North Atlantic Aerosols and Marine Ecosystems Study

NAAMES was led by scientists from Oregon State University and the National Aeronautics and Space Administration (NASA). They conducted four field campaigns from 2015-2018 that were designed to target specific phases of the annual phytoplankton cycle: minimum, climax, intermediary decreasing biomass, and increasing intermediary biomass. The campaigns were designed to observe each unique phase, in order to resolve the scientific debates on the timing of bloom formations and the patterns driving annual bloom re-creation. The NAAMES project also investigated the quantity, size, and composition of aerosols generated by primary production in order to understand how bloom cycles affect cloud formations and climate. Scientists employed multiple complementary research methods, including intensive field sampling via research ships, airborne aerosol sampling via airplane, and remote sensing via satellites.

The findings from NAAMES, while still forthcoming, have shed light on aerosols and cloud condensation nuclei, phytoplankton annual cycles, phytoplankton physiology, and mesoscale biology. Several methodological advances have also been published, including new remote sensing algorithms and advances in satellite remote sensing.

Background

Competing hypotheses of plankton blooms

NAAMES sought to better understand the impact of bioaerosol emissions on cloud dynamics and climate. It also aimed to test two competing hypotheses on plankton blooms:

Critical Depth Hypothesis - a resource-based view

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The critical depth hypothesis is a resource-based view of the North Atlantic annual phytoplankton blooms. It is the traditional explanation for the cause of spring blooms and has been documented as a foundational concept in oceanography textbooks for over 50 years. It focuses on the environmental conditions necessary to initiate a bloom such as high nutrients, shallower mixing, increased light, and warmer temperatures.

The central argument for the critical depth hypothesis is that blooms are a consequence of increased phytoplankton growth rates resulting from shoaling of the mixed layer above the critical depth. The critical depth is a surface mixing depth where phytoplankton biomass growth equals phytoplankton biomass losses. In this hypothesis, losses are both constant and independent of growth. The decline in biomass may be due to grazing, sinking, dilution, vertical mixing, infection, or parasitism. When the surface mixed layer becomes shallower than the critical depth, initiation of the seasonal bloom occurs due to phytoplankton growth exceeding loss. There is a correlation of phytoplankton growth with springtime increases of light, temperature, and shallower stratification depths.

Climate warming may increase stratification or decrease mixed layer depth during the winter, which would enhance the vernal bloom or increase phytoplankton biomass if this hypothesis governed spring phytoplankton bloom dynamics. A primary criticism of this resource-based view is that spring blooms occur in the absence of stratification or shoaling of the mixed layer.

Dilution-recoupling Hypothesis - an ecosystem-based view

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The dilution-recoupling hypothesis is an ecosystem-based view of the North Atlantic annual phytoplankton bloom. This hypothesis focuses on the physical processes that alter the balance between growth and grazing. The spring bloom is considered to be one feature of an annual cycle, and other features during the cycle "set the stage" for this bloom to occur.

This ecosystem-based view is based upon a dilution experiment where the addition of seawater dilutes predators but does not change the growth of phytoplankton. Thus, growth rates increase with dilution. Although the dilution effect is transient, predator-prey interactions can be maintained if the rate of the addition of water equals the rate of growth. The deepening of the surface mixed layer dilutes the predator-prey interactions and decouples growth and grazing. When the mixed layer stops deepening, the increase in growth rate becomes apparent, but now growth and grazing become coupled again. The shoaling of the mixed layer concentrates predators, thereby increasing grazing pressure. However, the increase in light availability counters grazing pressure, which allows growth rates to remain high. In late spring, when the mixed layer is even more shallow, nutrient depletion or overgrazing ends the bloom—losses exceed growth at this point in the cycle.

Climate warming would increase stratification and suppress winter mixing that occurs with the deepening of the mixed layer. The suppression of winter mixing would decrease phytoplankton biomass under this hypothesis.

Physical oceanographic processes

Mixed Layer Depth Debate

Meso-scale Eddies

Mesoscale eddies play a significant role in modulating the Mixed Layer Depth (MLD). Fluctuations created by mesoscale eddies modulate nutrients in the base of the mixed layer. These modulations, along with light availability, drive the abundance of phytoplankton in the region. The availability of phytoplankton significantly affects the marine food web and ocean health.

The fast-moving currents in the Gulf Stream meander and pinch-off to create eddies. These eddies retain the physical properties of their parent water mass (e.g. temperature, density, salinity, and other ocean dynamic properties) when they separate. As the eddies migrate, their physical properties change as they mix with the surrounding water. In the Gulf Stream, migrating eddies are known as anticyclonic or cyclonic eddies based on the direction in which they spin (clockwise vs. counter-clockwise). The two eddies differ in motion, physical properties, and, consequently, their effects on biology and chemistry of the ocean.

The Coriolis force combined with high velocity currents drive eddy motion. This motion creates a 'bulge,' i.e., high sea surface height (SSH) in the center of the Anticyclonic eddies. In contrast, cyclonic eddies exhibit a low SSH in the center. The SSH in both anticyclonic and cyclonic decreases and increases, respectively, as the distance from the center increases. Upwelling and downwelling processes in the eddies create a cold and warm core. Downwelling in the anticyclonic eddy prevents colder water from entering the surface, thus creating a warm-core in the center. Whereas in the cyclonic eddy, the upwelling entrains deep cold water and forms a cold-core.

Previous studies show the deepening effects of MLD under anticyclonic eddies and shoaling of MLD in cyclonic eddies. These phenomena may be due to increased heat loss to the atmosphere in anticyclonic eddies. This loss of heat causes the sinking of dense water, referred to as convective mixing, and the deepening of the MLD. In contrast, in cyclonic eddies the water temperature at the core is less cold than the Anticyclonic eddy. This therefore does not lead to deepening of the MLD. Studies conducted in the region via a network of Argo floats and model simulations created through satellite data have shown cases of the opposite phenomena. The deepening and shoaling of MLD via eddies is ubiquitous and varies seasonally. Such anomalies are most significant in the winter. Thus, the role of meso-scale eddies in MLD is complex, and a function of simultaneous processes where enhanced wind shear induced currents contribute to a shallowing of the MLD in anticyclonic eddies.

Relevant Atmospheric Processes

Marine Boundary Layer

The marine boundary layer (MBL) is the part of the atmosphere in direct contact with the ocean surface. The MBL is influenced by the exchange of heat, moisture, gases, particulates, and momentum, primarily via turbulence. The MBL is characterized by the formation of convective cells (or vertical flow of air) above the ocean surface, which perturbs the direction of the mean surface wind and generates texture, roughness, and waves on the sea's surface. Two types of boundary layers exist. One is a stable, convective layer found between the lower 100m of the atmosphere extending up to approximately 3 km in height, and is referred to as the convective boundary layer (CBL). The other boundary layer forms as a result of a surface atmospheric inversion. This generally occurs closer to the surface in the absence of turbulence and vertical mixing, and is determined through the interpretation of vertical humidity and temperature profiles. The MBL is often a localized and temporally dynamic phenomenon, and therefore its height into the air column can vary considerably from one region to another, or even across the span of a few days. The North Atlantic is a region where diverse and well-formed MBL clouds are commonly formed, and where MBL layer height can be between 2.0-and 0.1 km in height

Regional Atmospheric Processes

The westerlies are prevailing winds in the middle latitudes (between 35 and 65 degrees latitude), which blow in regions north or southward of the high-pressure sub-tropical regions of the world. Consequently, aerosols sampled over the North Atlantic Ocean will be influenced by air masses originating in North America, and therefore be characterized by both the natural terrestrial and anthropogenic inputs. Relevant to NAAMES are the emissions from industry and urban environments in eastern North America, which emit substantial quantities of sulfates, black carbon, and aromatic compounds. Such substances can be transported hundreds of kilometers over the sea. This contribution of continental influences may create a false positive signal in the biological fluorescence signals being measured and could affect cloud microphysical properties in the open North Atlantic Ocean. Furthermore, aerosols such as black carbon mixed with carbon dioxide and other greenhouse gases are emitted through the impartial combustion of fossil fuels from ship engines. These unburned hydrocarbons are present in the marine boundary layer of the North Atlantic and most other remote oceanic regions. As these particles age or are chemically transformed as a function of time in the air, they may alter microphysical and chemical properties as they react with other airborne particles.

Role of aerosols

Aerosols

Aerosols are very small, solid particles or liquid droplets suspended in the atmosphere or inside another gas and are formed through natural processes or by human actions. Natural aerosols include volcanic ash, biological particles, and mineral dust, as well as black carbon from the natural combustion of biomass, such as wildfires. Anthropogenic aerosols are those that have been emitted from human actions, such as fossil fuel burning or industrial emissions. Aerosols are classified as either primary or secondary depending on whether they have been directly emitted into the atmosphere (primary) or whether they have reacted and changed in composition (secondary) after being emitted from their source. Aerosols emitted from the marine environment are one of the largest components of primary natural aerosols. Marine primary aerosols interact with anthropogenic pollution, and through these reactions produce other secondary aerosols.

One of the most significant yet uncertain components of predictive climate change models is the impact of aerosols on the climate system. Aerosols affect Earth's radiation balance directly and indirectly. The direct effect occurs when aerosol particles scatter, absorb, or exhibit a combination of these two optical properties when interacting with incoming solar and infrared radiation in the atmosphere. Aerosols that typically scatter light include sulfates, nitrates, and some organic particles, while those that tend to exhibit a net absorption include mineral dust and black carbon (or soot). The second mechanism by which aerosols alter the planet's temperature is called the indirect effect, which occurs when a cloud's microphysical properties are altered causing either an increase in reflection of incoming solar radiation, or an inhibited ability of clouds to develop precipitation. The first indirect effect is an increase in the amount of water droplets, which leads to an increase in clouds that reflect more solar radiation and therefore cool the planet's surface. The second indirect effect (also called the cloud's lifetime effect) is the increase in droplet numbers, which simultaneously causes an increase in droplet size, and therefore less potential for precipitation. That is, smaller droplets mean clouds live longer and retain higher liquid water content, which is associated with lower precipitation rates and higher cloud albedo. This highlights the importance of aerosol size as one of the primary determinants of aerosol quantity in the atmosphere, how aerosols are removed from the atmosphere, and the implications of these processes in climate. Fine particles are generally those below 2 micrometers (μm) in diameter. Within this category, the range of particles that accumulate in the atmosphere (due to low volatility or condensation growth of nuclei) are from 0.1-1 μm, and are usually removed from the air through wet deposition. Wet deposition can be precipitation, snow or hail. On the other hand, coarse particles, such as old sea-spray and plant-derived particles, are removed from the atmosphere via dry deposition. This process is sometimes also called sedimentation. However, different types of biogenic organic aerosols exhibit different microphysical properties, and therefore their removal mechanisms from the air will depend on humidity. Without a better understanding of aerosol sizes and composition in the North Atlantic Ocean, climate models have limited ability to predict the magnitude of the cooling effect of aerosols in global climate.

Sea-spray Aerosols

Although the amount and composition of aerosol particles in the marine atmosphere originate both from continental and oceanic sources and can be transported great distances, freshly emitted sea-spray aerosols (SSA) constitute one of the major sources of primary aerosols, especially from moderate and strong winds. The estimated global emission of pure sea-salt aerosols are on the order of 2,000-10,000 Tg per year. The mechanism by which this occurs starts with the generation of air bubbles in breaking waves, which then rise to the atmosphere and burst into hundreds of ultra-fine droplets ranging from 0.1-1.0 μm in diameter. Sea-spray aerosols are mostly composed of inorganic salts, such as sodium and chloride. However, these bubbles sometimes carry organic material found in seawater, forming secondary organic compounds (SOAs) such as dimethyl sulfide (DMS). This compound plays a key role in the NAAMES project.

An important biogeochemical consequence of SSA are their role as cloud condensation nuclei. These are particles that provide the surfaces necessary for water vapor to condensate below supersaturation conditions. The freezing of organic matter in these aerosols promotes the formation of clouds in warmer and drier environments than where they would otherwise form, especially at high latitudes such as the North Atlantic Ocean. Organic matter in these aerosols help nucleation of water droplets at these regions, yet plenty of unknowns remain, such as what fraction contain ice-freezing organic materials, and from what biological sources. Nevertheless, the role of phytoplankton blooms as a source of enhanced ice nucleating particles has been confirmed in laboratory experiments, implying the important role of these aerosols in cloud radiative forcing. Primary marine aerosols created through bubble-bursting emission have been measured in the North Atlantic during spring 2008 by the International Chemistry Experiment in the Arctic Lower Troposphere (ICEALOT). This research cruise measured clean, or background, areas and found them to be mostly composed of primary marine aerosols containing hydroxyl (58% ±13) and alkene (21% ±9) functional groups, indicating the importance of chemical compounds in the air with biological origin. Nonetheless, the small temporal scale of these measurements, plus the inability to determine the exact source of these particles, justifies the scientific need for a better understanding of aerosols over this region.

Bioaerosols

Bioaerosols are particles composed of living and non-living components released from terrestrial and marine ecosystems into the atmosphere. These can be forest, grasslands, agricultural crops, or even marine primary producers, such as phytoplankton. Primary biological aerosol particles (PBAPs) contain a range of biologic materials, including bacteria, archaea, algae, and fungi, and have been estimated to comprise as much as 25% of global total aerosol mass. Dispersal of these PBAPs occur via direct emission into the atmosphere through fungi spores, pollen, viruses, and biological fragments. Ambient concentrations and sizes of these particles vary by location and seasonality, but of relevance to NAAMES are the transient sizes of fungi spores (0.05 to 0.15 μm in diameter) and larger sizes (0.1 to 4 μm) for bacteria. Marine organic aerosols (OA) have been estimated through their correlation to chlorophyll pigments to vary in magnitude between 2-100 Tg per year. However, recent studies of OA are correlated with DMS production and to a lesser extent chlorophyll, suggesting that organic material in sea salt aerosols are connected to biological activity in the sea's surface. The mechanisms contributing to marine organic aerosols thus remain unclear, and were a main focus of NAAMES.

There is some evidence that marine bioaerosols containing cyanobacteria and microalgae may be harmful to human health. Phytoplankton can absorb and accumulate a variety of toxic substances, such as methylmercury, polychlorinated biphenyls (PCBs), and polycyclic aromatic hydrocarbons. Cyanobacteria are known to produce toxins that can be aerosolized, which when inhaled by humans can affect the nervous and liver systems. For example, Caller et al. (2009) suggested that bioaerosls from cyanobacteria blooms could play a role in high incidences of amyotrophic lateral sclerosis (ALS). In addition, a group of toxic compounds called microcystins are produced by some cyanobacteria in the genera Microcystis, Synechococcus, and Anabaena. These microcystins have been found in aerosols by a number of investigators, and such aerosols have been implicated as causing isolated cases of pneumonia, gastroenteritis, and non-alcoholic fatty liver disease. Dinoflagellates are also thought to be involved in bioaerosol toxicity, with the genus Ostreopsis causing symptoms such as dyspnea, fever, rhinorrhea, and cough. Importantly, marine toxic aerosols have been found as far as 4 km inland, but investigators recommend additional studies that trace the fate of bioaerosols further inland.

The fungi phylum of Ascomycota has been understood as the major contributor (72% in relative proportion to other phyla) to marine bioaerosols, at least in the Southern Ocean. Of these, Agaricomycetes constitutes the majority (95%) of fungi classes inside this phylum. Within this group, the Penicillium genus is most frequently detected in marine fungi aerosols. Fungi bioaerosols can also serve as ice nuclei, and therefore also impact the radiative budget in remote ocean regions, such as the North Atlantic Ocean.

In addition to sea-spray aerosols (see section above), biogenic aerosols produced by phytoplankton are also important source of small (typically 0.2 μm) cloud condensation nuclei (CCN) particles suspended in the atmosphere. The Intergovernmental Panel on Climate Change (IPCC), forecasted an increase in global surface ocean temperatures by +1.3 to +2.8 degrees Celsius over the next century, which will cause spatial and seasonal shifts in North Atlantic phytoplankton blooms. Changes in community dynamics will greatly affect the bioaerosols available for cloud condensation nuclei. Therefore, cloud formation in the North Atlantic is sensitive to bioaerosol availability, particle size, and chemical composition.

Marine Bioaerosols and Global Radiation Balance

Marine aerosols contribute significantly to global aerosols. Traditionally, biogeochemical cycling and climate modeling have focused on sea-salt aerosols, with less attention on biogenically-derived aerosol particles such as sulfates and related chemical species emitted from phytoplankton. For example, in the eastern North Atlantic during the spring 2002 bloom, high phytoplankton activity was marked more by organic carbon (both soluble and insoluble species) than by sea-salts. The organic fraction from phytoplankton contributed as much as 63% of the aerosol mass in the atmosphere, while during winter periods of low biological activity it only accounted for 15% of the aerosol mass. Those data provided early empirical evidence of this emission phenomena, while also showing that organic matter from ocean biota can enhance cloud droplet concentrations by as much as 100%.

Data to test the CLAW Hypothesis

There is growing evidence describing how oceanic phytoplankton affect cloud albedo and climate through the biogeochemical cycle of sulfur, as originally proposed in the late 1980s. The CLAW hypothesis conceptualizes and tries to quantify the mechanisms by which phytoplankton can alter global cloud cover and provide planetary-scale radiation balance or homeostasis regulation. As solar irradiance drives primary production in the upper layers of the ocean, aerosols are released into the planetary boundary layer. A percentage of these aerosols are assimilated into clouds, which then can generate a negative feedback loop by reflecting solar radiation. The ecosystem-based hypothesis of phytoplankton bloom cycles (explored by NAAMES) suggests that a warming ocean would lead to a decrease in phytoplankton productivity. Decreased phytoplankton would cause a decrease in aerosol availability, which may lead to fewer clouds. This would result in a positive feedback loop, where warmer oceans lead to fewer clouds, which allows for more warming.

One of the key components of the CLAW hypothesis is the emission of dimethylsulfoniopropionate (DMSP) by phytoplankton. Another chemical compound, dimethyl sulfide (DMS), has been identified as a major volatile sulfur compound in most oceans. DMS concentrations in the world's seawater have been estimated to be, on average, on the order of 102.4 nanograms per liter (ng/L). Regional values of the North Atlantic are roughly 66.8 ng/L. These regional values vary seasonally and are influenced by the effects of continental aerosols. Nonetheless, DMS is one of the dominant sources of biogenic volatile sulfur compounds in the marine atmosphere. Since its conceptualization, several research studies have found empirical and circumstantial evidence supporting the CLAW hypothesis in mid-latitudes of the Atlantic Ocean. The NAAMES campaign sought to provide an empirical understanding of the effects of marine bioaerosols on cloud formation and global radiation balance by quantifying the mechanisms underlying the CLAW hypothesis.

Emissions from the sea surface micro-layer

Dissolved organic compounds containing remnants of polysaccharides, proteins, lipids, and other biological components are released by phytoplankton and bacteria. They are concentrated into nano-sized gels on the surface of the oceans. Specifically, such compounds are concentrated in the sea surface micro-layer (SML), the uppermost film of water in the ocean. The SML is considered a "skin" within the top 1 millimeter of water where the exchange of matter and energy occurs between the sea and atmosphere. The biological, chemical, and physical processes occurring here may be some of the most important anywhere on Earth, and this thin layer experiences the first exposure to climatic changes such as heat, trace gases, winds, precipitation, and also wastes such as nanomaterials and plastics. The SML also has important roles in air-sea gas exchange and the production of primary organic aerosols.

A study using water samples and ambient conditions from the North Atlantic Ocean found that a polysaccharide-containing exopolymer and a protein are easily aerosolized in surface ocean waters, and scientists were able to quantify the amount and size resolution of the primary sea to air transport of biogenic material. These materials are small enough (0.2μm) to be largely emitted from phytoplankton and other microorganisms. However, predicting aerosol quantity, size distribution, and composition through water samples are currently problematic. Investigators suggest that future measurements focus on comparing fluorescence detection techniques that are able to detect proteins in aerosols. NAAMES filled this research gap by providing a fluorescent-based instrument (See section on Atmospheric Instruments below), both in the air column and near the sea's surface.

NAAMES Objectives

• Identify the different features of the annual cycle of phytoplankton blooms in the North Atlantic and determine the different physical processes affecting those features.

To accomplish this objective, a combination of ship-based, airborne, and remote sensing measurements was used. NAAMES conducted multiple campaigns that occurred during the various phases of the cycle in order to capture the important transitory features of the annual bloom for a comprehensive view.

• Understand how the different features of the North Atlantic annual phytoplankton cycle interact to "set the stage" for annual blooms.

This objective seeks to reconcile the competing resource-based and ecosystem-based hypotheses. NAAMES goal was to provide the mechanistic field studies necessary to understand a more holistic view of the annual bloom cycle.

• Determine how the different features of the annual phytoplankton cycle affect marine aerosols and cloud formation.

The effects of aerosols on clouds is an understudied topic despite the major implications it could have for predicting future climate change. This objective addressed this gap by using combined measurement methods to understand the contribution of various aerosols to cloud formation produced during each major phase of the annual phytoplankton cycle.