed by wet and dry deposition.

The dramatic increase in our knowledge and the appreciation of the details of reactive halogen chemistry in the troposphere is reflected in the number of reviews that dealt with this topic in the last 10 years (e.g. Graedel and Keene, 1995, Wayne et al., 1995, Carpenter, 2003, Platt and Hönninger, 2003, von Glasow and Crutzen, 2003 and O'Dowd and Hoffmann, 2005). A group of experts on halogen chemistry initiated the project HitT at a workshop in Heidelberg in May 2004. As an outcome a White Paper (3 MByte .pdf) was written with contributions from other scientists from the broader community that briefly summarizes the current knowledge on halogens in the troposphere and lists detailed research questions as well as initiatives to address the questions. This paper is a condensed version of that White Paper.

Reactive halogen compounds in the troposphere are mainly of natural origin and influence atmospheric chemistry and physics in several regards. The main implications of RHC for the troposphere are:

• catalytic ozone destruction and therefore change (decrease) in oxidation power of the troposphere and its radiative forcing
• change in HO₂:OH and NO:NO₂ ratios with implications for many photochemical reaction cycles including a decrease in ozone production
• increase in CH₄ and NMHC oxidation by the chlorine radical, therefore increasing the oxidation power of the atmosphere for these compounds
• new particle formation and particle growth by iodine compounds with potential consequences for cloud formation and lifetime and inland transport of particulate iodine
• deposition and enhancement of bioavailability of mercury especially in polar regions
• increase in DMS oxidation and shift in the final products potentially leading to a decrease in cloud albedo

Figure 1 gives an overview of the regions where halogen chemistry has been shown to be involved in tropospheric photochemistry. The research topics are arranged by these domains.

The characterization, quantification, and understanding of the abundances and cycles of RHC has already begun in many parts of the troposphere. However, beyond the present, often only pioneering and exploratory efforts, a comprehensive and coherent approach is needed, which is beyond the national scope and requires international and interdisciplinary cooperation.

In order to address the HitT science questions the combination of many measurements in comprehensive field campaigns is required. This can usually only be achieved if the know-how, instrumentation, and logistics from several countries is combined. One of the tasks of HitT is to suggest campaigns and to provide a platform for collaboration so that optimum use of the resources can be made. Our efforts will be combined with existing/planned activities by e.g. IGAC, SOLAS, iLEAPS, OASIS, AICI and others. We aim to bring scientists from different fields (chemists, physicists, meteorologists, (micro-)biologists; laboratory, field, model, different focus on topics) and countries together to identify, discuss, and publish the most important topics with regard to reactive halogen chemistry. We want to make use of synergies between different fields/domains (e.g. measurement techniques and models developed for one domain can be applied to others as well) because of the interdisciplinary and international nature of the subject.

The following research topics have been identified in the HitT White Paper (3 MByte .pdf) to receive priority in the next years. Details on each are given in the next section:

1) Sources and distribution of RHC:
Determine the emission fluxes of and key release processes for RHCs and their precursors from the open and coastal oceans, polar regions, land surfaces, volcanoes, and urban-industrial areas. In order to achieve this goal existing techniques have to be refined and new, faster and more sensitive methods for measuring RHCs have to be developed.

2) Transformation and transport:
Develop a detailed understanding of the multiphase chemical processes that determine the distribution of RHCs and their precursors at different spatial and temporal scales throughout the troposphere and the physical processes including aerosol- and cloud-microphysics and transport. This effort should ultimately lead to a realistic representation in numerical models.

3) Implications of RHC:
Integrate different measurement techniques and models to determine the regional and global role of RHCs in a series of physico-chemical processes in the troposphere, including: tropospheric oxidation processes (esp. sulfur), the ozone budget, HOx and NOx radical cycles, and aerosol nucleation and evolution.

The RHS sources in the different domains involve similar physico-chemical processes so that important synergy can be achieved if the respective communities dealing with the different domains collaborate closely. In order to understand reactive tropospheric halogen chemistry and especially to properly assess the regional and global effects with numerical models, a thorough qualitative and quantitative understanding of the sources and transformation processes is essential.

Our research plan consists of three main parts: 1) focused investigation of processes in the single domains and 2) investigation of reaction mechanisms in the laboratory, and 3) development of research tools: improved instruments for field measurements and improved numerical models (box to global three-dimensional).

The foremost research question for all domains is the regional and global importance of these processes. All suggested research ultimately aims to provide the basis to answer this question.

We restrict the discussion of the research topics to only a few key questions per domain. Most of these cannot be addressed separately but only by including several other questions - as explained in the White Paper.

Polar boundary layer
In this domain, the profound effects of RHS in the troposphere were first discovered -- the ozone holes in the polar boundary layer caused by bromine chemistry. The upcoming International Polar Year (IPY) provides a great opportunity to further our understanding of halogen processes in the polar regions and their impact on the polar regions, esp. the atmosphere. A lot of HitT related research is already part of other IPY initiatives, for example as part of OASIS or AICI.

RHCs are present in the polar atmosphere in very high concentrations during the ozone depletion events. The complete lifecycle of such an event, especially the start of it, is still subject to debate. Links to the cycles of other elements, foremost mercury, have great relevance for life at high latitudes. Part of IPY are large, coordinated field campaigns including aircraft, ships, drift stations, buoys, and numerical modeling on various scale as well as supporting laboratory studies. This domain will be part of Phase 1 of HitT. The continuation of existing and establishment of further long timeseries is important to understand the natural variability and possible human influence.

Salt Lakes
Salt lakes can be used as "natural laboratories" to study the release and cycling of halogens from a large, extended source. In order to advance beyond the current studies, coordinated field-model studies that take the orography into consideration should be pursued. The field part would ideally be set up in a stationary lagrangian mode starting upwind from salt deposits and investigating both the vertical and horizontal extend of halogen activation. A combination of ground, sonde and possible airship measurements is anticipated for a major campaign in Phase 2 of HitT. Satellite data should be used to monitor salt lakes, salt pans, and other regions with exposed salts for the occurrence of RHS. A key question to be answered is to what extent the released reactive halogens are reaching the free troposphere.

Marine Boundary Layer
Halogens are released in vast amounts from the ocean in the form of sea salt aerosol and organic halogens, which are mainly produced by algae. Upwelling regions and coastal regions are the main areas where organic halogens are being released. Measurements of RHC have been made for decades but we still lack global maps of the relevant compounds which - according to numerical models - are important in concentrations that provide great challenges for current instrumentation. In addition, for many important compounds no measurement technique is available. There are indications that the distribution of RHC in coastal regions is very heterogeneous. Furthermore, many open questions remain regarding the chemical release and cycling mechanism.

As about 70% of the earth's surface is covered by oceans, this domain is key in assessing the global importance of RHC for the troposphere. We have to overcome the above mentioned technical problems in order to answer these questions. In addition to new techniques, improvement and combinations of existing techniques and different indirect analysis procedures should be pursued.

Several SOLAS projects are already ongoing that fall into this field, so that this domain will be a focus of Phase 1 and also 2.

One point that has only been addressed in very few studies so far, is the advection of marine air masses that contain RHC both in the gas phase and aerosol particles into coastal cities, where they are mixed with anthropogenic pollutants which might include further halogens. Many of the existing and future Megacities are located in coastal regions, making this issue one of increasing relevance.

Halogens in Plumes
Volcanoes, biomass burning and dust storms are sources of concentrated plumes and also of RHCs. Due to the high concentration of gases and aerosol particles reactive cycling is expected to be occurring. Field measurements of halogen oxides are so far only available for volcanic plumes but it can be expected that similar processes also occur in other types of plumes. We need a combination of field, laboratory, and model investigations for these very specific conditions.

Monitoring of volcanoes has been done for a long time, new networks are being build (e.g., NOVAC) and attempts are being made to use satellite data to detect halogen oxides in volcanic plumes. Additional pilot studies with regard to halogens in volcanic plume shave been made, so that these plumes will be foci of Phases 1 and 2. Dust and biomass burning plumes have not been studied so far with regard to RHC, so that pilot studies are planned for Phase 1 and more detailed studies for Phases 2 and 3.

Free Troposphere
Except for two direct measurements, so far only indirect determinations of halogens in the free tropospheric have been reported. Global model calculations show that BrO in the 0.5-2 ppt range would have a distinct impact on photochemistry, reducing the zonal mean concentration of ozone by 5-20% (von Glasow et al., 2004, Yang et al., 2005). In order to reduce the existing uncertainties in this domain, we urgently need more measurements of BrO as well as precursor compounds. This can to some extent be done by remote sensing instruments but in-situ, high precision measurements are also needed.

Given the potentially high relevance for our understanding of tropospheric chemistry, this topic should be one of our highest priorities.

Other continental sources
Soils and vegetation have been shown to release organic halogen compounds. So far, the focus was on long-lived compounds that might impact the stratospheric halogen load, so we have to add to these investigations a focus on short-lived compounds. In addition to field measurements, laboratory studies should be undertaken to provide more information on regions where these sources are relevant as well as more detailed source processes for the inclusion into numerical models. Almost all sources for RHC that we mentioned so far are natural. Anthropogenic sources include cooling towers, industry etc. and are so far ill-characterized. A lot more information is needed - best in the form of inventories - in order to assess the relevance on photochemistry.

Many important chemical and photochemical processes involving halogen species in the MBL are associated with very large uncertainties, which impede assessment of their role in e.g. ozone destruction, sulfur oxidation, liberation of sea-salt halides and particle formation. For gas-phase processes involving chlorine and bromine species, the kinetic / photochemical database is in good shape, for iodine species this is not the case. To be able to completely address the impact of halogen chemistry on the sulfur cycle, more information on the break-down products of esp. DMSO has to be gathered. Similarly, still important gaps in our knowledge of the halogen-mercury coupling exist. Our understanding of multiphase processes, aqueous phase processes and (iodine related) particle formation has large gaps for all halogens. In the White Paper a detailed list of topics for future laboratory studies is listed. Molecular modeling is also very helpful in addressing fundamental questions regarding reaction mechanisms.

As already mentioned above, several gaps in our understanding of tropospheric halogen chemistry are due to the very small concentrations of RHC and involved instrumental problems. This is true not only for the gas phase but also for measurements of particulate phase compounds. A detailed list of instrument development requirements is given in the White Paper, here we only repeat a few of these.

Gas phase
New/alternative techniques for the detection of X, XO, HX, HOX, XY, XONO₂, XNO₂, and oxidized Hg should be developed. Exact speciation will be needed as much as low detection limits. The instrumentation should be designed to measure high spatial (horizontal and vertical; meter-scale and BL) variation of XO. Important in this regard is also to develop an algorithm for the detection of IO from satellites.

Aerosols
The poor temporal and vertical resolution of current measurement techniques for the chemical composition of super-micron-diameter aerosols is a major impediment to progress in resolving the nature and importance of halogen-radical chemistry in the atmosphere. Instruments that enable us to probe the vertical profiles and diurnal temporal variations of particles should be developed. Furthermore, the size range of current aerosol mass spectrometers should be extended both to smaller and larger sizes.

In the White Paper we also list promising measurement strategies that would help to address the above mentioned research questions.

A variety of numerical models to study tropospheric halogen chemistry has been developed, ranging from box to 1D and 3D models. Model studies can play a vital role in answering the open questions, address single processes, and can help locate measurement sites, where, according to current knowledge as implemented in the models, promising situations should be present. The models are very complex as they have to include not only extensive gas-phase reactions mechanisms but also particulate phase reactions. Even though process in the global multi-phase modeling of halogen chemistry has been made, this is an area where more research is needed. With the help of box and 1D models further details of the physico-chemical processes involved can be studied and reaction mechanisms of reduced complexity can be provided for global 3D models. More details and specific suggestion can be found in the White Paper (3 MByte .pdf).

Key to expanding our knowledge are field campaigns and long-term observations, which will be accompanied by and closely coordinated with laboratory and modeling studies. In the following we outline HitT activities for the next 9 years, divided into three 3-year periods. This timetable, esp. during Phase 1, refers to a large number of already funded projects. One of the main goals for HitT will be to bring the scientists from these projects together for discussions, scientific workshops, and technical and scientific exchange to ensure that the best possible use is made of synergies and cross-links between the different projects. HitT will serve as a forum to link ongoing and planned projects and to produce ideas and prepare proposals for future projects. The focus of our outline is on Phase 1, the topics of Phases 2 and 3 will to a large degree be determined by the outcomes of Phase 1.

The International Polar Year (IPY, 2007 - 2008) will give us a chance to study halogen-related scientific questions in both polar regions with a multitude of small and large campaigns. Several of these campaigns have already been set up with a strong halogen component (e.g., O₃ and BrO measurements aboard the sailing vessel Tara and autonomous buoys; a NSF-funded Summit campaign focusing on snow processes and the impact of halogens on HOx, NOx and free tropospheric chemistry; COBRA (NERC funded): halogen campaign in the Hudson Bay with a focus on iodine and particles). Furthermore, we have strong links with the tasks OASIS (Ocean air snow interactions) and AICI (Air ice chemistry interactions). Both these tasks are planning field campaigns with strong halogen components and we are establishing links between these and our task.

The science questions that will be addressed with regard to halogens in the polar boundary layer and the information that we expect from the investigations during IPY are:

• Details about the start of ODEs.
• What is the actual source for the bromine - frost flowers, brine?
• What is the magnitude and relevance of the export of halogens to the free troposphere?
• Importance of snow for cycling and release of halogens.
• Has iodine been overlooked in previous studies? What is its source?
• Is there a potential for iodine related new particle formation?
• To what degree do dark processes (polar night) play a role in the initiation of ODEs?

The second focus of Phase 1 will be the marine boundary layer, as a number of national programs that deal with halogens in the troposphere have already been funded. This includes the EU-funded projects MAP and OOMPH which both deal particle formation in the MBL and the national contributions to SOLAS (Surface Ocean Lower Atmosphere Studies), where especially the UK (e.g., RHaMBLe, the Cape Verde observatory) and German programs (SOPRAN, MAPHINS) have large halogen components.

The science questions for the MBL include:

• How widespread are reactive halogens in the MBL?
• Are there interactions between dust deposition and release of halogens from the ocean?
• Is coastal iodine-related new particle formation of relevance on a global scale? Can iodine compounds increase the nucleation rates over the open ocean?
• What is the importance of halogens for the ozone budget in the MBL?
• How relevant is halogen chemistry for the sulfur cycle from a global, quantitative viewpoint?

Additional halogen-related projects that are ongoing or in the final stage of proposal writing are i) NOVAC, an EU-funded global network of volcano observatories, which are mainly for the monitoring of SO₂ emissions but which is also envisioned to be used to study halogen oxide emissions (project started in 2006); ii) HALOPROC, a German project involving 8 institutions focusing on halogenation processes in soils, salt lakes, and aerosols (preproposal accepted, final proposal submitted); iii) as part of CARIBIC II, a measurement container on commercial aircraft, a MAX-DOAS system is installed, which is being used for the investigation of free tropospheric BrO.

Pilot studies that would be very well suited for Phase 1 and that can be easily added to other projects include the investigation of halogen chemistry in dust and biomass burning plumes. Several large campaigns for each of these topics are being planned (e.g., as part of UK-SOLAS) and it would be easy to add a small, exploratory halogen component. Furthermore, more pilot studies should be made in the free troposphere. All these pilot projects should be in preparation for large field campaigns in Phases 2 and 3 of HitT.

Laboratory studies, instrument development, and numerical modeling are part of many of the mentioned large projects, several smaller projects are also ongoing and one important goals of HitT is to link these into the larger scientific community.

Deliverables for Phase 1:
We plan an implementation workshop for April 2007 with the main topics: coordination of IPY and SOLAS-related work; preparation of pilot studies for Phase 1 and strategic planning for the following phases; and coordination of planned model and laboratory studies. After the end of the IPY, we plan another workshop, possibly together with OASIS/AICI, to synthesize the results in a review article that should be published in a peer-reviewed international journal. A similar workshop will be planned for research in the MBL. Further platforms for scientific exchange are sessions at international conferences, a halogen session will be part of the EGU meeting 2007 in Vienna (organized by Rolf Sander and Roland von Glasow).

As already mentioned above, the foci of Phases 2 and 3 will depend on the outcome of Phase 1 (and 2). Several of the SOLAS projects mentioned under Phase 1 will still be ongoing in Phase 2 and some even in Phase 3 (e.g., SOPRAN, MAPHINS) so the MBL will remain one focus of these phases. The other foci will be chosen based on the pilot studies in Phase 1 and 2. Further large campaigns could include Megacities, esp. coastal Megacities to investigate the link between anthropogenic pollution and halogens and poss. related ozone production, salt lakes and regions with extended exposure of salty soils and halogen chemistry in plumes (volcanic, dust, biomass burning).

Long term observatories will remain important part of HitT. This includes ground-based, satellite and recurrent aircraft measurements. We will continue to organize science workshops and sessions at international conferences to ensure the close collaboration of the scientific community and to provide platforms for discussions.

In order to analyze interannual variations and to detect trends, longterm observatories are essential. Several networks are in existence (e.g., the GAW network) and some stations have very long records (e.g., Cape Grim); others are still being established (e.g., the UK-SOLAS/D-SOLAS observatory on the Cape Verde Islands or the volcano project NOVAC). Satellite platforms are very suitable in this regard (see, e.g., the time series of BrO clouds on the cover of this paper), however, more algorithm development is still needed. Commercial aircraft equipped with scientific instruments are also used as quasi-longterm observatories as they provide information from regions that are traversed on a regular basis (e.g., MOSAIC, IGAOS, CARIBIC). It is our goal to contribute to more halogen-related measurements at these sites and to try to identify compounds from the standard measurements that might be used as proxies for RHCs.

As outlined in section 2, the development of instruments for the field, laboratory techniques and numerical models are needed. Partly they are ongoing but HitT will also try and encourage the continuation and expansion of these activities. Many of the domains that we listed deal with processes in the boundary layer, where vertical gradients are often of key importance. The best platform to investigate such gradients would a an airship as it has a very large payload, can do vertical profiles but - probably the most important advantage - it is a platform that is best suited for Lagrangian studies, so that the evolution of an air mass can be studied. An European project is at a very advanced state for the actual equipment of an existing airship with instrumentation. Several of the PIs on this project are also actively involved in halogen research, so that we will actively put forward initiatives to use airships as measurement platforms.

Most projects involve PhD students as key participants. Furthermore, we will participate in summer schools like the SOLAS summer schools with specific halogen related topics and actively encourage the participation of students at workshops and halogen sessions at international conferences.

This is an umbrella for funded campaigns many of which have their own excellent data plans, e.g. UK data centres.

This task proposal has been written on the basis of a White Paper which was written by a large number of experts from several countries who initiated this program on a workshop held in Heidelberg, Germany in May 2004. The full White paper can be downloaded from here (3 MByte .pdf).

In this list we only included a few, mainly review papers, for a detailed bibliography, refer to the White Paper.

A Core Project of the International Geosphere-Biosphere Programme (IGBP)

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