PIs: John P. Burrows, Astrid Bracher

The Arctic cryosphere and ocean have a unique role in the Earth’s radiation budget and the bio–geochemical cycling in the planetary boundary layer. Although conventionally the Arctic is considered pristine, it is increasingly being influenced by anthropogenic activity. Our understanding of the impact of the observed decline of sea ice on the atmospheric composition, in particular in the Atmospheric Boundary Layer (ABL), the interactions with ice, snow and ocean and the ecosystems and related feedback is limited and inadequate. Changes in the extent and type of sea ice, and thus amount of open ocean water, influence the inorganic production of halogen oxides and the productivity of the biomass in the ocean. The halogen free radicals play a key role in the oxidizing capacity of the Arctic ABL through complex inorganic and organic heterogeneous reactions and/or bio–geochemical photochemical mechanisms. The amount and type of phytoplankton and Coloured Dissolved Organic Matter (CDOM) are also influenced by the changing amount of ice. They are sources of organo–halogens released from the surface ocean to the atmosphere. The oxidation and photolysis of organo–halogens generate halogen radicals. Thus the amounts and distributions of halogen radicals and oxidation chemistry in the Arctic, are also expected to respond to Arctic Amplification. Further the absorption by chlorophyll and its pigments warms the ocean and photosynthesis by phytoplankton draws down carbon dioxide.

The changes and links between reactive halogens and phytoplankton and their response to changing physical conditions (sea ice coverage and thickness, which depend on its age, temperature, stratification, radiation, etc. of the surface ocean and ABL) are the research foci of this investigation. To meet the objectives of the project, consistent and consolidated data products, retrieved from remote sensing instrumentation on satellite platforms, will be produced and studied to provide insight about the changes of trace constituents and phytoplankton types and CDOM absorption over the past two decades. The scientific objectives of this proposal are:

• To produce validated data sets using observations from the satellite instruments GOMEc, SCIAMACHYand GOME–2 and merged products on (i) the distribution of phytoplankton types and CDOM absorption in the Arctic Ocean, and (ii) halogen oxides (BrO and IO) in the Arctic, primarily in the troposphere but also in the stratosphere

• To evaluate changes in the quantities of halogen amounts, phytoplankton types and CDOM over the last two decades and thereby to establish, investigate and understand the changes in the oxidative capacity of the Arctic ABL,

• To assess the impact of Arctic Amplification and the related changing coverage and thickness of sea ice, (increasing) temperatures, solar radiation on the phytoplankton composition, loading of CDOM and halogen chemistry of the ABL in the Arctic region, their changes and the feedback between each other.

This project utilizes, in the first instance, the long–term remote sensing measurements of the upwelling radiation at the top of the atmosphere in the solar spectral region. The series of hyperspectral measurements of UV, visible and near infrared radiation began in 1995 with the instrument GOME (ESA ERS–2 1995 to 2003), and has been continued with SCIAMACHY (ESA Envisat 2002 to 2012) and GOME–2 (MetOp–A 2006 to present MetOp–B 2012 to present). These instruments fly in sun synchronous orbits, having equator crossing times in the early morning. Data from the OMI instrument on AURA, (2004 until today) and TROPOMI in Sentinel-5-P (launched in 2016), which provide higher spatial resolution in a sun synchronous early afternoon orbit, will also be used for halogen oxide and ocean colour retrievals. In addition, multi spectral measurements of the upwelling solar radiation having much higher spatial resolution but much lower spectral resolution are available from the sensors ATRS–2 (ERS–2 1995 to 2003), MERIS and AATSR (both on Envisat 2002 to 2012 ), SeaWiFS (on OrbView–2 1997 to 2007), MODIS (on Aqua 2000 to present), AVHRR–3 (MetOp–A 2006 to present and MetOp–B 2012 to present) and OLCI (Sentinel–3 planned for launch in 2016). Data from these instruments will also be used to complement the hyperspectral retrieval results and validate ocean–ice–ecosystem system model data.

Central questions which will be answered in the project are:

• What are the recent decadal changes in Arctic atmospheric composition and oceanic phytoplanktonand coloured dissolved organic matter (CDOM) composition, and

• How are they linked to each other and to changes of sea ice and other environmental and meteorologicalparameters?

• Key atmospheric constitutents in the ABL and absorbers of radiation in the surface ocean, and their interactions to be used as input for Cluster D and E modelling
• Surface properties and radiative forcing needed to improve retrievals and models, respectively