Shallow Mixing Layer Research Articles

Source attribution of carbon monoxide over Northern India during crop residue burning period over Punjab

National Capital Territory of Delhi and its satellite cities suffer from poor air quality during the post-monsoon months of October–November. In this study, a novel attempt is made to estimate the contribution of different emission sources (industrial, residential, power generation, transportation, biomass burning, photochemical production, lateral transport, etc.) towards the criteria air pollutant carbon monoxide (CO) concentration over North India. Multiple simulations of the WRF-Chem model with a tagged tracer approach with different inputs (6 anthropogenic emission inventories and 3 biomass burning emission inventories) were used. The model performance was evaluated against the MOPITT retrieved CO surface concentration. Analysis of model simulated CO over North India suggests that anthropogenic emissions contribute around 32–49% to surface CO concentration while crop residue burning contributes 27–44% of which 80% originates from Punjab. For Delhi, the contribution from anthropogenic sources is dominant (53–77%) of which 10–28% is from the domestic sector and 14–55% is from the transport sector. Agricultural waste burning contributes about 15–30% to Delhi's surface CO concentration (of which 75% originates from Punjab). Crop residue burning emission is a chief source of CO over Punjab with a contribution of about 56–76%. The results suggest that industrial, transport, and domestic sector activities are more responsible for increased CO levels over New Delhi and surrounding regions than crop residue burning over Punjab. Furthermore, critical meteorological parameters like 10 m wind speed, boundary layer height, 2 m temperature, total precipitation, and relative humidity were evaluated against CO concentration to understand their impact on CO distribution. Results conclude that deteriorating air quality over the North Indian region is caused by a combination of prevailing meteorological factors (such as slow winds, shallow mixing layer, and cold temperatures) and man-made emissions.

Evaluating shallow mixing layer in partially-distributed canopy flows using DA-LES: bed friction, water shallowness and canopy denseness

ABSTRACT Shallow mixing layers developed in partially-distributed canopy flows (PCFs) are important for water conveyance, sediment transport and solute dispersion. In this paper, we address the response of shallow mixing layers in PCFs to several influential factors associated with bed friction, water shallowness and canopy denseness using depth-averaged Large Eddy Simulation (DA-LES). The results show that under certain canopy denseness, shallow mixing layers in PCFs display asymptotic behaviour for the variation of bed friction and water shallowness, consistent with classic shallow mixing layers. The canopy drag encourages the asymmetric growth of the inner and outer parts of the mixing layer, and this effect decreases when bed friction and water shallowness become pronounced. Characteristic velocity and length scales show similarity well delineated by exponential decay function regarding a stabilization efficiency parameter. The exception happens for the inner length scale of the mixing layer when only changing canopy density. We conclude that changing the canopy denseness can encourage the overall shift of mixing layer scaling regarding a broad range of bed friction and water shallowness. Furthermore, the bed friction parameter thresholding triggering flow instability of mixing layers in PCFs is found about 0.03, lower than 0.09 for splitter-plate induced mixing layers.

Assessing the role of atmospheric dispersion vs. emission strength in the southern Po Valley (Italy) using dispersion-normalised multi-time receptor modelling

In this paper, we applied the Dispersion Normalised Positive Matrix Factorisation (DN-PMF) approach recently proposed in the literature to provide a more realistic picture of the relative importance of emission strength vs. atmospheric dispersion conditions. The disentanglement of such effects is of great concern in pollution hot spots like the Po Valley (Italy), where particulate matter limit values are exceeded despite the existing abatement measures. To explore the potentiality of the DN-PMF approach – still scarcely applied in the literature – a well-chemically characterised PM1 (atmospheric particles with aerodynamic diameter <1 μm) dataset comprising samples collected at different time resolutions at an urban background site (Bologna) in the southern Po Valley was used. Indeed, it is well known that shallow mixing layers promote pollutant accumulation but this observation is not enough to exclude an enhancement of emission strength which could be tackled by appropriate abatement strategies.The source apportionment of sub-micron sized aerosols having a quite long atmospheric residence time in a complex environment like the Po Valley - which is also strongly impacted by secondary aerosol formation on a basin-scale - is generally quite challenging when using receptor models. Due to the availability of a huge dataset with variables having multiple time resolutions, in this work the DN-PMF was implemented in a multi-time resolution approach (MT) to achieve a better source identification and to gain knowledge about the relative importance of atmospheric dilution vs. emissions. A comparison between results obtained by the application of the regular multi time resolution (REG-MT) vs. the DN-MT approach is presented here for the five factors identified (nitrate-dominated, sulphate-dominated, biomass burning, mineral dust, and urban aerosol). The first interesting outcome is that REG-MT and DN-MT results do not point at significant differences in temporal patterns for aerosol components and sources impacting at the basin-scale (i.e. sulphate- and nitrate-dominated aerosol, biomass burning) thus suggesting that the diel modulation of these PM1 emissions is somehow masked by the stronger variability of the mixing layer. Conversely, contributions from local sources with more pronounced diel variation like traffic are quite well reproduced by DN-MT and the ambient concentrations are enhanced compared to REG-MT. This is an important piece of information highlighting that PM1 concentrations from local sources have been likely underestimated by REG-MT assessments.To our knowledge, this is one of the very few applications of DN-MT and the first one at a European site where the huge effort made to implement air pollution containment measures is still not very much effective in reducing PM levels; moreover, in this paper a detailed discussion about the possible interpretation of the output of DN-MT in terms of temporal patterns is reported.

An analytical and numerical study on influence of the load transfer shallow mixing layer on stress-induced and settlement of ground improved by CDM columns

In the literature, there are a few analytical methods for evaluating stress induced on the CDM column heads and most of them involve the use of geosynthetics embedded layers, except the method of ALiCC. However, the ALiCC method still has some limitations in actual designs. This paper presents numerical analysis (by using Plaxis software) and analytical analysis (by using the ALiCC method) on the influence of thickness and stiffness of the SM layer (without the use of geosynthetics embedded layers) on stress induced on column heads and settlement of the improved ground. A typical parametric case and actual project case were taken into analyses. Analysis results from the examined cases indicate that, as expected, when the thickness or stiffness of the SM layer increases, the settlement of the ground decreases and the stress induced on the column head increases. An important finding from this study is that the maximum stress induced in the CDM column is typically not on the head of the columns but at the middle depths where soil layers are softer than the SM layer and the bearing layer at the column toes.

An Investigation of Impacts of Surface Waves-Induced Mixing on the Upper Ocean under Typhoon Megi (2010)

Surface waves play an essential role in regulating the mixing processes in the upper ocean boundary, and then directly affect the air–sea exchange of mass and energy, which is important for the intensity prediction of tropical cyclones (TCs). The relative and integrated impacts of the wave breaking (WB) and the wave orbital motion (WOM) on the mixing and ocean response to TC forcing are investigated under typhoon Megi (2010), using the modeled data from a fully coupled air–sea–wave model. It is shown that the WOM can effectively increase the turbulence mixing in the upper ocean, thus significantly deepening the mixing layer depth and cooling the sea surface temperature. The WB can modulate the mixing layer depth and sea surface temperature to some extent in the cold tail zone with a shallow mixing layer (owing to typhoon forcing), whereas the WOM plays a predominant role. On the aspect of ocean currents driven by typhoon winds, the WOM-induced mixing significantly weakens the current velocity and shear strength in the upper ocean mixing layer, while the relative contribution for turbulence production between the WOM and the current shear differs at different vertical regions. Moreover, the effect of the WOM on the upper ocean turbulent mixing are dependent on the location with respect to the typhoon center, the local vertical thermal structure, and surface wave states.

Shallow mixing layers over hydraulically smooth bottom in a tilted open channel

Shallow mixing layers (SMLs) behind a splitter plate were studied in a tilted rectangular open-channel flume for a range of flow depths and the initial shear parameter ${\lambda = (U_{2}-U_{1})/(U_{2}+U_{1})}$ , where $U_1$ and $U_2$ are streamwise velocities of the slow and fast streams, respectively. The main focus of the study is on (i) key parameters controlling the time-averaged SMLs; and (ii) the emergence and spatial development of Kelvin–Helmholtz coherent structures (KHCSs) and large- and very-large-scale motions (LSMs and VLSMs) and associated turbulence statistics. The time-averaged flow features of the SMLs are mostly controlled by bed-friction length scale $h/c_f$ and shear parameter $\lambda$ as well as by time-averaged spanwise velocities $V$ and momentum fluxes $UV$ , where $h$ and $c_f$ are flow depth and bed-friction coefficient, respectively. For all studied cases, the effect of shear layer turbulence on the SML growth is comparatively weak, as the fluxes $UV$ dominate over the spanwise turbulent fluxes. Initially, the emergence of KHCSs and their length scales largely depend on $\lambda$ . The KHCSs cannot form if ${\lambda \lessapprox 0.3}$ and the turbulence behind the splitter plate resembles that of free mixing layers. Further downstream, shear layer turbulence becomes dependent on the bed-friction number $S = c_f \delta _v /(4 h \lambda )$ , where $\delta _v$ is vorticity thickness. When $S \gtrapprox 0.01$ , the KHCSs are longitudinally stretched and the scaled transverse turbulent fluxes decrease with increasing $S$ . The presence and streamwise development of LSMs/VLSMs away from the splitter plate depends on the $\lambda$ -value, particularly when $\lambda &gt; 0.3$ , resembling LSMs/VLSMs in conventional open-channel flows when $\lambda$ is small.

Physical mechanisms for biological carbon uptake during the onset of the spring phytoplankton bloom in the northwestern Mediterranean Sea (BOUSSOLE site)

Abstract. Several trigger mechanisms have been proposed for the onset of the phytoplankton spring bloom. Among these is that phytoplankton cells begin to bloom when they experience higher average light levels in shallower mixed layers, a result of the surface net heat fluxes becoming positive and wind strength decreasing. We study the impact of these two forcings in the northwestern Mediterranean Sea. We take advantage of hourly measurements of oceanic and atmospheric parameters collected at two neighbouring moorings during the months of March and April in the years 2016 to 2019, combined with glider data in 2016. We identify the onset of the surface phytoplankton growth as concomitant with the start of significant biological activity detected by a sudden decrease in dissolved inorganic carbon derived from measurements in the upper 10 m of the water column. A rapid reduction in wind stress following high-wind events is observed at the same time. A resulting shallow mixing layer favours carbon uptake by phytoplankton lasting a few days. Simultaneously, the air–sea net heat flux switches from negative to positive, linked to changes in the latent air–sea heat flux, which is proportional to the wind speed. This results in an increased thermal stratification of the ocean's surface layers. In 2016, glider data show that the mixing layer is significantly shallower than the mixed layer at the onset of the surface phytoplankton bloom. We conclude that decreases in the mixing- and mixed-layer depths lead to the onset of the phytoplankton growth due to the relaxation of wind speed following storms. We estimate net daily community production in the mixing layer over periods of 3 d between 2016 and 2019 as between 38 and 191 mmol C m−2. These results have important implications, as biological processes play a major role in the seasonal evolution of surface pCO2 and thereby the rate of reduction in atmospheric CO2 by exchange at the air–sea interface.

Depth- averaged large eddy simulation of shallow turbulent mixing layer

This paper presents the DA-LES large-scale simulation model of turbulent shallow flows, which is based on dual filtering, the first filter removes vertical scales and the second filtering, as a filter spatial, removes all scales from the movement of sizes smaller than the mesh size. To validate this model, we consider the mixing layer problem in a shallow flow of two different velocities imposed on the input Uijttewaal, Booij,Van Prooijen and Uijttewaal [1,2]. In this work, we consider the finite volume method based on an unstructured mesh for turbulent equations, the numerical results are presented and compared to experimental data.

Shallow mixing interfaces between parallel streams of unequal densities

As opposed to the case of shallow mixing layers forming between parallel streams of unequal velocities and equal densities, the spatial development of the mixing interface (MI) between two parallel streams of unequal velocities and sufficiently large density contrast is controlled by the formation of a spatially developing, lock-exchange-like flow in transverse planes. Buoyancy effects are driven by the presence of a vertical density interface near the splitter plate. Eddy-resolving numerical simulations conducted in a wide and very long channel are used to investigate the mean flow structure and the effects of the lock-exchange-like flow and the associated coherent structures on mixing and the capacity of the flow to entrain sediment from the channel bed. When the two streams have unequal densities, quasi-two-dimensional Kelvin–Helmholtz (KH) vortices still form near the MI origin, but their coherence is lost over a much shorter distance compared with the case with no density contrast, and their cores are severely stretched in the transverse direction. A main cell of recirculating (cross-) flow forms in the mean flow, in between the fronts of the near-bed intrusion of heavier fluid and the free-surface intrusion of lighter fluid. The instantaneous flow fields contain streamwise-oriented vortical cells along the interface separating the regions containing heavier and lighter fluids. These vortical cells play an important role in enhancing mixing, similar to the KH billows forming in a classical lock-exchange flow. The regions of highest turbulence amplification are situated next to the boundaries of the main recirculating cell. Once the oscillating fronts of the intrusions get sufficiently close to the channel sidewalls, streamwise cells of secondary flow form near the channel boundaries. For cases with a strong density contrast, the free-surface mixing pattern is not a good indicator of mixing between the two streams. For the same flow velocities in the incoming channels, the two streams mix faster with increasing density difference between the two streams. This is because, as opposed to the KH vortices, coherent structures induced by the formation of the lock-exchange-like flow maintain their coherence and capacity to induce mixing at very large distances from the splitter plate. Meanwhile, the redistribution of the streamwise momentum leading to uniform, fully developed flow over the whole cross-section is delayed by buoyancy effects. Away from the splitter plate, the region with the highest sediment entrainment potential is situated next to the edge of the main recirculation cell on the high-speed side of the channel. For the same flow velocities in the incoming channels, the sediment entrainment capacity of the flow is much larger in the simulations conducted with density contrast between the incoming streams and peaks for the case when the faster stream contains the denser fluid.

Wake flow patterns and turbulence around naturally deposited and installed trees in a gravel bed river

AbstractLarge wood structures, such as wood fragments, debris jams, or entire trees, create flow and habitat diversity in rivers. A key flow feature associated with such structures is the wake, characterised by a core zone of reduced velocity and shear layers at its margins. Wakes are largely controlled by geometric and structural properties of the wood. In the present study, the flow patterns and turbulence created by different wood structures were compared at two study sites: naturally eroded and fragmented oaks (Site A) and artificial poplar installations (Site B). Flow and turbulence were quantified using pointwise velocity measurements with acoustic Doppler velocimeters (ADVs) and surface particle tracking velocimetry (SPTV). The measured flow patterns exhibited similarities with shallow porous wakes that feature fluid advection through the structure into the wake core downstream. Two additional features of wood structures were identified in the present study: (i) the growth of the shear layers was hindered by bed friction like for shallow mixing layers and (ii) the presence of a tree stem and sediment deposit in the wake centre delayed or even suppressed the interaction of the shear layers and vortex street formation similar to a wake‐splitter plate. Methodologically, the combined ADV/SPTV measurement approach and the use of analytical models for shallow mixing layers proved to be highly valuable to decipher the complex flow patterns around wood structures in the field.

Destratification and Restratification of the Spring Surface Boundary Layer in a Subtropical Front

AbstractDestratification and restratification of a ~50-m-thick surface boundary layer in the North Pacific Subtropical Front are examined during 24–31 March 2017 in the wake of a storm using a ~5-km array of 23 chi-augmented EM-APEX profiling floats (u, υ, T, S, χT), as well as towyo and ADCP ship surveys, shipboard air-sea surface fluxes, and parameterized shortwave penetrative radiation. During the first four days, nocturnal destabilizing buoyancy fluxes mixed the surface layer over almost its full depth every night followed by restratification to N ~ 2 × 10−3 rad s−1 during daylight. Starting on 28 March, nocturnal destabilizing buoyancy fluxes weakened because weakening winds reduced latent heat flux. Shallow mixing and stratified transition layers formed above ~20-m depth. A remnant layer in the lower part of the surface layer was insulated from destabilizing surface forcing. Penetrative radiation, turbulent buoyancy fluxes, and horizontal buoyancy advection all contribute to its restratification, closing the budget to within measurement uncertainties. Buoyancy advective restratification (slumping) plays a minor role. Before 28 March, measured advective restratification is confined to daytime; is often destratifying; and is much stronger than predictions of geostrophic adjustment, mixed-layer eddy instability, and Ekman buoyancy flux because of storm-forced inertial shear. Starting on 28 March, while small, the subinertial envelope of measured buoyancy advective restratification in the remnant layer proceeds as predicted by mixed-layer eddy parameterizations.

Shallow mixing layers between non-parallel streams in a flat-bed wide channel

Abstract

Tracking and Assessing Oil Spill Toxicity to Aquatic Organisms: A Novel Approach.

An in situ exposure and effects bioassay system was developed for assessing the toxicity of oil spills to aquatic organisms. The assessment tool combines components of 2 previously developed systems, the sediment ecotoxicity assessment ring (SEA Ring) and the drifting particle simulator. The integrated drifting exposure and effects assessment ring (DEEAR) is comprised of a Global Positioning System (GPS) float, a drifter drogue, the SEA Ring, and the Cyclops-7 fluorescent sensor. Polyethylene passive sampling devices (PED) were mounted for an additional means to characterize water quality conditions and exposures. The DEEAR is optimized for evaluating oil exposure and toxicity in the shallow surface mixing layer of marine waters. A short-term preliminary test was conducted in San Diego, California, USA, to verify the operation of the GPS tracking, the iridium communications, and the integrated SEA Ring exposure system. Further, a proof-of-concept demonstration was conducted offshore in the Santa Barbara Channel, where natural oil seeps produce surface slicks and sheens. Two DEEAR units were deployed for 24 h-one within the oil slick and one in an area outside observable slicks. An aerial drone provided tracking of the surface oil and optimal sites for deployment. The DEEAR proof-of-concept demonstrated integrated real-time tracking and characterization of oil exposures by grab samples, PED, and fluorescent sensors. Oil exposures were directly linked to toxic responses in fish and mysids. This novel integrated system shows promise for use in a variety of aquatic sites to more accurately determine in situ oil exposure and toxicity. Environ Toxicol Chem 2021;40:1452-1462. © 2021 SETAC.

Study of a hydrodynamic threshold system for controlling dinoflagellate blooms in reservoirs

Hydrodynamic conditions often affect the eutrophication process and play a key role in algal growth in reservoirs. A promising approach for controlling algal blooms in reservoirs is to create adverse hydrodynamic conditions by implementing reservoir operation strategies. However, research on this method is still nascent and does not support practical applications due to the lack of quantitative hydrodynamic thresholds. In this paper, field observations of algal growth from April 2015 to August 2016 were conducted, and a three-dimensional (3D) model that couples hydrodynamics and water temperatures for the Zipingpu Reservoir was established. Low flow velocities (V) and low Reynolds numbers (Re) in the Longchi tributary are favorable for dinoflagellate growth and accumulation, which can explain why dinoflagellate blooms are more likely to occur in the tributary. A temperature of 18-22°C is considered a precondition for Peridiniopsis penardii blooms, suggesting that freshwater dinoflagellate species may prefer lower temperatures than marine dinoflagellate species. Shallow mixing layer depth (Zmix) is conducive to Peridiniopsis penardii gathering in the upper water layers and promotes growth. The shallow euphotic layer depth (Zeu) was speculated to promote the dominance of this species by stimulating its heterotrophy and inhibiting other algal autotrophy. Furthermore, a boundary line analysis was introduced to characterize the relationships between algal biomass and hydrodynamic indicators. Thus, the thresholds for V, Re, and Zmix/Zeu were determined to be 0.034ms-1, 6.7×104, and 1.7, respectively. Either accelerating horizontal flow to exceed the thresholds of V and Re or facilitating vertical mixing to exceed the threshold of Zmix/Zeu can prevent dinoflagellate blooms. Therefore, the summarized hydrodynamic threshold system is suggested to be an effective standard for controlling dinoflagellate blooms in the reservoir. Moreover, this study can provide a useful reference for understanding the mechanism of freshwater dinoflagellate blooms.

Near- and far-field structure of shallow mixing layers between parallel streams

Abstract

Wind Intensity Is Key to Phytoplankton Spring Bloom Under Climate Change

The onset of the spring bloom (OSB) occurs when phytoplankton growth exceeds losses and is promoted by a transition from deep convection to a shallow mixing layer concurrent with increasing light intensities in nutrient-enriched waters. We have combined remotely sensed chlorophyll-a data and high-resolution sea-surface winds to quantify and understand high-latitude spring-bloom dynamics and the effect of varying winds. Increasing winds strengthen turbulent mixing and may eventually cause the mixing depth to extend beyond the depth at which light is favorable for net growth and delay the OSB. We find that wind intensity accounts for up to 60 % of the interannual variation in the OSB as revealed by remotely-sensed chlorophyll-a values at the key spawning ground (62-63 oN) of one of the worlds’ largest herring stocks. The OSB is, on average, one month later and with about half the variability farther north at the main spawning ground (67-68 oN) of one of the world’s largest cod stocks. Since the atmospheric reanalysis considered here extends wind time series much further back in time (1958) than remote sensing (1998), the former may act as a good proxy for investigating OSB trends on the time scales of multi-decadal variability and climate change. We find a weak but non-significant signal of delay in the OSB across these extended time periods. More importantly, our results clearly show that predictions of future productivity and ecosystem dynamics under global warming based on earth system models require accurate representation of winds.

An Overview of Low‐Level Jet Winds and Corresponding Mixed Layer Depths During PECAN

AbstractThirty southerly low‐level jet (LLJ) events were observed during the Plains Elevated Convection at Night (PECAN) field campaign in the Great Plains region of the United States during summer 2015. Here we present Doppler lidar wind data from three PECAN instrumentation sites to explore characteristics of LLJs and the boundary layer as well as some of the heterogeneities possible within the wind field of a LLJ. Southerly LLJs were observed on 66% of nights at the southwestern site (Greensburg, KS) but only 56% and 53% of nights at the eastern and northern sites, respectively (Hesston and Ellis, KS). The northernmost site had a relative abundance of weaker jets or nonjet conditions due to fronts or convective systems that only affected part of the observation domain. Plotting mean wind fields of each LLJ type reveals that the strongest LLJs tend to develop under very similar conditions but begin to show variability in wind profile evolution after several hours. A robust mixed layer height retrieval algorithm is used to investigate the interplay between the jets and the turbulent convective boundary layer, showing that stronger LLJs are preceded by deeper afternoon mixed layers and often have a later decoupling of mixing between the upper convective mixed layer and the near‐surface layer. Only the strongest LLJs generated a shallow mixing layer overnight. Comparing jet strength and direction to pristine nocturnal convection initiation shows that the strongest southerly LLJs yielded the most pristine nocturnal convection initiation events per night, and the pristine nocturnal convection initiation occurred farther north.

Flow structure within a vegetation patch in a gravel-bed river

Abstract Investigation of the interactions between submerged vegetation patch and flow structure is of crucial importance for river engineering. Most of hydraulic models have been presented for fully developed flows over uniform vegetation in the laboratory conditions; however, the mentioned interactions are complex in river flows where the flow is not developed along small patch. This reveals a gap between developed and non-developed flow along the vegetation patch. This study was conducted in a gravel-bed river in the central Iran. The results reveal that the flow structure in evolving flow (non-developed flow) along the patch resembles that in shallow mixing layer. Accordingly, a shallow mixing layer model and modified equations are combined to quantify evolving area along the patch. The evolving shallow mixing layer equations for the flow along a non-uniform vegetation patch reach a reasonable agreement with field data. However, the spreading coefficient of this model less than one was reported in literature, 0.06 and 0.12. In addition, the flow immediately downstream the vegetation patch behaves similar to a jet and is parameterized by two conventional models, conventional logarithmic law and mixing layer theory. These models present a reasonable agreement with the measured velocity profiles immediately downstream the patch.

Optical Properties of Near-Surface Urban Aerosols and their Chemical Tracing in a Mediterranean City (Athens)

ABSTRACT One-year measurements (October 2016–September 2017) of aerosol optical properties in the Athens urban environment were analyzed; for closure purposes, the results were supported by data of chemical composition of the non-refractory submicron aerosol fraction acquired with an Aerosol Chemical Speciation Monitor (ACSM). Both the spectral scattering (bsca) and absorption (babs) coefficients exhibit a pronounced annual variability with higher values (63.6 Mm–1 at 550 nm and 41.0 Mm–1 at 520 nm, respectively) in winter, due to domestic heating releasing increased carbonaceous emissions and the shallow mixing layer trapping aerosols near the surface. Much lower values (33.5 Mm–1 and 22.9 Mm–1 for bsca and babs, respectively) are found during summer, indicating rather aged aerosols from regional sources. The estimations of the dry spectral single scattering albedo (SSA), scattering (SAE) and absorption (AAE) Angstrom exponents focus on the seasonality of the urban aerosols. The high SAE (~2.0) and low SSA (0.62 ± 0.11) values throughout the year indicate the dominance of fine-absorbing aerosols from fossil-fuel combustion, while the high AAE (~1.5) in winter suggests enhanced presence of biomass-burning aerosols. Pronounced morning and late evening/night peaks are found in both bsca and babs during winter, coinciding with the morning traffic rush hour and increased residential wood burning in the evening, while in the other seasons, the diurnal patterns flatten out. The wind speed strongly affects the aerosol loading and properties in winter, since for winds below 3 m s–1, a high increase in bsca and babs is observed, consistent with low dilution processes and hazy/smoggy conditions. Our closure experiments indicate a good agreement (R2 = 0.91, slope = 1.08) between the reconstructed and measured bsca values and reveal that organic matter contributes about half of the sub-micron mass in winter, followed by sulfate (~40%) and nitrate (10%, only in winter) aerosols.

The evolution of light and vertical mixing across a phytoplankton ice-edge bloom

During summer, phytoplankton can bloom in the Arctic Ocean, both in open water and under ice, often strongly linked to the retreating ice edge. There, the surface ocean responds to steep lateral gradients in ice melt, mixing, and light input, shaping the Arctic ecosystem in unique ways not found in other regions of the world ocean. In 2016, we sampled a high-resolution grid of 135 hydrographic stations in Baffin Bay as part of the Green Edge project to study the ice-edge bloom, including turbulent vertical mixing, the under-ice light field, concentrations of inorganic nutrients, and phytoplankton biomass. We found pronounced differences between an Atlantic sector dominated by the warm West Greenland Current and an Arctic sector with surface waters originating from the Canadian archipelago. Winter overturning and thus nutrient replenishment was hampered by strong haline stratification in the Arctic domain, whereas close to the West Greenland shelf, weak stratification permitted winter mixing with high-nitrate Atlantic-derived waters. Using a space-for-time approach, we linked upper ocean dynamics to the phytoplankton bloom trailing the retreating ice edge. In a band of 60 km (or 15 days) around the ice edge, the upper ocean was especially affected by a freshened surface layer. Light climate, as evidenced by deep 0.415 mol m–2 d–1 isolumes, and vertical mixing, as quantified by shallow mixing layer depths, should have permitted significant net phytoplankton growth more than 100 km into the pack ice at ice concentrations close to 100%. Yet, under-ice biomass was relatively low at 20 mg chlorophyll-a m–2 and depth-integrated total chlorophyll-a (0–80 m) peaked at an average value of 75 mg chlorophyll-a m–2 only around 10 days after ice retreat. This phenological peak may hence have been the delayed result of much earlier bloom initiation and demonstrates the importance of temporal dynamics for constraints of Arctic marine primary production.