Early white paper draft version- October 2013
This preliminary draft is intended as a consultation document.
Comments/edits welcome. Please do not use or cite.
Theme 4: Interconnections between aerosols, clouds, and ecosystems
Co-authors: Trish Quinn, Ilan Koren and Rafel Simo
1. Brief statement defining the theme:
Interconnections between ocean-derived aerosols, clouds, and marine ecosystems are not well understood. Assessing the system as a whole is required for an accurate understanding of how a change in one component is manifested in another as well as the potentially complex web of associated feedbacks. In addition, accurate projections of the evolution of climate and the ocean biosphere can only be achieved through a better understanding of these potential interactions and feedbacks. The intent of this theme is to assess interactions between key components of marine aerosols, clouds, and ecosystems and associated feedbacks.
2. The scientific and societal basis justifying research on this issue. Why is it critical and why does it need to be done now? What is the end goal? Why is international coordination required?
Although clouds play a major role in climate and account for approximately two thirds of Earth’s albedo, they are the least understood component of the climate system and carry the largest uncertainty in global warming projections (Forster et al., 2007). Interactions between aerosol and clouds and impacts of the biosphere on both aerosols and clouds contribute to this uncertainty. Links between oceanic ecosystems and clouds may act as either amplifiers or buffers of climate variability.
Changes in cloud properties may impact ecosystems, including plankton physiology and dynamics, by altering incident radiation, precipitation, surface winds, the ocean mixed layer energy budget, and sea surface temperature. At the same time, aerosols alter the microphysical (e.g., cloud droplet number concentration and size distribution) and macrophysical (e.g., extent and lifetime) properties of clouds by acting as seeds for cloud droplet and ice crystal formation, i.e., by serving as cloud condensation nuclei (CCN) and ice nuclei (IN). A large fraction of the emission and production of ocean-derived CCN occurs in remote regions where concentrations of continentally derived CCN are low. In these regions, clouds are particularly susceptible to small changes in aerosol concentration.
Due to the scarcity of measurements and limited modelling capabilities, the emission, formation, transformation, and climate effects of ocean-derived aerosols are poorly understood. Hence, this theme will focus on first order problems including the biological, physical and chemical processes that determine the emission, production, and composition of ocean-derived aerosols and their effects on clouds. A first step is to obtain the data necessary to develop empirically constrained parameterizations of the emission flux and production rates of ocean-derived sea spray aerosol (SSA) and gaseous precursors of secondary aerosol (SA) and their impacts on cloud properties. The goal is to develop parameterizations for use in chemical transport models (CTMs), cloud resolving models (CRMs) and global climate models (GCMs) to accurately estimate impacts of ocean-derived aerosols on cloud properties and associated feedbacks on marine ecosystems.
A concerted effort involving shipboard measurements, remote sensing, and modelling studies is required to achieve these goals. The research will be interdisciplinary in nature involving oceanographers and atmospheric scientists. Current limitations in funding and ship time require that resources be pooled and that the effort be internationally coordinated.
3. Background – major scientific concepts, key prior work defining the issues:
Primary ocean-derived SSA is produced from the entrainment of air bubbles as waves break on the ocean surface. When injected to the atmosphere, the bubbles burst and yield SSA composed of both inorganic sea salt and organic matter. SSA is highly enriched in organic matter relative to seawater, especially for particles less than 500 nm in diameter (Keene et al., 2007; Facchini et al., 2008; Bates et al., 2012). The composition of the organic fraction is not fully known but has been reported to be composed of viruses, bacteria, microalgal debris, biogenic polymeric and gel-forming organic material (Facchini et al., 2008; Hawkings and Russell, 2010; Orellana et al., 2011). The processes controlling the source of the organics are not well understood and the impact of organics on the ability of SSA to act as CCN or IN and nucleate cloud droplets is very uncertain. This uncertainty is due, in large part, to a scarcity of measurements of freshly emitted SSA. Current model estimates of the flux and climate impact of SSA either do not take into account the organic component or parameterize the organic component based on surface seawater chlorophyll concentrations (e.g., Rinaldi et al., 2013). Chlorophyll is a measure of phytoplankton biomass but does not account for species composition, physiological status, productivity and non-phytoplankton planktonic activity, all of which may play a role in the production of organic matter available for incorporation into SSA.
Secondary aerosols (SA) form by nucleation of low volatility, oxidized products of trace gases and subsequent growth by condensation of semi volatile species on the seed particles. The most studied SA production process in the marine atmosphere is the oxidation of biogenic dimethylsulfide (DMS) into sulfuric and sulfonic acids. This process is the basis for the CLAW hypothesis whereby emissions of DMS, a by product of phytoplankton processes, lead to enhanced CCN concentrations and cloud albedo resulting in a biological regulation of climate (Charlson et al., 1987). The impact of a change in cloud albedo on DMS emission relies on particle nucleation in the boundary layer. The lack of observations of MBL nucleation over the open ocean along with evidence for primary (wind-driven) and free tropospheric sources of MBL CCN (including DMS) has led to the realization that sources of CCN to the MBL are much more complex than originally thought (Carslaw et al., 2010; Quinn and Bates, 2011; Clarke et al., 2013).
Nucleation events have been observed at coastal sites (Modini et al., 2009; O’Dowd et al., 2010; Chang et al., 2011) and sulfuric and sulfonic acids have been shown to nucleate new particles in the presence of organic condensable species in smog chamber studies (Metzger et al. 2010; Dawson et al. 2012). How these results apply to open ocean conditions is yet to be determined. Recent improvements in observational tools (Kulmala et al., 2013) should reveal the actual contribution of nucleation to total CCN numbers.
The growth of primary organic aerosols by condensation of surface active and hygroscopic compounds is also suggested as a CCN source (Andreae and Rosenfeld, 2008; Clarke et al., 2013). A very recent work suggests that bursts of nanoparticles can occur by in-cloud downsizing of primary organic aerosols (Karl et al., 2013). All in all, the contribution of primary and secondary sources to CCN numbers is yet to be fully assessed. The task stands as a formidable challenge due to the reaction of freshly emitted SSA with existing atmospheric gases and particles soon after emission resulting in a blurring of the distinction between SSA and SA.
Further complication comes from the transport of gases and aerosols that are derived in continental atmospheres and advected into the marine atmosphere resulting in complex internal and external particle mixtures (Andreae and Rosenfeld, 2008). Attempts to evaluate the impact of the ocean on cloud formation and properties and the radiative budget on a global scale must be able to distinguish between ocean and continental sources of aerosols that exist in the marine atmosphere.
The effects of marine ecosystem changes associated with global change (such as water warming and stratification, regional oligotrophication or eutrophication, and ocean acidification) on the formation and properties of ocean-derived aerosol and clouds remains uncertain. Equally uncertain are the feedbacks of naturally driven or global change associated changes in clouds and aerosols on marine ecosystems.
4. Approaches – what will it take to make substantive progress on the issue? What will be achieved in the 10 years of Future SOLAS?
· Simultaneous observations of surface seawater and freshly emitted SSA properties are required to determine the processes controlling the organic enrichment of freshly emitted SSA.
· New approaches for determining the emission flux of SSA and SA precursors, especially at high wind speeds, are required to reduce associated uncertainties.
· Development of techniques for the identification of the most important players among marine SA precursors (beyond DMS, isoprene and iodine) and to determine their sources, volatility, and aerosol yields. Amines and semi volatile hydrocarbons are suggested as target candidates.
· New techniques that allow for counting and characterizing nascent ultra-small aerosols to better assess the frequency and mechanisms of particle nucleation in the marine boundary layer.
· Measurements able to elucidate processes that modify aerosol in the MBL including growth, aging, photochemistry and internal mixing. Implementation of these processes in models.
· Simultaneous studies of surface ocean plankton taxonomy/ecophysiology/bloom dynamics, surface concentrations of aerosol precursors and aerosol characteristics to constrain and model the biological and environmental drivers of biogenic aerosol emission. Time-series studies (both short term –through bloom phases- and long term –through seasons and years) and across-provinces studies will be fundamental tools.
· Development of methods to discriminate between ocean- and continentally-derived aerosols found in the marine atmosphere to allow for the assessment of the impact of the marine biosphere on tropospheric aerosols and clouds.
· High quality and high-resolution measurements of the physical properties of the surface ocean mixed layer and the atmospheric MBL to decouple ocean-derived aerosol affects on marine clouds from physical effects.
· In situ and high-resolution satellite observations of aerosols, winds and cloud properties to improve process understanding and develop parameterizations of marine – cloud interactions. Participation by the marine aerosol community in the development of new remote sensing platforms and sensors, ensuring their relevance to ocean-aerosol-clouds feedbacks.
· Development of high-resolution numerical models to integrate cloud microphysics into small-scale process dynamics.
5. Community readiness – is there an existing community engaged on this issue? Are there institutional or other barriers to progress? Is infrastructure or human capacity building required in order to achieve the goals?
There is a growing effort among existing oceanographic and atmospheric science communities to address this issue, largely triggered by SOLAS during the last decade. Yet field studies with balanced contributions from both sides of the ocean-atmosphere interface are rare and should be emphasized in the future. Development of a common language (both concepts and terminology) to be shared by the two communities is in its infant stages but is needed for progress in address interconnections between aerosols, clouds, and ecosystems. In addition, the education of a new generation of scientists capable of looking across the interface will eventually be reflected in the building of truly coupled ocean-atmosphere modules in Earth System models.
There is a clear need to maintain and reinforce a dedicated international, interdisciplinary program like SOLAS to build frameworks that will bring the two disciplines together to facilitate the exchange of ideas and enhance the results of future experiments.
6. External connections – what partnerships are required in order to achieve the goals? What mechanisms will be used to accomplish the interactions?
Desired partnerships:
· IGAC
· Atmospheric Chemical Transport and Climate Modeling Communities
· Ocean Ecosystem Community
· ICCP (The International Commission on Clouds and Precipitation) http://www.iccp-iamas.org
7. Sustainability – articulate relationship (if any) between this project and the FE goals of Global Development and Transformation Towards Sustainability.
· The production of climate-active aerosols and clouds by the oceans must be considered when accounting for ecosystem services. The pelagic ocean provides aerosols that scatter sunlight as well as water vapor and seeds for cloud condensation, in addition to food provision, CO2 sequestration, O2 production, waste dumping and recycling, transportation and recreation, and cultural reference.
· Aerosols stand as one of the largest paradoxes in global change mitigation efforts. Since the Industrial Revolution, global dimming by anthropogenic aerosols has acted as the most powerful counterforce to greenhouse gas derived warming (IPCC 2007). Since the decade of 1980s when the harmful effects that aerosols have on health, visibility and cultural heritage were fully recognized, the development of cleaner and more efficient combustion technologies has led to reductions in anthropogenic aerosol emissions, at least in the most industrialized countries. The benefits of this reduction have (and will) come along with an acceleration of warming by reduction of the atmospheric dimming. An accurate assessment of the effects of aerosol emission policies on climate requires a solid knowledge of the current and projected roles of natural (including marine) aerosols on the energy balance at the regional and global scales.
References:
Andreae, M.O., D. Rosenfeld, Aerosol–cloud–precipitation interactions. Part 1. The nature and sources of cloud-active aerosols, Earth-Science Rev., 89, 13–41, 2008.
Bates, T.S., P.K. Quinn, A.A. Frossard, L.M. Russell, J. Hakala, T. Petäjä, M. Kulmala, D.S. Covert, C.D. Cappa, S.-M. Li, K.L. Hayden, I. Nuaaman, R. McLaren, P. Massoli, M.R. Canagaratna, T.B. Onasch, D. Sueper, D.R. Worsnop, and W.C. Keene, Measurements of ocean derived aerosol off the coast of California, J. Geophys. Res., 117(D00V15), doi:10.1029/2012JD017588, 2012.
Carslaw et al., A review of natural aerosol interactions and feedbacks within the Earth system, Atmos. Chem. Phys., 10, 1701 – 1737, 2010.
Charlson, R.J., Lovelock, J.E., Andreae, M.O., & Warren, S.G. Oceanic phytoplankton, atmospheric sulphur, cloud albedo, and climate, Nature, 326, 655 – 661, 1987.
Chang, R. Y.-W., S. J. Sjostedt, J. R. Pierce, T. N. Papakyriakou, M. G. Scarratt, S. Michaud, M. Levasseur, W. R. Leaitch, and J. P. D. Abbatt, Relating atmospheric and oceanic DMS levels to particle nucleation events in the Canadian Arctic, J. Geophys. Res., 116, D00S03, doi:10.1029/2011JD015926, 2011.
Clarke, A.D. et al., Free troposphere as a major source of CCN for the equatorial pacific boundary layer: long-range transport and teleconnections, Atm. Chem. Phys., 13, 7511-7529, 2013.
Dawson, M.L., M.E. Varner, V. Perraud, M.J. Ezell, R.B. Gerber, and B.J. Finlayson-Pitts, Simplified mechanism for new particle formation from methanesulfonic acid, amines, and wáter via experiments and ab initio calculations, Proc. Nat. Acad. Sci. USA, 109, 18719–18724, 2012.