US SOLAS Research Direction: Boundary Layer Physics The biogeochemical cycles of many climate relevant compounds (CRCs) are influenced by the physics of the surface ocean boundary layer (SOBL) and lower atmospheric boundary layer (LABL). These highly dynamic regimes are forced from both above and below, in addition to interacting in complex ways. Fluxes through these boundary layers and across the air-sea interface can be considered "generic" processes that transcend the studies of specific biogeochemical cycles undertaken by US SOLAS. Significant advances have been made over the past decades in field and laboratory observations and modeling of boundary layer processes. New instrumentation has been developed to probe these layers and we now are in a position to effectively implement this work. U.S. SOLAS studies will focus on the impact of boundary layer transfer processes on the cycling of CRCs. Of principal interest will be the transport across the dynamic boundaries of the LABL and SOBL, and across the air- water interface. Concentrations of CRCs in the boundary are controlled by transport into the layer and reactions within the layers. Turbulent diffusion and advection, which follow well-defined physical laws, limit transport across the relevant interfaces. However, our mechanistic climate understanding of the processes are limited and must be improved to understand transfer dynamics. In particular, the following processes are of critical importance to understand control of the relevant compounds: -- Physical transport of key compounds into or out of the euphotic zone. Concentrations of several trace gases are controlled by biological productivity that in turn is limited by (trace) nutrient supply. This requires knowledge of bulk diffusion coefficients and advective velocities. More importantly, it requires an understanding of the processes controlling advection and diffusion in the SOBL such that future changes can be predicted. Of particular interest are episodic events such as eddies, storms, and breaking internal waves which might contribute a disproportionate share of the total transport. -- Transport of CRCs into and out of the LABL. The climatic impact of CRCs is to a large extent dependent on the ability to escape the LABL into the upper troposphere. The dynamics are poorly understood. Like for the SOBL episodic events, such as storms, could play a large role in transport of CRCs. Regional and seasonal differences in transport patterns and mechanisms can be significant. -- Air-sea gas transfer. Gas transfer is controlled by physical processes in the air-water boundary region of thickness of less than about 100 micron. Several theoretical models, some based or validated by laboratory studies, have been devised that should be applied and tested for the ocean. The effect of bubbles and surfactants on gas transfer and the ability to "measure" gas transfer from space are several of the research areas that should be explored further to improve estimates of air-water gas fluxes. -- Air-sea transfer of non-gaseous materials. Fluxes of several elements from the SOBL to the LABL are believed to be controlled by seaspray and subsequent aerosol formation. Deposition patterns of (trace) nutrients that are often of continental origin depend on transport in the LABL and the deposition mechanisms onto the sea surface (for instance, wet and dry deposition). This can lead to fractionation of earosols by size and/or composition, in turn effecting the bioavailablility of chemical species transported by this mechanism. Mechanistic understanding necessary to adequately model these transport processes is currently lacking. Several new or improved tools make it possible to explore these issues in further detail. In particular remote sensing from local to global scales will offer the ability to quantify both the fluxes across the boundaries and the processes controlling the fluxes. Tools include active and passive spectral (from UV through IR) methods to probe the interfaces, high-resolution physical and chemical profilers in tandem with conventional sampling methods. Improvements in high-resolution (i.e. eddy-resolving) models and techniques to parameterize important processes in prognostic large scale models will provide a means to use this knowledge in predictions of the impact of CRCs on future climate. US SOLAS Research Direction: Atmospheric Connections with the Marine Nitrogen Cycle Nitrogen availability in the marine euphotic zone is a key modulator of primary productivity and export production and, therefore, of C dynamics in diverse and expansive areas of the world's ocean. Intertwined sets of physical processes and biological reactions of N contribute to N availability and the relative fertility of the upper ocean. While we recognize that the relationships, and key processes of the N cycle relating to upper ocean C dynamics vary among ocean environments, our knowledge remains rudimentary with respect to the quantitative relationships, controls and feedbacks between these two cycles. The marine N cycle is intimately connected to the atmosphere. Several important species of N (e.g. N2, N2O, NO, NH3) are gaseous and can exchange readily with atmospheric pools. Indeed, the main pool of N globally is as atmospheric N2. Some combined forms of nitrogen (NO3, NH4+, DON) can enter the sea through atmospheric deposition. Moreover, Fe, a critical trace nutrient required for many of the reactions of the N cycle, has an important atmospheric path to the sea through deposition of Fe-rich aeolian dust. Fe additions have been shown to promote NO3 drawdown in some HNLC areas such as the equatorial Pacific and are suspected to be an important control on N2 fixation in the oligotrophic tropics as well. The marine N cycle can contribute directly to atmospheric N dynamics as well. Nitrous oxide, a product of marine nitrification and denitrification as well as a potent greenhouse gas, may have a marine source (strength). NO and alkyl N species may also form in the upper ocean. Source strengths of atmospheric N deposition are expected to increase with future population growth and the further development in coastal areas. Inputs of aeolian Fe have varied dramatically over glacial-interglacial periods (flesh out more w/r to NO3 uptake; N2 fixation and denitrification balance?). Anticipated changes in terrestrial physiography through shifts in climate and land use are occurring and dramatic, and the rate of change is anticipated to accelerate - with anticipated effects on the marine N and, thereby, C cycles. The direct (e.g. N2O) and indirect (e.g. through controls on organic C production, sequestration and air/ sea CO2 flux) interactions of the marine N cycle with drivers suggests the potential for feedbacks between the marine N cycle and global climate dynamics. Hence, with its intimate connection to C cycling and its role in the generation of radiatively important trace gases, the N cycle of the upper ocean should be an appropriate and critical focus for the Surface Ocean Lower Atmosphere Study. Moreover, mesoscale manipulative experiments examining biological and chemical responses and gas exchanges across the interface in response to perturbations such as ocean Fe fertilization should clearly consider N dynamics. US SOLAS Research Direction: Formation mechanisms of cloud condensation nuclei (CCN) in the lower atmosphere Understanding the formation mechanisms of CCN in the atmosphere is extremely important for determining both past and future climate. The issue was first highlighted in Charlson et al. [1987]'s paper which hypothesized a link between biogenic DMS emission and global climate change. In short, DMS released by marine phytoplankton enters the troposphere and is oxidized there to sulfate particles, which then act as cloud condensation nuclei (CCN) for marine clouds. Changes in CCN concentration affect the cloud droplet number concentration, which influences cloud albedo and, consequently, climate. Large-scale climate change, in turn, affects the phytoplankton number and speciation in the oceans and thereby closes a feedback loop. In spite of considerable progress in the past 10 years, fundamental gaps remain in our understanding of the processes that regulate the concentration of DMS in seawater as well as the processes that regulate CCN formation. For DMS formation, generally applicable models of DMS-plankton relationships are still in their infancy. Important factors which govern DMS accumulation in the surface ocean result from the overall food web dynamics, including phytoplankton production, macro- and microzooplankton grazing, bacterial, and viral activities, as well as the physicochemical dynamics of the upper ocean. The recent revolution in understanding of how certain trace nutrients (e.g. Fe) limit primary productivity in the ocean has also led to improved knowledge of DMS biogeochemistry. Increases in phytoplankton production and biomass were accompanied by increases in DMSP and DMS concentrations. It is noteworthy elevated iron and MSA (a product of DMS degradation) and lowered CO2 levels in the last glacial period are consistent with a scenario where ocean productivity was higher then, due to enhanced atmospheric inputs of iron and other trace metals. Increases in DMS concentrations necessarily lead to increases in the ocean-to-atmosphere DMS flux. However, there is still a factor of two uncertainty associated with estimating this flux, once the ocean concentrations are determined. Moreover, our understanding of how to translate this flux into CCN concentrations is very limited. Both the nature of the DMS oxidation mechanism as well as the quantification of the extent to which the oxidation products MSA and H2SO4 contribute to new CCN is largely unknown. Furthermore, changes in climate are expected to lead to changes in the fluxes of other components as well: sea salt aerosol and emissions associated with organic aerosols. These altered fluxes will influence the efficiency with which DMS produces CCN, but how that might happen, and even the direction of change is largely unexplored. In view of these outstanding issues a number of hypotheses can be formulated which might form the basis of a national program: For ocean processes which determine DMS concentrations in surface waters: * Future climate change will affect biogenic emissions by affecting the concentrations in water and the exchange rate with the atmosphere. * Marine DMS concentration is mainly controlled by planktonic processes and so, changes in the planktonic community structure will affect its concentration. * Ecosystem shifts towards the dominance of regenerated-based microbial food chains will increase the DMS production efficiency. * Nutrient input will increase the net DMS production. * Increase of the frequency and intensity of mixing events will de-couple DMS production and loss and, so, increase the net DMS production. For atmospheric processes which determine CCN concentrations from known concentrations of DMS in surface waters: * In certain areas marine biogenic emissions make a major contribution to the mass and number of marine aerosols and, as such, control the number of CCN and cloud albedo. * Future climate change will affect the pathways that link biogenic emissions to CCN and cloud albedo * Changes in wind speed associated with future climate change will quantitatively change sea salt and DMS emissions, thereby increasing CCN and cloud albedo * CCN concentrations are partly controlled by mixing processes between the marine boundary layer and the atmosphere and so, changes in climate will alter CCN through changes in the rate of mixing * Organic compounds in the aerosol limit the formation of CCN, so that changes in the emissions of organics will lead to substantive changes in CCN. Specific gaps in linking DMS and CCN are (from SOLAS International): * DMS oxidation chemistry * Nucleation theory and which species are contributing to nucleation * Air-sea exchange parameterization * Role of organics in limiting CCN formation US SOLAS Research Direction Lower atmosphere - upper ocean halogen dynamics: Processes and Feedbacks Emissions of seasalt aerosols and biogenic gases from the ocean surface are the largest sources of halogenated compounds to the atmosphere. The ocean is also a sink for some halogenated compounds whose budgets are impacted by human activities. The oceans are fertilized by nutrients deposited from the atmosphere where their chemical form can be changed by processes involving halogens. Seasalt production flux and gas transfer coefficients are strong functions of wind velocity. Consequently, exchange fluxes and atmospheric concentrations of halogen compounds are highly sensitive to variability in global wind fields. In contrast to CFCs, whose inertness gives them long atmospheric lifetimes and significant global warming potentials, short-lived "active" halogen compounds derive their climate relevance indirectly by affecting the cycling and lifetimes of radiatively active gases, sulfur gases (and their conversion to aerosols and cloud condensation nuclei (CCN)), organic compounds and fixed nitrogen in the troposphere. To the extent that UV penetration into the troposphere is a climate variable, contributions by halogen compounds of intermediate lifetime (months) to equivalent chlorine in the stratosphere must be considered as well. The following paragraphs briefly summarize these phenomena and highlight some key unknowns. Marine boundary layer halogen radical chemistry. Investigations of surface marine air in the Arctic and at lower latitudes over the past decade have provided compelling evidence that halogen radical chemistry significantly influences the composition of the MBL. Atomic halogens (primarily Br and perhaps I) destroy substantial ozone in some regions and thereby impact oxidizing efficiency and the corresponding lifetimes of other important MBL constituents. Atomic Cl efficiently oxidizes many alkanes, including methane (CH4), which influences net O3 production through subsequent reactions. The overall atmospheric lifetime of CH4 may be shortened by roughly 10% relative to a no-Cl case, which would influence its global warming potential. Cl atoms and BrO are also important oxidants for dimethylsulfide ((CH3)2S), which may accelerate S cycling and affect aerosol radiative forcing. Although potentially quite important, details of many chemical pathways involving halogen radical chemistry in marine air are uncertain. Consequently, the overall influence of these processes and their possible feedbacks on the upper ocean remain very poorly constrained. Reliable predictive capability does not currently exist. Seasalt-hydrogen chloride interactions and CCN production. Deliquesced seasalt aerosol is a large and rapidly cycling reservoir of liquid water in the MBL. The phase partitioning of hydrogen chloride (HCl) between gas and seasalt aerosol buffers the aerosol solutions and regulates important pH-dependent chemical transformations occurring in them. For instance, it is well known that sulfur dioxide (SO2) is converted to sulfuric acid (H2SO4) both in the gas phase and in aerosols and cloud droplets. Only H2SO4 molecules produced in the gas phase can nucleate into new "sulfate" aerosols, which can eventually grow and can become CCN. H2SO4 molecules produced in condensed phases can only increase the S content and size of existing aerosols (making them more likely to become CCN) and droplets. Recent model studies suggest that the efficient oxidation of S by hypochlorous acid (HOCl) in moderately acidic seasalt solutions may shift this competition in favor of the condensed phase, thereby limiting production of new sulfate aerosols and CCN. Observations are needed to assess the importance of this hypothesized oxidation pathway and to quantify the amounts of S processed in gas versus condensed phases over the oceans. It should be noted as well that seasalt aerosols are themselves excellent CCN. They can contribute significantly to CCN populations over the Southern Ocean where seasalt aerosol production is high relative to S sources. Measurements are needed to determine their importance elsewhere. Seasalt-hydrogen chloride interactions and the nitrogen cycle. Buffering resulting from HCl phase partitioning affects the phase partitioning of nitrate/nitric acid and ammonium/ammonia in the MBL and, in turn, the deposition flux of these fixed nitrogen compounds to the ocean surface. This deposition flux may be a significant nitrogen source for marine biota especially in polluted coastal areas. Marine halogen compunds and stratospheric ozone. Because O3 attenuates the UV radiation flux through the atmosphere, change in the production and delivery of halogens to the upper troposphere and lower stratosphere could alter the UV spectrum through the troposphere down to the surface. Currently methyl bromide (CH3Br) and methyl chloride (CH3Cl) contribute about 25% of the equivalent chlorine to the stratosphere and, consequently, exert significant control on stratospheric O3. Any increase in the atmospheric burden of these gases, particularly CH3Br, will offset the effect of anthropogenic CFC declines achieved through the Montreal Protocol and its subsequent amendments. For example, less than a 0.5 pptv (5%) increase in the atmospheric burden of CH3Br could reverse the current downward trend in the atmospheric burden of these O3-depleting gases. The oceans are currently thought to be a minor source of CH3Cl, however they act as both a source and a sink for CH3Br. Globally averaged they are currently estimated to be a significant net sink of approximately 80 Gg yr-1whose magnitude is uncertain by a factor of 3.5. Recent CH3Br budget estimates imply missing sources of about 60 Gg yr-1 to balance sinks totaling about 200 Gg yr-1. Most terms in the budget are uncertain by at least a factor of 2. In addition, shorter-lived organic (e.g., CH2Br2, CH3I) and inorganic (e.g., Br2, BrCl, Cl2, BrO, ClO, IO) halogen compounds are emitted directly from the ocean to the atmosphere, produced indirectly via photochemical transformation of precursor species, or produced by chemical processes involving sea salt aerosol in the MBL. These compounds can be mixed efficiently by deep convection (especially in the tropics) to the upper troposphere and lower stratosphere and contribute to O3 destruction in those regions. Thus, climatically driven changes in either the emission/production fluxes of these compounds or in deep convection could significantly impact tropospheric and stratospheric O3.