Marine Atmospheric Boundary Layer Research Articles

CALIPSO Observed Low Clouds Over the Subtropical Northeast Pacific: Summer Climatology and Interannual Variability

AbstractThis study examines the spatial distribution and temporal variations of low‐cloud top over the subtropical northeast Pacific (NEP), using observations from the Cloud‐Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO). A comparison between CALIPSO observations and in‐situ soundings reveals a distinct bimodal distribution of low‐cloud tops, characterized by peaks near the inversion base and transition layer, corresponding to shallow cumulus clouds below the stratocumulus deck and boundary‐layer decoupling phenomena. Climatologically, while traversing along a great circle from Los Angeles to Honolulu, the frequency of low cloud occurrence experiences a gradual reduction from 80% to 30%. Meanwhile, cloud top height increases from 0.6 to 1.6 km, and surface roughness varies from 50 to 300 m. This study marks the first presentation of long‐term characteristics of low clouds over a vast, remote area, expanding beyond discrete field campaign observations. Accurately characterizing the marine atmospheric boundary layer (MABL) including its depth and degree of decoupling, enables further investigation into the interannual variations of low clouds, using 14 years of CALIPSO observations. Significant interannual variability in areal fraction and vertical bimodality of low clouds emerges downstream of the anomalously cold water; increased cloud cover with weakened bimodality is associated with a shallow weakly decoupled MABL. This research emphasizes the need to consider the downstream effects in studying low cloud‐sea surface temperature feedback within climate models.

Effect of wind speed on marine aerosol optical properties over remote oceans with use of spaceborne lidar observations

Abstract. Marine aerosol affects the global energy budget and regional weather. The production of marine aerosol is primarily driven by wind at the sea–air interface. Previous studies have explored the effects of wind on marine aerosol, mostly by examining the relationships between aerosol optical depth (AOD) and surface wind speed. In this paper, utilizing the synergy of aerosol and wind observations from Aeolus, the relationships between the marine aerosol optical properties at 355 nm and the instantaneous co-located wind speeds of remote oceans are investigated at two vertical layers (within and above the marine atmospheric boundary layer (MABL)). The results show that the enhancements of the extinction and backscatter coefficients caused by wind are larger within the MABL than above it. The correlation models between extinction and backscatter with wind speed were established using power-law functions. The slope variation points occur during extinction and backscatter coefficients increasing with wind speed, indicating that the wind-driven enhancement of marine aerosol involves two phases: a rapid-growth phase with high wind dependence, followed by a slower-growth phase after the slope variation points. We also compared the AOD–wind relationship acquired from Aeolus with CALIPSO-derived results from previous research. The variation in the lidar ratio with wind speed is examined, suggesting a possible “increasing–decreasing–increasing” trend of marine aerosol particle size as wind speed increases. This study enhances the comprehension of the correlation between marine aerosol optical properties and wind speed by providing vertical information and demonstrating that their relationships are more complex than a linear or exponential relation.

Polydisperse sea spray effect on the vertical momentum transport in hurricanes

Abstract This study focuses on the influence of the sea spray polydispersity on the vertical transport of momentum in a turbulent marine atmospheric boundary layer in high-wind conditions of a hurricane. The Eulerian multifluid model treating air and spray droplets of different sizes as interacting inter-penetrating continua is developed and its numerical solutions are analyzed. Several droplet size distribution spectra and correlation laws relating wind speed and spray production intensity are considered. Polydisperse model solutions have confirmed the difference between the roles small and large spray droplets play in modifying the turbulent momentum transport that have been previously identified by monodisperse spray models. The obtained results have also provided a physical explanation for the previously unreported phenomenon of the formation of thin low-eddy-viscosity “sliding” layers in strongly turbulent boundary layer flows laden with predominantly fine spray.

Observed sub-daily variations in air–sea turbulent heat fluxes under different marine atmospheric boundary layer stability conditions in the Gulf Stream

Abstract Based on data collected from 14 buoys in the Gulf Stream, this study examines how hourly air–sea turbulent heat fluxes vary on sub-daily timescales under different boundary layer stability conditions. The annual mean magnitudes of the sub-daily variations in latent and sensible heat fluxes at all stations are 40 and 15 W·m−2, respectively. Under near-neutral conditions, hourly fluctuations in air–sea humidity and temperature differences are the major drivers of sub-daily variations in latent and sensible heat fluxes, respectively. When the boundary layer is stable, on the other hand, wind anomalies play a dominant role in shaping the sub-daily variations in latent and sensible heat fluxes. In the context of a convectively unstable boundary layer, wind anomalies exert a strong controlling influence on sub-daily variations in latent heat fluxes, whereas sub-daily variations in sensible heat fluxes are equally determined by air–sea temperature difference and wind anomalies. The relative contributions by all physical quantities that affect sub-daily variations in turbulent heat fluxes are further documented. For near-neutral and unstable boundary layers, the sub-daily contributions are О(2) and О(1) W·m−2 for latent and sensible heat fluxes, respectively, and they are less than О(1) W·m−2 for turbulent heat fluxes under stable conditions.

On the relationship between mesoscale cellular convection and meteorological forcing: comparing the Southern Ocean against the North Pacific

Abstract. Marine atmospheric boundary layer (MABL) clouds cover vast areas over the ocean and have important radiative effects on the Earth's climate system. These radiative effects are known to be sensitive to the local organization, or structure, of the mesoscale cellular convection (MCC). A convolutional neural network model is used to identify the two idealized classes of MCC clouds, namely open and closed, over the Southern Ocean (SO) and Northwest Pacific (NP) from high-frequency geostationary Himawari-8 satellite observations. The results of the climatology show that MCC clouds are evenly distributed over the mid-latitude storm tracks for both hemispheres, with peaks poleward of the 40∘ latitude. Open-MCC clouds are more prevalent than closed MCC in both regions. An examination of the presumed meteorological forcing associated with open- and closed-MCC clouds is conducted to illustrate the influence of large-scale meteorological conditions. We establish the importance of the Kuroshio western boundary current in the spatial coverage of open and closed MCC across the NP, presumably through the supply of strong heat and moisture fluxes during marine cold-air outbreaks events. In regions where static stability is higher, we observe a more frequent occurrence of closed MCCs. This behavior contrasts markedly with that of open MCCs, whose formation and persistence are significantly influenced by the difference in temperature between the air and the sea surface. The occurrence frequency of closed MCC over the SO exhibits a significant diurnal cycle, while the diurnal cycle of closed MCC over the NP is less noticeable.

Seasonal Variations in Eddy-Induced Atmospheric Perturbations in the South China Sea

Abstract This study investigates the impact of eddy-induced sea surface temperature (SST) anomalies on the overlying atmosphere in the South China Sea, utilizing observational and reanalysis datasets. The results reveal that SST anomalies caused by anticyclonic or cyclonic eddies have a significant impact on the acceleration or deceleration of surface winds, with a stronger response in summer compared to winter. Moreover, atmospheric responses, such as heat flux, precipitation, and marine atmospheric boundary layer (MABL) depth above anticyclonic or cyclonic eddies, exhibit in-phase seasonal variations as surface winds. The study also explores the mechanisms leading to atmospheric response, pointing toward the vertical mixing mechanism as the dominant cause, supported by both the in-phase relationship between SST and surface wind anomalies and the linear relationship between the wind stress divergence anomaly and downwind SST gradient anomaly. Seasonal variations in coupling intensity are attributed to varying background atmospheric conditions, with more effective vertical turbulence mixing and stronger coupling intensity caused by more unstable MABL and enhanced large-scale vertical wind shear during summer than winter. Besides, the atmosphere above eddies is under a quasi-equilibrium condition, in which the surface wind stress increases monotonically with the depth of MABL. Given that the MABL depth response to eddy-induced SST anomaly is stronger in summer than in winter, it is reasonable to expect a more intense wind response during this season. Thus, the MABL depth coupling works together with the vertical mixing mechanism to explain the proportional relationship between SST and wind anomalies, and why the atmospheric response is stronger in summer than in winter. Significance Statement The atmosphere exerts a significant influence on the ocean, with strong winds cooling the sea surface temperature on a large scale (more than 1000 km). Conversely, the ocean tends to drive the atmosphere at a mesoscale level (10–1000 km), whereby a warm sea surface temperature causes surface wind to accelerate. This study aims to better understand how mesoscale eddies in the South China Sea influence the overlying atmosphere in different seasons. Our research explores factors that control mesoscale air–sea coupling, and highlights the importance of stability and depth change of marine atmospheric boundary layer. These results will establish a foundation for future research examining how the atmospheric perturbations caused by mesoscale eddies can in turn impact both ocean circulation and eddies.

An Online Assimilation Method to Improve the Numerical Forecast of Sea Fog Using Microwave Radiometer‐Retrieved Cloud Water Path

AbstractNumerical forecast of the sea fog is sensitive to the initial moist stratification within the marine atmospheric boundary layer (MABL). This study develops an online assimilation method to improve the MABL thermal and moist structures in sea fog ensemble forecasts based on the Weather Research and Forecasting model and Grid‐point Statistical Interpolation/EnKF system. It uses the satellite‐retrieved cloud water path (CWP) as the indicator of sea fog and low‐level stratus to determine the best ensemble members at each grid point. The relative humidity and cloud water profiles are extracted from the best members to generate a series of pseudo‐observations, which are assimilated to update all the members by EnKF method. The new method significantly improves the ensemble forecast of five widespread advection fog events over the Yellow Sea, which can be attributed to the decrease of both missed and spurious fog areas. The case study shows that assimilating the information of both humidity and cloud water outperforms assimilating either of them, while the impact of directly assimilating CWP observation is insignificant. The analysis increments of cloud water, thermal and moist structures in MABL together contribute to the correction of forecasted sea fog. The generation of pseudo‐observations can use the dynamic compatibility of the model to alleviate the impact of erroneous data in the observation, leading to the low sensitivity of the new method to CWP retrieval error.

CO2 fluxes under different oceanic and atmospheric conditions in the Southwest Atlantic Ocean

The Southwest Atlantic Ocean (SAO) is one of the largest global carbon sink areas. Therefore, the main objective of this study is to investigate turbulent CO2 flux behavior and quantify it in the presence of an intense horizontal sea surface temperature (SST) gradient in the SAO under different atmospheric conditions. In-situ, satellite, and reanalysis data were used from October 14 to 27, 2018 to achieve this objective. The study area was divided into four areas based on satellite observations of SST, salinity, and chlorophyll. The CO2 flux was calculated using the eddy covariance method. During the experiment the area absorbing the most CO2 was the Brazil Current (BC) owing to its proximity to the chlorophyll-rich and less saline waters of the La Plata River and the cold and less saline waters from the Malvinas Current (MC). Moreover, intense wind speeds increased the CO2 flux between the ocean and atmosphere. The Brazil Malvinas Confluence (BMC) also behaved as a CO2 sink, and the modulation of CO2 fluxes was due to the intense horizontal gradient of SST together with the moderate surface wind and turbulence. During the experiment, the MC sequestered less carbon than other regions because of the presence of high-pressure atmospheric systems near the region, resulting in high atmospheric stability, that inhibited mass exchange between the ocean and atmosphere. Vertical mixing mechanisms were identified at the BMC on the cold side, over MC waters. However, in the BC waters, the marine atmospheric boundary layer was modulated by the high-pressure atmospheric system, which suppressed the turbulent mixing. However, the intense mass exchange between the ocean and atmosphere was inhibited, and the area behaved as a mild CO2 sink because of the high-pressure system. This research contributes to a better understanding of the role of the SAO in the global carbon balance in a climate change scenario, and we showed that area can act as a CO2 sink or source, depending on the large-scale atmospheric conditions acting.

Evaluation of sea surface roughness parameterization in meso-to-micro scale simulation of the offshore wind field

Accurate parameterization of the wind-induced effects on the Marine Atmospheric Boundary Layer (MABL) is of fundamental importance for many applications including weather forecasting and offshore wind energy. Roughness parameterizations derived from different datasets are widely used but their performance is not sufficiently assessed under realistic marine environmental conditions. To this end, a multi-scale atmosphere–wave coupled model is constructed for the first time and used in the simulation of the MABL flow over the North Sea. Results show that our model is able to capture the large-scale variation of the mean wind meanwhile reproducing the appropriate turbulence energy cascade in the frequency domain. The performance of different roughness parameterizations is evaluated in comparison with the measurement data from the FINO1 platform. The Taylor-Yelland method outperforms the others on statistics of the mean wind while yielding the highest bias of the Turbulence Kinetic Energy (TKE) from the observations. However, we find that the discrepancy between our simulation and observation mainly depends on the meso-scale model. The magnitude of the differences caused by roughness parameterizations is negligible, especially in low wind speed conditions. The micro-scale simulation is demonstrated as not sensible to the roughness input in the considered wind–wave condition.

Classification of Turbulent Mixing Driven Sources in Marine Atmospheric Boundary Layer With Use of Shipborne Coherent Doppler Lidar Observations

AbstractA method to identify the turbulent mixing sources within the marine atmospheric boundary layer (MABL) based on the shipborne coherent Doppler lidar measurements is introduced in this paper. Combining with the coherent Doppler lidar signal‐to‐noise ratio, vertical velocity skewness, turbulence kinetic energy dissipation rate, and wind shear intensity, the categories of turbulent mixing sources and the specific turbulent mixing sources could be determined. The method is applied into two voyages of MABL observation during May 2021 in the South China Sea and during April 2022 in the Bohai Sea and Yellow Sea. The turbulent mixing processes are captured and the classification of the turbulence driven sources within the MABL are realized. The temporal‐spatial evolution characteristics of the turbulence mixing process in the MABL are investigated under different weather conditions containing clear‐sky day, cloudy‐sky day, and sea‐fog day. The convective mixing process is recognized in the daytime of the clear‐sky day and the intermittent cloud‐driven turbulence exists below the cloud layer. Additionally, the turbulent mixing is weak which could not act as the main driven source during the sea‐fog day. Furthermore, the dominant turbulence scale analyses of different turbulence sources are conducted based on the cospectra of the vertical velocity and the horizontal speed measurements. The turbulence parameters of different turbulence sources are statistical analyzed and compared in different sea areas. The classification method has the broad application prospects on the study of the air‐sea interaction.

Coupling wind LiDAR fixed and volumetric scans for enhanced characterization of wind turbulence and flow three‐dimensionality

SummaryOver the last decades, pulsed light detection and ranging (LiDAR) anemometry has gained growing attention in probing the marine atmospheric boundary layer (MABL) due to its ease of use combined with compelling spatio‐temporal resolution. Among several scanning strategies, fixed scans represent the most prominent choice when high‐frequency resolution is required; however, no information is provided about the spatial heterogeneity of the wind field. On the other hand, volumetric scans allow for the characterization of the spatial variability of the wind field with much lower temporal resolution than fixed scans. In this work, the recently developed “LiDAR Statistical Barnes Objective Analysis” (LiSBOA) algorithm for the optimal design of LiDAR scans and retrieval of wind velocity statistics is tailored for applications in the MABL. The LiDAR data, collected during a recent experimental campaign over Lake Lavon in Texas, show a good consistency of mean velocity profiles between fixed and LiSBOA‐interpolated volumetric data, thus further encouraging the use of coupled fixed and volumetric scans for simultaneous characterizations of wind turbulence statistics along the vertical direction and volumetric heterogeneity of the wind field.

Internal boundary layer characteristics at the southern Bulgarian Black Sea coast

Abstract. The marine Atmospheric Boundary Layer (ABL) over the southern Bulgarian Black Sea coast is studied based on remote sensing measurements with a monostatic Doppler sodar system located at about 400 m inland. Long-term profile data (August 2008–October 2016) with high spatial (10 m) and temporal (20 min running averages at every 10 min) resolution was analysed to reveal the complex vertical structure of the coastal ABL at marine airflow. The processes of air masses transformation due to the sharp change in physical characteristics of the underlying surface lead to Internal Boundary Layer (IBL) formation. Its spatial scales as a sublayer of the coastal ABL depend on the distance from the shore. In the absence of temperature and humidity profile measurements, the turbulent profiles of marine air masses of different fetch over land (400 to 2500 m) were used to examine the characteristics of the IBL. Different fetch or distance passed by the marine airflow before reaching the sodar is considered selecting intervals of wind directions. IBL heights between 60 and 150 m depending on the fetch are obtained.

Observed Air–Sea Turbulent Heat Flux Anomalies during the Onset of the South China Sea Summer Monsoon in 2021

Abstract This study presents observational findings of air–sea turbulent heat flux anomalies during the onset of the South China Sea summer monsoon (SCSSM) in 2021 and explains the mechanism for high-resolution heat flux variations. Turbulent heat flux discrepancies are not uniform throughout the basin but indicate a significant regional disparity in the South China Sea (SCS), which also experiences evident year-to-year variability. Based on buoy- and cruise-based air–sea measurements, high-temporal-resolution (less than hourly) anomalies in the latent heat flux during the SCSSM burst are unexpectedly determined by sea–air humidity differences instead of wind effects under near-neutral and mixed marine atmospheric boundary layer (MABL) stability conditions. However, latent heat anomalies are mainly induced by wind speed under changing MABL conditions. The sensible heat flux is much weaker, with its anomalies dominated by sea–air temperature differences regardless of the boundary layer condition. The observational results are used to examine the discrepancies in turbulent heat fluxes and associated air–sea variables in reanalysis products. The comparisons indicate that latent and sensible heat fluxes in the reanalysis are overestimated by approximately 55 and 3 W m−2, respectively. These overestimations are mainly induced by higher estimates of sea–air humidity/temperature differences. The relative humidity is underestimated by approximately 4.2% in the two high-resolution reanalysis products. The higher SST (near-surface specific humidity) and lower air temperature (specific air humidity) eventually lead to higher estimates of sea–air humidity/temperature differences (1.75 g kg−1/0.25°C), which are the dominant factors controlling the variations in the air–sea turbulent heat fluxes. Significance Statement Air–sea interactions are significant in predicting the onset of East Asian monsoon systems, including the SCSSM. During the SCSSM in 2021, four buoys and cruise observations are used to investigate anomalies in the latent and sensible heat fluxes. The physical mechanism of the variations in turbulent heat fluxes under different MABL stability conditions is explored in this study. The humidity and wind speed anomalies play roles under mixed boundary conditions in determining the high-resolution variations in latent heat fluxes. Based on these observational results, the heat fluxes and associated air–sea variables from reanalysis products are compared to identify the differences in the operational systems. These comparison results can help improve the reanalysis to obtain better monsoon predictions.

On the development of a generalized atmospheric boundary layer velocity profile for offshore engineering applications considering wind–wave interaction

Within industrial standards, effects due to wind–wave interaction on the marine atmospheric boundary layer (ABL) may be included via the Charnock sea-surface roughness parameter for open-sea and near-coastal waters. This roughness parameter does not accommodate the wide variety of wave states possible, nor does it modify the ABL to include speed-up effects resulting from an undulating wave profile. In an attempt to provide an updated definition of the general ABL profile for offshore wind engineering applications, an experimental and numerical study is performed to assess the interaction of the ABL on a fixed wave geometry. By extracting the mean velocity field during the wind–wave interaction, bespoke values of the velocity profile power exponent and wind risk factor can be obtained. A range of wave heights and wave lengths are considered from which a Kriging surrogate model is trained to supply the relevant wind profile parameters depending on the wave state. These newly garnered parameters enable the practitioner to define a reference wind velocity, and then adjust the velocity profile characteristics to contain the influence of the wave. This approach provides a new inlet velocity condition that can be used within computational wind engineering investigations without the need to explicitly model the wave surface, as well as flexibility in specifying the underlying wave conditions. Application of the re-calibrated velocity profile shows that wind forces are significantly greater throughout an offshore wind turbine (OWT) swept blade area when large (H= 15 m) and small (H= 1.5 m) wave heights are compared.

Atmospheric Sea Spray Modeling in the North‐East Atlantic Ocean Using Tunnel‐Derived Generation Functions and the SUMOS Cruise Data Set

AbstractThis study contributes to the communal effort to improve understanding of sea spray generation and transport. For the first time, laboratory‐derived sea spray generation functions (SSGFs) are parameterized in the Meso‐NH mesoscale atmospheric model and are field tested. Formulated from the MATE19 laboratory experiments (Bruch et al., 2021, https://doi.org/10.1007/s10546-021-00636-y) the two SSGFs are driven by the upwind component of the wave‐slope variance (herein B21A), or both and the wind friction velocity cubed (herein B21B). In this first attempt to incorporate the SSGFs in Meso‐NH, the simulations are run without a wave model, and the wave‐wind SSGFs are assumed wind‐dependent. Model evaluation is achieved with a new set of sea spray and meteorological measurements acquired over the 0.1–22.75 μm radius range and U10 1–20 m s−1 wind speeds onboard R/V Atalante during the 25 day SUMOS field campaign in the Bay of Biscay. The B21B SSGF offers particularly good sensitivity to a wide range of environmental conditions over the size range, with an average overestimation by a factor 1.5 compared with measurements, well below the deviations reported elsewhere. B21A also performs well for larger droplets at wind speeds above 15 m s−1. Associated with airflow separation and wave breaking, wave‐slope variance allows to represent multiple wave scales and to scale sea spray generation in the laboratory and the field. Using Meso‐NH simulations we find that sea spray may be transported inland and to altitudes well above the marine atmospheric boundary layer.

Stability mechanisms and air temperature advection on the marine atmospheric boundary layer modulation at the southwestern Atlantic Ocean

AbstractHydrostatic adjustment and vertical mixing are the two main mechanisms used to describe the stability of the marine atmospheric boundary layer (MABL) in oceanic regions with intense horizontal temperature gradients. To analyze the occurrence of these mechanisms, we performed simulations using an active coupled regional ocean–atmosphere numerical model in the southwestern Atlantic Ocean (SWA) region in October 2014 in the presence of an atmospheric frontal system. A novel in‐situ dataset was sampled by radiosondes in the Brazil–Malvinas Confluence (BMC) region and used with the model dataset. The vertical mixing mechanism and a prefrontal warm‐air temperature advection were identified, which modulated the shallower and more stable MABL. The hydrostatic adjustment mechanism was not evident because of the near‐surface wind convergence field modification caused by the large‐scale atmospheric system observed in our experiment. The coupled model simulation presented good agreement compared to in‐situ and satellite data. This contribution to the knowledge of the ocean–atmosphere interaction processes at the SWA reinforced that coupled models can be a helpful tool to investigate the air–sea interactions and physical mechanisms that explain MABL stability.

Environmental Control of Wind Response to Sea Surface Temperature Patterns in Reanalysis Dataset

Abstract Sea surface temperature (SST) is characterized by abundant warm and cold structures that influence the overlying atmospheric boundary layer dynamics through two different mechanisms. First, turbulence and large eddies in the lower troposphere are affected by atmospheric stability, which can be modified by local SST, resulting in enhanced vertical mixing and larger surface winds over warmer waters. Second, the thermodynamic adjustment of air density to the underlying SST structures and the subsequent changes in atmospheric pressure drive secondary circulations. This paper aims to disentangle the effects of these processes and explore the environmental conditions that favor them. Two main environmental variables are considered: the large-scale air–sea temperature difference (proxy for stability) and wind speed. Using 5 years of daily reanalyses data, we investigate the 10-m wind response to SST structures. Based on linear regression between wind divergence and SST derivatives, we show that both mechanisms operate over a large spectrum of conditions. Ten-meter wind divergence is strongly impacted by the local SST via its effect on vertical mixing for midwind regimes in slightly unstable to near-neutral conditions, whereas the secondary circulation is important in two distinct regimes: low wind speed with a slightly unstable air column and high background wind speed with a very unstable air column. The first regime is explained by the prolonged Lagrangian time that the air parcel stays over an SST structure while the second one is related to strong heat fluxes at the air–sea interface, which greatly modify the marine atmospheric boundary layer properties. Location and frequency of the environmentally favorable conditions are discussed, as well as the response in low-cloud cover and rainfall. Significance Statement The main objective of this study is to explore the wind response to thermal structures at the sea surface under different environmental conditions using the latest atmospheric reanalysis. Recent literature suggests that fine-scale air–sea interactions affect a large spectrum of atmospheric dynamics, from seasonal to weather-type regimes. It is thus important to characterize the atmospheric response to ocean surface variability. Our findings describe the environmental conditions for which the two main physical processes through which the atmosphere responds to sea surface temperature structures are active the most and can guide the development of high-resolution observing missions and campaigns in specific geographical locations and seasons to retrieve data that can be used to improve parameterization in models.

Modeling atmospheric microplastic cycle by GEOS-Chem: An optimized estimation by a global dataset suggests likely 50 times lower ocean emissions

•A new atmospheric transport model for microplastics is developed based on GEOS-Chem•Ocean has a 50 times smaller emission (171 [308–764] Gg year−1) than previously thought•Almost all (99%) the microplastic particles are emitted as coarse aerosols (70 μm)•Net land-to-ocean transport is found (25 Gg year−1), much smaller than river discharge Microplastics are tiny plastic particles that are widespread in our environment and pose potential risks to ecosystems and human health. The atmosphere plays a vital role in moving microplastics over long distances and continuously engages in exchanges with the land and ocean. However, identifying the sources of atmospheric microplastics remains challenging. Previous research suggested that the ocean is the main source, but our study, which combines global data and atmospheric models, reveals that the ocean contributes less than initially thought. Road-related sources, such as tire and brake wear and poorly managed plastic waste, also contribute significantly. Our findings underscore the necessity of regulating litter and dumping of plastic waste near roads. Further data collection and laboratory research are needed to better understand the atmospheric microplastic cycle. The atmosphere plays a vital role in microplastic (MP) transport, facilitating continuous exchanges with land and ocean. However, the sources of atmospheric MP remain unclear. Previous studies suggested that the ocean is the primary source, with global emissions reaching up to 8,600 Gg year−1. Here, we use global atmospheric abundance data, a newly developed atmospheric model, and optimal estimation to constrain the atmospheric sources. We find that the global atmospheric MP emissions are 324 (73–1,450) Gg year−1. The ocean source is estimated to have a much smaller global emission (171 [38–764] Gg year−1] than previously believed, followed by road-related sources (115 [26–513] Gg year−1) including the suspension of tire and brake wears and mismanaged plastic waste. We simulate a net land-to-ocean transport by the atmosphere (25 Gg year−1). This highlights the importance of controlling terrestrial sources, and more data are needed to improve our understanding of the atmospheric MP cycle. The atmosphere plays a vital role in microplastic (MP) transport, facilitating continuous exchanges with land and ocean. However, the sources of atmospheric MP remain unclear. Previous studies suggested that the ocean is the primary source, with global emissions reaching up to 8,600 Gg year−1. Here, we use global atmospheric abundance data, a newly developed atmospheric model, and optimal estimation to constrain the atmospheric sources. We find that the global atmospheric MP emissions are 324 (73–1,450) Gg year−1. The ocean source is estimated to have a much smaller global emission (171 [38–764] Gg year−1] than previously believed, followed by road-related sources (115 [26–513] Gg year−1) including the suspension of tire and brake wears and mismanaged plastic waste. We simulate a net land-to-ocean transport by the atmosphere (25 Gg year−1). This highlights the importance of controlling terrestrial sources, and more data are needed to improve our understanding of the atmospheric MP cycle. Plastics are durable, versatile, and ubiquitous in modern life. While their global production has increased from 1,700 Gg year−1 in 1950 to 367,000 Gg year−1 in the 2020s, plastic waste management has not kept up, resulting in more than 40,000–80,000 Gg mismanaged plastic waste (MMPW) globally per year.1Borrelle S.B. Ringma J. Law K.L. Monnahan C.C. Lebreton L. McGivern A. Murphy E. Jambeck J. Leonard G.H. 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Evidence of free tropospheric and long-range transport of microplastic at Pic du Midi Observatory.Nat. Commun. 2021; 12: 7242https://doi.org/10.1038/s41467-021-27454-7Crossref Scopus (46) Google Scholar,10Sonke J.E. Koenig A.M. Yakovenko N. Hagelskjær O. Margenat H. Hansson S.V. De Vleeschouwer F. Magand O. Le Roux G. Thomas J.L. A mass budget and box model of global plastics cycling, degradation and dispersal in the land-ocean-atmosphere system.Microplast. nanoplast. 2022; 2: 28https://doi.org/10.1186/s43591-022-00048-wCrossref Google Scholar Atmospheric long-range transport has been suggested as the major pathway of MPs observed in the remote environment such as protected areas of the US,11Brahney J. Hallerud M. Heim E. Hahnenberger M. Sukumaran S. Plastic rain in protected areas of the United States.Science. 2020; 368: 1257-1260https://doi.org/10.1126/science.aaz5819Crossref PubMed Scopus (376) Google Scholar pristine mountain catchments in the French Pyrenees,12Allen S. Allen D. Phoenix V.R. Le Roux G. Jiménez P.D. Simonneau A. Binet S. Galop D. Atmospheric transport and deposition of microplastics in a remote mountain catchment (vol 12, pg 339, 2019).Nat. Geosci. 2019; 12: 679https://doi.org/10.1038/s41561-019-0409-4Crossref Scopus (13) Google Scholar the Tibetan Plateau,13Dong H. Wang L. Wang X. Xu L. Chen M. Gong P. Wang C. Microplastics in a remote lake basin of the Tibetan plateau: impacts of atmospheric transport and glacial melting.Environ. Sci. Technol. 2021; 55: 12951-12960https://doi.org/10.1021/acs.est.1c03227Crossref PubMed Scopus (49) Google Scholar and the polar regions.14Bergmann M. Mützel S. Primpke S. Tekman M.B. Trachsel J. Gerdts G. White and wonderful? Microplastics prevail in snow from the Alps to the Arctic.Sci. Adv. 2019; 5: eaax1157https://doi.org/10.1126/sciadv.aax1157Crossref PubMed Scopus (601) Google Scholar Records of atmospheric MP levels remain sparse, but limited data suggest that atmospheric deposition rates range from 50 to 700 MP m−2 d−1 with atmospheric concentrations of 10−2 to 101 MP m−3.15Allen D. Allen S. Abbasi S. Baker A. Bergmann M. Brahney J. Butler T. Duce R.A. Eckhardt S. Evangeliou N. et al.Microplastics and nanoplastics in the marine-atmosphere environment.Nat. Rev. Earth Environ. 2022; 3: 393-405https://doi.org/10.1038/s43017-022-00292-xCrossref Scopus (34) Google Scholar Two categories of sources are hypothesized as the major contributors to atmospheric MPs: (1) marine sources, such as aerosolization of marine plastics caused by sea spray, and (2) terrestrial sources, such as the resuspension of road-related plastic particles from tires, brakes, and road surfaces; movement of previously deposited or discharged plastics in soils and agriculture lands; and direct atmospheric emissions from human activities occurring around population centers.8Brahney J. Mahowald N. Prank M. Cornwell G. Klimont Z. Matsui H. Prather K.A. Constraining the atmospheric limb of the plastic cycle.Proc. Natl. Acad. Sci. USA. 2021; 118e2020719118https://doi.org/10.1073/pnas.2020719118Crossref Scopus (140) Google Scholar While most bottom-up inventories focus on plastic emissions to the total environment, the inverse modeling method is often applied to constrain the atmospheric emissions with observed data. For example, Brahney et al.8Brahney J. Mahowald N. Prank M. Cornwell G. Klimont Z. Matsui H. Prather K.A. Constraining the atmospheric limb of the plastic cycle.Proc. Natl. Acad. Sci. USA. 2021; 118e2020719118https://doi.org/10.1073/pnas.2020719118Crossref Scopus (140) Google Scholar suggest the current global atmospheric emissions of MPs as 8,600 (0–22,000) Gg year−1 mainly from the ocean source, and Long et al.16Long X. Fu T.M. Yang X. Tang Y. Zheng Y. Zhu L. Shen H. Ye J. Wang C. Wang T. Li B. Efficient atmospheric transport of microplastics over Asia and adjacent oceans.Environ. Sci. Technol. 2022; 56: 6243-6252https://doi.org/10.1021/acs.est.1c07825Crossref Scopus (11) Google Scholar found a large tire dust source (280 Gg yea−1) over the continent of Asia. Residence time is the average time for MP particles to stay in the atmosphere before their removal by dry and wet deposition, which is calculated as the ratio of its atmospheric abundance to total emission flux. The wet deposition of an MP is similar to other aerosol components such as dust and sea salt,11Brahney J. Hallerud M. Heim E. Hahnenberger M. Sukumaran S. Plastic rain in protected areas of the United States.Science. 2020; 368: 1257-1260https://doi.org/10.1126/science.aaz5819Crossref PubMed Scopus (376) Google Scholar while the size, shape, and chemical compositions are important factors influencing the dry deposition velocity of MP particles. The shapes can be pellets, fibers, and sheets with sizes ranging from 10−6 to 10−3 m.15Allen D. Allen S. Abbasi S. Baker A. Bergmann M. Brahney J. Butler T. Duce R.A. Eckhardt S. Evangeliou N. et al.Microplastics and nanoplastics in the marine-atmosphere environment.Nat. Rev. Earth Environ. 2022; 3: 393-405https://doi.org/10.1038/s43017-022-00292-xCrossref Scopus (34) Google Scholar The density ranges, approximately from 0.9 to 1.4 g cm−3, may have an overall smaller influence on deposition velocity than the other factors. Therefore, aerodynamic diameters are often used, as they are directly associated with the residence time of MPs in the atmosphere. In modeling studies, aerodynamic sizes of different MP particles are considered with a residence time from hours to days.8Brahney J. Mahowald N. Prank M. Cornwell G. Klimont Z. Matsui H. Prather K.A. Constraining the atmospheric limb of the plastic cycle.Proc. Natl. Acad. Sci. USA. 2021; 118e2020719118https://doi.org/10.1073/pnas.2020719118Crossref Scopus (140) Google Scholar,16Long X. Fu T.M. Yang X. Tang Y. Zheng Y. Zhu L. Shen H. Ye J. Wang C. Wang T. Li B. Efficient atmospheric transport of microplastics over Asia and adjacent oceans.Environ. Sci. Technol. 2022; 56: 6243-6252https://doi.org/10.1021/acs.est.1c07825Crossref Scopus (11) Google Scholar Given their importance, the sources of atmospheric MPs and their residence times remain unclear. This study develops an atmospheric transport model for MPs based on the Goddard Earth Observing System (GEOS)-Chem model to determine the most likely sources and residence time of atmospheric MPs. The global atmospheric MP emissions are determined to be 324 (73–1,450) Gg per year. Marine plastic aerosolization is still considered a significant source, but with considerably lower global emissions (171 [38–764] Gg per year), followed by road-related sources (115 [26–513] Gg per year), which include tire and brake wear suspension and improperly managed plastic waste. The findings reveal a predominantly coarse size distribution for these sources, with almost all (99%) of the MP particles classified as very coarse aerosols (70 μm). We calculate a net land-to-ocean transport of atmospheric MP at 25 Gg per year, a considerably smaller amount than riverine discharge and erosion of coastal waste to the ocean (approximately 1,000 Gg per year). Our findings underscore the necessity of regulating litter and dumping of plastic waste near roads. To enhance our comprehension of the atmospheric MP cycle, more data on atmospheric abundance in unexamined regions and direct measurements of emissions from various source categories are required. The GEOS-Chem model considers the emissions, transport, and deposition of MPs in the atmosphere and includes six aerosol-like MP tracers with aerodynamic sizes ranging from 0.3 to 70 μm and residence times ranging from 0.04 to 6.5 days following Brahney et al.8Brahney J. Mahowald N. Prank M. Cornwell G. Klimont Z. Matsui H. Prather K.A. Constraining the atmospheric limb of the plastic cycle.Proc. Natl. Acad. Sci. USA. 2021; 118e2020719118https://doi.org/10.1073/pnas.2020719118Crossref Scopus (140) Google Scholar This size range mainly belongs to MPs with the smallest one to nanoplastics, but we call both MPs for simplicity. We lump all shapes of MP particles into these six bins, as the aerodynamic size is the most important influencing factor for their residence time, and pellets also dominate the air samples among other shapes such as fragments and fibers.16Long X. Fu T.M. Yang X. Tang Y. Zheng Y. Zhu L. Shen H. Ye J. Wang C. Wang T. Li B. Efficient atmospheric transport of microplastics over Asia and adjacent oceans.Environ. Sci. Technol. 2022; 56: 6243-6252https://doi.org/10.1021/acs.est.1c07825Crossref Scopus (11) Google Scholar Possible sources include aerosolized marine plastic, traffic-related sources, resuspension of MMPW and agricultural plastic waste, and generic sources associated with residential activities. The model results are compared with available observed data for atmospheric MPs (both atmospheric concentrations and deposition fluxes). We adopt an optimal estimation approach following Brahney et al.8Brahney J. Mahowald N. Prank M. Cornwell G. Klimont Z. Matsui H. Prather K.A. Constraining the atmospheric limb of the plastic cycle.Proc. Natl. Acad. Sci. USA. 2021; 118e2020719118https://doi.org/10.1073/pnas.2020719118Crossref Scopus (140) Google Scholar and Long et al.16Long X. Fu T.M. Yang X. Tang Y. Zheng Y. Zhu L. Shen H. Ye J. Wang C. Wang T. Li B. Efficient atmospheric transport of microplastics over Asia and adjacent oceans.Environ. Sci. Technol. 2022; 56: 6243-6252https://doi.org/10.1021/acs.est.1c07825Crossref Scopus (11) Google Scholar and incorporate all available observational data over the global land and the ocean environment.15Allen D. Allen S. Abbasi S. Baker A. Bergmann M. Brahney J. Butler T. Duce R.A. Eckhardt S. Evangeliou N. et al.Microplastics and nanoplastics in the marine-atmosphere environment.Nat. Rev. Earth Environ. 2022; 3: 393-405https://doi.org/10.1038/s43017-022-00292-xCrossref Scopus (34) Google Scholar Due to the close coupling between emission strength and size distribution (i.e., residence time), they are optimized simultaneously in the optimization procedure (more details in the experimental procedures). We find a global total atmospheric MP emission of 324 Gg year−1, with terrestrial and marine sources contributing 154 and 171 Gg year−1, respectively (Figure 1). Among the terrestrial sources, the road-related source is dominant (115 Gg year−1), followed by agriculture dust (38 Gg year−1), residential sources (0.80 Gg year−1), and MMPW resuspension (0.11 Gg year−1). The spatial pattern of ocean emissions follows that of the modeled surface ocean plastic mass, scaled by the wind speed and sea surface temperature (see experimental procedures).17Jaeglé L. Quinn P.K. Bates T.S. Alexander B. Lin J.T. Global distribution of sea salt aerosols: new constraints from in situ and remote sensing observations.Atmos. Chem. Phys. 2011; 11: 3137-3157https://doi.org/10.5194/acp-11-3137-2011Crossref Scopus (384) Google Scholar The highest emissions are in the subtropical centers of the five gyres (i.e., North Pacific, South Pacific, North Atlantic, South Atlantic, and the Indian Ocean), where convergent ocean circulation causes an accumulation of floating plastic particles (Figure 1B).18Peng Y. Wu P. Schartup A.T. Zhang Y. Plastic waste release caused by COVID-19 and its fate in the global ocean.Proc. Natl. Acad. Sci. USA. 2021; 118e2111530118https://doi.org/10.1073/pnas.2111530118Crossref Scopus (156) Google Scholar Road-related emissions follow that of traffic activities with higher emissions in North America, West Europe, East Asia, and South Asia (Figure 1A). The other three categories have much smaller emissions compared with these two. The optimizer can also infer the size distribution of emissions. The GEOS-Chem model simulates the mass concentrations of six MP bins that have distinct atmospheric residence times. A unit mass of emissions with different sizes thus has different travel distances and a varied impact on the MP concentrations at receptors. By searching within the possible combinations for the size distributions of emissions to minimize the model-observation difference, the optimizer suggests that almost all (99% in mass) MP particles are emitted as very coarse aerosols (70 μm), reflecting the fact that most of them are from the ocean and road-related sources. The fractions from smaller sizes (0.3–35 μm) are much smaller, with negligible contributions. Indeed, these sources are related to mechanical processes forced by strong wind events or wind/wave breaking of sea surface spray. The size distribution of emissions reflects the influence of inertial or cohesive forces on roadside and ocean surfaces. The results are also consistent with observed size distributions of MPs in the marine atmospheric boundary layer (MABL or MBL), with mass concentrations often dominated by sizes >50 μm.19Wang X. Li C. Liu K. Zhu L. Song Z. Li D. Atmospheric microplastic over the South China sea and East Indian ocean: abundance, distribution and source.J. Hazard Mater. 2020; 389121846https://doi.org/10.1016/j.jhazmat.2019.121846Crossref Scopus (118) Google Scholar,20Ding Y. Zou X. Wang C. Feng Z. Wang Y. Fan Q. Chen H. The abundance and characteristics of atmospheric microplastic deposition in the northwestern South China Sea in the fall.Atmos. Environ. 2021; 253118389https://doi.org/10.1016/j.atmosenv.2021.118389Crossref Scopus (48) Google Scholar This size distribution pattern contrasts with combustion processes, which are typically associated with smaller or finer particles like sulfate and black carbon. The residential sources indeed may have a smaller particle size, but our study could not sufficiently constrain it due to the very low overall global emissions. This is generally consistent with previous studies.8Brahney J. Mahowald N. Prank M. Cornwell G. Klimont Z. Matsui H. Prather K.A. Constraining the atmospheric limb of the plastic cycle.Proc. Natl. Acad. Sci. USA. 2021; 118e2020719118https://doi.org/10.1073/pnas.2020719118Crossref Scopus (140) Google Scholar,16Long X. Fu T.M. Yang X. Tang Y. Zheng Y. Zhu L. Shen H. Ye J. Wang C. Wang T. Li B. Efficient atmospheric transport of microplastics over Asia and adjacent oceans.Environ. Sci. Technol. 2022; 56: 6243-6252https://doi.org/10.1021/acs.est.1c07825Crossref Scopus (11) Google Scholar For example, Brahney et al.8Brahney J. Mahowald N. Prank M. Cornwell G. Klimont Z. Matsui H. Prather K.A. Constraining the atmospheric limb of the plastic cycle.Proc. Natl. Acad. Sci. USA. 2021; 118e2020719118https://doi.org/10.1073/pnas.2020719118Crossref Scopus (140) Google Scholar considered three cases with different prescribed size distributions and found that the best case has 85% global ocean emissions in the 70 μm bin. Long et al.16Long X. Fu T.M. Yang X. Tang Y. Zheng Y. Zhu L. Shen H. Ye J. Wang C. Wang T. Li B. Efficient atmospheric transport of microplastics over Asia and adjacent oceans.Environ. Sci. Technol. 2022; 56: 6243-6252https://doi.org/10.1021/acs.est.1c07825Crossref Scopus (11) Google Scholar also found that 87% terrestrial emissions and 90% ocean emissions are larger than 50 μm. One drawback of this study is that we use only bulk mass concentrations of observed MPs in the cost function of our optimizer (see experimental procedures), as many studies did not report size distributions or used inconsistent definition of size categories.9Allen S. Allen D. Baladima F. Phoenix V.R. Thomas J.L. Le Roux G. Sonke J.E. Evidence of free tropospheric and long-range transport of microplastic at Pic du Midi Observatory.Nat. Commun. 2021; 12: 7242https://doi.org/10.1038/s41467-021-27454-7Crossref Scopus (46) Google Scholar In addition, many of these studies adopted Fourier transform infrared spectroscopy to identify polymers. This technique is typically limited to size ranges greater than ∼30 μm, which thus causes an underestimation of MP particles smaller than this size.21Allen S. Allen D. Moss K. Le Roux G. Phoenix V.R. Sonke J.E. Examination of the ocean as a source for atmospheric microplastics.PLoS One. 2020; 15e0232746https://doi.org/10.1371/journal.pone.0232746Crossref Scopus (125) Google Scholar Nevertheless, our optimizer can greatly benefit from more consistent observations with detailed size distributions by using a cost function considering such information. We find a global terrestrial emission source of 154 Gg year−1. Figure 2 compares the model results driven by the optimized emission inventory against these observations. These data include atmospheric deposition fluxes for US national parks and European and Asian coastal cities. Overall, the modeled average deposition flux over all the sites is 3.6 ± 3.8 kg km−2 year−1, which is not significantly different from the observations (3.5 ± 3.2 kg km−2 year−1) (Figures 2C and 2D). The measured atmospheric deposition flux over US national parks (2.8 ± 2.3 kg km−2 year−1), which are remote and mountainous regions, is slightly lower than the data over urban areas in Asia (3.5 ± 2.8 kg km−2 year−1). Our model simulates a lower deposition flux of 1.5 ± 1.2 kg km−2 year−1 in US national parks but at a comparable level (3.9 ± 2.3 kg km−2 year−1) over Asia. The model (14 ± 15 ng m−3) also reproduces the observed (18 ± 24 ng m−3) atmospheric MP concentrations over land-based sites. Indeed, our model cannot fully simulate the variability in the observations (e.g., Figure 2D). The optimized terrestrial emission and model performance are similar to those of Brahney et al.8Brahney J. Mahowald N. Prank M. Cornwell G. Klimont Z. Matsui H. Prather K.A. Constraining the atmospheric limb of the plastic cycle.Proc. Natl. Acad. Sci. USA. 2021; 118e2020719118https://doi.org/10.1073/pnas.2020719118Crossref Scopus (140) Google Scholar (183 Gg year−1), even though we include observations not only over US national parks, which were used by that study, but also data from other continents including atmospheric concentrations and deposition fluxes. This, on one hand, suggests that our estimate for terrestrial sources is quite robust, i.e., not sensitive to a specific subset of data. On the other hand, this may mean that both our optimized results are local optimums due to the lack of size distribution information, which is the key to further improve the model performance. The optimized results suggest that road-related (115 Gg year−1) and agricultural (38 Gg year−1) sources contribute the most to total terrestrial emissions, while the contributions from MMPW and residential sectors are much smaller. This is generally consistent with Brahney et al.8Brahney J. Mahowald N. Prank M. Cornwell G. Klimont Z. Matsui H. Prather K.A. Constraining the atmospheric limb of the plastic cycle.Proc. Natl. Acad. Sci. USA. 2021; 118e2020719118https://doi.org/10.1073/pnas.2020719118Crossref Scopus (140) Google Scholar and Long et al.,16Long X. Fu T.M. Yang X. Tang Y. Zheng Y. Zhu L. Shen H. Ye J. Wang C. Wang T. Li B. Efficient atmospheric transport of microplastics over Asia and adjacent oceans.Environ. Sci. Technol. 2022; 56: 6243-6252https://doi.org/10.1021/acs.est.1c07825Crossref Scopus (11) Google Scholar but we estimate a lower emission from agricultural sources than Brahney et al.8Brahney J. Mahowald N. Prank M. Cornwell G. Klimont Z. Matsui H. Prather K.A. Constraining the atmospheric limb of the plastic cycle.Proc. Natl. Acad. Sci. USA. 2021; 118e2020719118https://doi.org/10.1073/pnas.2020719118Crossref Scopus (140) Google Scholar (69 Gg year−1). We scale the MP emissions of this source by the fraction of agricultural land use and dust emission fluxes, which are quite sporadic but had no observations available nearby (see experimental procedures; Figure 1C). We thus call for more observations near these source regions to better constrain this source. The residential sector contains sources that include synthetic fibers from clothing, artificial turf, personal care, and cosmetic products.22Wang T. Li B. Zou X. Wang Y. Li Y. Xu Y. Mao L. Zhang C. Yu W. Emission of primary microplastics in mainland China: invisible but not negligible.Water Res. 2019; 162: 214-224https://doi.org/10.1016/j.watres.2019.06.042Crossref Scopus (102) Google Scholar We scale the spatial pattern of this source by population density following Brahney et al.8Brahney J. Mahowald N. Prank M. Cornwell G. Klimont Z. Matsui H. Prather K.A. Constraining the atmospheric limb of the plastic cycle.Proc. Natl. Acad. Sci. USA. 2021; 118e2020719118https://doi.org/10.1073/pnas.2020719118Crossref Scopus (140) Google Scholar We find that the relatively high MP deposition flux over US national parks with low population density drives down the optimized emission for this category. It is surprising to find that the aerosolization of MMPW is a negligible source. MMPW emissions are based on MMPW generation (see experimental procedures), which has a larger emission from populous developing countries such as India and China (Figure 1E).1Borrelle S.B. Ringma J. Law K.L. Monnahan C.C. Lebreton L. McGivern A. Murphy E. Jambeck J. Leonard G.H. Hilleary M.A. et al.Predicted growth in plastic waste exceeds efforts to mitigate plastic pollution.Science. 2020; 369: 1515-1518https://doi.org/10.1126/science.aba3656Crossref PubMed Google Scholar Wind speed and soil moisture also influence the emissions analogous to dust emissions, which makes higher emissions over dry and windy climate regions such as the Middle East (Figure 1D). Developed countries in Western Europe and North America have generally lower MMPW emissions due to better waste management practices.1Borrelle S.B. Ringma J. Law K.L. Monnahan C.C. Lebreton L. McGivern A. Murphy E. Jambeck J. Leonard G.H. Hilleary M.A. et al.Predicted growth in plastic waste exceeds efforts to mitigate plastic pollution.Science. 2020; 369: 1515-1518https://doi.org/10.1126/science.aba3656Crossref PubMed Google Scholar,23Jambeck J.R. Geyer R. Wilcox C. Siegler T.R. Perryman M. Andrady A. Narayan R. Law K.L. Plastic waste inputs from land into the ocean.Science. 2015; 347: 768-771https://doi.org/10.1126/science.1260352Crossref PubMed Scopus (6050) Google Scholar However, such a pattern is not well supported by the available observation data, and the optimizer generates a low weighting factor for this source category. An alternative explanation is that the optimized road-related source does not only include plastic emissions from tire and brake wears but also the aerosolization of MMPW dumped and/or littered roadside, which consists of two-thirds of all MMPW.24Kawecki D. Nowack B. Polymer-specific modeling of the environmental emissions of seven commodity plastics as macro- and microplastics.Environ. Sci. Technol. 2019; 53: 9664-9676https://doi.org/10.1021/acs.est.9b02900Crossref PubMed Scopus (116) Google Scholar Exposure to the environment including ultra-violet radiation, high temperatures, and being smashed by vehicles further break it down into microscopic fragments before being brought into the atmosphere by the energetic flows and turbulence caused by traffic.8Brahney J. Mahowald N. Prank M. Cornwell G. Klimont Z. Matsui H. Prather K.A. Constraining the atmospheric limb of the plastic cycle.Proc. Natl. Acad. Sci. USA. 2021; 118e2020719118https://doi.org/10.1073/pnas.2020719118Crossref Scopus (140) Google Scholar This is also supported by the fact that road dust consists of multiple chemical compositions rather than just tire materials (natural or synthetic rubber).25Dehghani S. Moore F. Akhbarizadeh R. Microplastic pollution in deposited urban dust, Tehran metropolis, Iran.Environ. Sci. Pollut. Res. 2017; 24: 20360-20371https://doi.org/10.1007/s11356-017-9674-1Crossref PubMed Scop

The Southern Ocean mixed layer and its boundary fluxes: fine-scale observational progress and future research priorities.

Interactions between the upper ocean and air-ice-ocean fluxes in the Southern Ocean play a critical role in global climate by impacting the overturning circulation and oceanic heat and carbon uptake. Remote and challenging conditions have led to sparse observational coverage, while ongoing field programmes often fail to collect sufficient information in the right place or at the time-space scales required to constrain the variability occurring in the coupled ocean-atmosphere system. Only within the last 10 years have we been able to directly observe and assess the role of the fine-scale ocean and rapidly evolving atmospheric marine boundary layer on the upper limb of the Southern Ocean's overturning circulation. This review summarizes advances in mechanistic understanding, arising in part from observational programmes using autonomous platforms, of the fine-scale processes (1-100 km, hours-seasons) influencing the Southern Ocean mixed layer and its variability. We also review progress in observing the ocean interior connections and the coupled interactions between the ocean, atmosphere and cryosphere that moderate air-sea fluxes of heat and carbon. Most examples provided are for the ice-free Southern Ocean, while major challenges remain for observing the ice-covered ocean. We attempt to elucidate contemporary research gaps and ongoing/future efforts needed to address them. This article is part of a discussion meeting issue 'Heat and carbon uptake in the Southern Ocean: the state of the art and future priorities'.

Comparative Analysis of Marine Atmospheric Boundary Layer Height Changes in the East China Sea Based on FNL and ERA5 Data

This study compiled the NCEP FNL and ERA5 data from 2010 to 2019 to study the characteristics of the height changes of the marine atmospheric boundary layer (MABL) in the East China Sea and selected coastal (region A), offshore (region B), and Kuroshio areas (region C) for comparative analysis. The results show that, in general, the MABL heights of both datasets show a decreasing trend year by year, and the FNL decreasing trend is more significant. The two datasets show the largest difference in the eastern regions, while they are more consistent in the Taiwan Strait, suggesting substantial regional variations. The FNL decreasing trend is more significant than the ERA5 decreasing trend in regions A, B and C from 2014 to 2017. The root mean square (RMS) of the difference between FNL and ERA5 MABL is the largest in winter, followed by summer, showing significant seasonal variations. Regional MABL variations in the ERA5 dataset were more pronounced in summer and the areas with greater regional variations coincided with the areas with mean absolute differences and the RMS large value area of the MABL variation. Both datasets show region A had the largest differences in fall while regions B and C had the largest differences in winter. In terms of daily variation, both MABL reached their highest at 14:00, with the largest differences at 14:00 in regions A and C and at 20:00 and 02:00 in region B. Region A had the largest daily variations as reflected by its largest mean absolute difference and RMS of the 14:00 MABL differences. The effect of seasonal and daily variations in different regions must be considered to study the variation of MABL height in the East China Sea.