Oceanic Reactive Carbon: Chemistry-Climate impacts (ORC3)

Oceanic organic carbon (OC) aerosol particles have been proposed to exert a profound effect on climate through modification of the properties of shallow marine stratocumulus clouds, yet their sources are highly uncertain. These aerosol may be generated directly by sea spray involving a bubble bursting mechanism, or by the emission and subsequent oxidation of biogenic VOC (BVOC) that produce semivolatile products and aerosol. There is evidence that the secondary component of marine OC is underpredicted by our current models. Oceanic terpenes, especially isoprene and the monoterpenes, are highly reactive BVOC produced by phytoplankton, and are prime candidates for secondary organic aerosol (SOA). Limited observations show high concentrations of these species over biologically active regions of the remote marine atmosphere, which implies that they may play a role in modifying marine cloud properties through SOA formation. A clear picture of the contribution of marine terpenes to SOA is hampered by a lack of observations in the remote MBL. Substantial differences exist between "bottom-up" methods of estimating global emissions, where lab-based photoplankton production rates are scaled to the global oceans, and "top-down" estimates, where the source is scaled to force a match between modelled and observed atmospheric concentrations. The bottom-up methods are generally a factor of 10-1000 smaller than top-down methods, suggesting that they do not capture the full range of marine processes giving rise to BVOC production. The much higher top-down estimates however are also subject to significant uncertainty since they are derived from only a limited quantity of concentration measurements in the MBL. Our preliminary model results suggest that a global monoterpene source of the magnitude required to reproduce observed marine concentrations may increase the aerosol number acting as cloud condensation nuclei (CCN) by up to a factor of two over large oceanic regions. This has profound significance for our understanding of the first aerosol indirect effect, since it alters our understanding of controls on the background natural aerosol. In addition, marine terpenes are highly reactive, and therefore may alter the atmospheric oxidising capacity (and therefore the lifetime of the greenhouse gas methane), especially in the marine tropics where most methane is destroyed.

Another potential source of marine SOA is glyoxal, a highly reactive species which has been observed in the remote atmosphere many 1000s km away from the coasts. This species has a lifetime on the order of hours and so transport from terrestrial sources cannot explain its presence in the remote marine atmosphere. Oxidation of glyoxal leads to the rapid production of peroxy radicals and condensable products which are believed to lead to the formation of SOA. However the presence of glyoxal in remote marine regions is so far unexplained. One as yet unexplored hypothesis is photo-oxidation of larger VOCs such as terpenes. The implications of a large marine glyoxal source for atmospheric composition and climate has also yet to be tested.

In this project we will substantially increase the observational database of monoterpenes and isoprene in the marine atmosphere and evaluate existing and make new observations of glyoxal. We will use these observations along with global models of chemistry and aerosol to quantify the impact of marine reactive oceanic carbon on atmospheric composition and climate.

Improved understanding of natural processes controlling background aerosol and atmospheric oxidative capacity in the Earth's climate system is of high priority, since they underpin our estimates of man-made impacts on climate and the Earth system response. There is an urgent need to evaluate the marine sources of reactive volatile organic compounds (VOCs) and to quantity their importance for CCN, oxidative capacity and global climate.

Users and benefits

The outcomes of the project will deliver impact in five distinct areas; 1) climate and Earth systems models, 2) West African capacity building, 3) technology end-users, 4) policy influence and 5) public understanding. For each we have devised an appropriate route to deliver the maximum impact.

1. Climate and Earth system models
This project addresses research issues which are primarily associated with understanding the biogeochemical influence of oceanic emissions and their impacts on the overlying atmosphere. There are however very strong links between this basic fundamental science and users of Earth system models interested in process understanding, and specificallly for those associated with climate simulations. The users of such data are well known to the PIs, including other academic researchers, the Met Office Hadley Centre, and international coordinating agencies such as WMO and IGBP.
We will organise an early stage user group meeting where users' main questions regarding this research topic can be identified, and their needs, in terms of products and deliverables, understood at a technical level. A second end-of-project user meeting will be used to make parametrisations and findings from the project publically available, and to discuss in detail with stakeholders how the research may be used or refined in downstream Earth system or climate models. We will engage with providers of Earth System model National Capability within NERC, and will communicate our preliminary findings to the UK Earth system model community through the new ACITES Atmospheric Chemistry network (PI Mat Evans, York).

2. Capacity Building
The project has substantial ties to scientists in Cape Verde, through the Atmospheric Observatory (operated by INMG, the national meteorological service) and INDP (the national fisheries institute). This research will require us to deepen our links with INDP (itself a regional centre of excellence in fisheries and marine science in West Africa) and offer an opportunity for the research to make an impact through capacity building and training of scientists from developing countries. We would aim to use this research project as a means to extend the work into additional fields associated with marine chemistry, and in particular make new links to the recently established University of Cape Verde. We will also work through the FCO Science and Innovation network representatives (based at the UK embassy in Dakar) to ensure wider engagement with other interested nations.

3. Technologies end users
TThe high time resolution TD-GC-MS system to be used for monoterpene measurements in the project is a key new technology to apply to understanding monoterpene behaviour in oceanic environments. This instrument has been developed primarily for use as an aircraft deployed facility, and so this application for in situ and seawater measurements is a new technical application . We will extend the engagement with potential end users of data fom this instrument to include marine scientists.

4. Policy
Downstream policy impacts of the research will be achieved through delivering capability for improved climate model simulations, which include the new basic knowledge generated from this project. The additional data generated by short term measurements from this project will supplement the long-term contributions made at the observatory to WMO-GAW and world data centres, and will also be reported through UK data submissions to UNFCCC.