Vertical Dispersion Research Articles

Dispersion of deep-sea hydrothermal vent effluents and larvae by submesoscale and tidal currents

Deep-sea hydrothermal vents provide sources of geochemical materials that impact the global ocean heat and chemical budgets, and support complex biological communities. Vent effluents and larvae are dispersed and transported long distances by deep ocean currents, but these currents are largely undersampled and little is known about their variability. Submesoscale (0.1–10 km) currents are known to play an important role for the dispersion of biogeochemical materials in the ocean surface layer, but their impact for the dispersion in the deep ocean is unknown. Here, we use a series of nested regional oceanic numerical simulations with increasing resolution (from δx=6km to δx=0.75km) to investigate the structure and variability of highly-resolved deep currents over the Mid-Atlantic Ridge (MAR) and their role on the dispersion of the Lucky Strike hydrothermal vent effluents and larvae. We shed light on a submesoscale regime of oceanic turbulence over the MAR at 1500 m depth, contrasting with open-ocean – i.e., far from topographic features – regimes of turbulence, dominated by mesoscales.Impacts of submesoscale and tidal currents on larval dispersion and connectivity among vent populations are investigated by releasing neutrally buoyant Lagrangian particles at the Lucky Strike hydrothermal vent. Although the absolute dispersion is overall not sensitive to the model resolution, submesoscale currents are found to significantly increase both the horizontal and vertical relative dispersion of particles at O(1–10) km and O(1–10) days, resulting in an increased mixing of the cloud of particles. A fraction of particles are trapped in submesoscale coherent vortices, which enable transport over long time and distances. Tidal currents and internal tides do not significantly impact the horizontal relative dispersion. However, they roughly double the vertical dispersion. Specifically, particles undergo strong tidally-induced mixing close to rough topographic features, which allows them to rise up in the water column and to cross topographic obstacles.The mesoscale variability controls at first order the connectivity between hydrothermal sites and we do not have long enough simulations to conclude on the connectivity between the different MAR hydrothermal sites. However, our simulations suggest that the connectivity might be increased by submesoscale and tidal currents, which act to spread the cloud of particles and help them cross topographic barriers.

Predicting response latency using EEG alpha-band power and low-cost wearable physiological sensors.

Predicting response latency using EEG alpha-band power and low-cost wearable physiological sensors.

The Gaia-ESO Survey: Churning through the Milky Way

Context. There have been conflicting results with respect to the extent that radial migration has played in the evolution of the Galaxy. Additionally, observations of the solar neighborhood have shown evidence of a merger in the past history of the Milky Way that drives enhanced radial migration. Aims. We attempt to determine the relative fraction of stars that have undergone significant radial migration by studying the orbital properties of metal-rich ([Fe/H] > 0.1) stars within 2 kpc of the Sun. We also aim to investigate the kinematic properties, such as velocity dispersion and orbital parameters, of stellar populations near the Sun as a function of [Mg/Fe] and [Fe/H], which could show evidence of a major merger in the past history of the Milky Way. Methods. We used a sample of more than 3000 stars selected from the fourth internal data release of the Gaia-ESO Survey. We used the stellar parameters from the Gaia-ESO Survey along with proper motions from PPMXL to determine distances, kinematics, and orbital properties for these stars to analyze the chemodynamic properties of stellar populations near the Sun. Results. Analyzing the kinematics of the most metal-rich stars ([Fe/H] > 0.1), we find that more than half have small eccentricities (e< 0.2) or are on nearly circular orbits. Slightly more than 20% of the metal-rich stars have perigalacticons Rp> 7 kpc. We find that the highest [Mg/Fe], metal-poor populations have lower vertical and radial velocity dispersions compared to lower [Mg/Fe] populations of similar metallicity by ~10 km s-1. The median eccentricity increases linearly with [Mg/Fe] across all metallicities, while the perigalacticon decreases with increasing [Mg/Fe] for all metallicities. Finally, the most [Mg/Fe]-rich stars are found to have significant asymmetric drift and rotate more than 40 km s-1 slower than stars with lower [Mg/Fe] ratios. Conclusions. While our results cannot constrain how far stars have migrated, we propose that migration processes are likely to have played an important role in the evolution of the Milky Way, with metal-rich stars migrating from the inner disk toward to solar neighborhood and past mergers potentially driving enhanced migration of older stellar populations in the disk.

Effect of surface roughness on turbulence, ventilation and pollutant dispersion over hypothetical urban area

Wind flows and pollutant dispersion over cities are strongly affected by urban morphology. Scientific evidence measuring the effect of surface roughness on transport processes is needed to effectuate air quality strategy. Based on idealised building models assembled by ribs and LEGO® bricks, a series of wind tunnel experiments are performed to investigate the effect of aerodynamic resistance on the ventilation ability and pollutant dispersion over rough surfaces. Water vapour is used as the tracer to examine the passive, inert pollutant dispersion from a line source in crossflows. Velocity and tracer concentrations are measured by hot-wire anemometers (HWAs) with X-wire probes and humidity sensors, respectively. The results show that the near-wall streamwise fluctuating velocity 1/2 , vertical fluctuating velocity 1/2 and momentum flux − are enhanced by the aerodynamic resistance induced by the surface-mounted roughness elements. The spatially averaged turbulence properties 1/2 , 1/2 and − are found peaked in the measurement cases AR = 1/12 (rib-type elements) and h:2l-T (cube-type elements). Both turbulent air exchange rate ACHʹ and vertical dispersion coefficient σz increase with increasing drag coefficient Cd, suggesting the prototypes of new parameterisations of ventilation and pollutant dispersion over urban areas.

Effect of surface roughness on turbulence, ventilation and pollutant dispersion over hypothetical urban area

Wind flows and pollutant dispersion over cities are strongly affected by urban morphology. Scientific evidence measuring the effect of surface roughness on transport processes is needed to effectuate air quality strategy. Based on idealised building models assembled by ribs and LEGO® bricks, a series of wind tunnel experiments are performed to investigate the effect of aerodynamic resistance on the ventilation ability and pollutant dispersion over rough surfaces. Water vapour is used as the tracer to examine the passive, inert pollutant dispersion from a line source in crossflows. Velocity and tracer concentrations are measured by hot-wire anemometers (HWAs) with X-wire probes and humidity sensors, respectively. The results show that the near-wall streamwise fluctuating velocity 1/2 , vertical fluctuating velocity 1/2 and momentum flux − are enhanced by the aerodynamic resistance induced by the surface-mounted roughness elements. The spatially averaged turbulence properties 1/2 , 1/2 and − are found peaked in the measurement cases AR = 1/12 (rib-type elements) and h:2l-T (cube-type elements). Both turbulent air exchange rate ACHʹ and vertical dispersion coefficient σz increase with increasing drag coefficient Cd, suggesting the prototypes of new parameterisations of ventilation and pollutant dispersion over urban areas.

Predicting the minimum height of forest fire smoke within the atmosphere using machine learning and data from the CALIPSO satellite

Forest fire smoke is a growing public health concern as more intense and frequent fires are expected under climate change. Remote sensing is a promising tool for exposure assessment, but its utility for health studies is limited because most products measure pollutants in the total column of the atmosphere, and not the surface concentrations most relevant to population health. Information about the vertical distribution of smoke is vital for addressing this limitation. The CALIPSO satellite can provide such information but it cannot cover all smoke events due to its narrow ground track. In this study, we developed a random forests model to predict the minimum height of the smoke layer observed by CALIPSO at high temporal and spatial resolution, using information about fire activity in the vicinity, geographic location, and meteorological conditions. These pieces of information are typically available in near-real-time, ensuring that the resulting model can be easily operationalized. A total of 15,617 CALIPSO data blocks were identified as impacted by smoke within the province of British Columbia, Canada from 2006 to 2015, and 52.1% had smoke within the boundary layer, where the population might be exposed. The final model explained 82.1% of the variance in the observations with a root mean squared error of 560m. The most important variables in the model were wind patterns, the month of smoke observation, and fire intensity within 500km. Predictions from this model can be 1) directly applied to smoke detection from the existing remote sensing products to provide another dimension of information; 2) incorporated into statistical smoke models with inputs from remote sensing products; or 3) used to inform estimates of vertical dispersion in deterministic smoke models. These potential applications are expected to improve the assessment of ground-level population exposure to forest fire smoke.

Dispersion and Persistence of Trichogrammatoidea bactrae (Nagaraja) over Tuta absoluta (Meyrick), in Tomato Greenhouses.

Inundative biological control depends on the ability of natural enemies to disperse and persist in the environment. The objective was to evaluate the dispersion and persistence of Trichogrammatoidea bactrae (Nagaraja) on Tuta absoluta (Meyrick) eggs. Inundative releases of this parasitoid were performed in experimental tomato greenhouses. For vertical dispersion, leaves of the upper and middle third of plants were artificially infested with T. absoluta eggs; for horizontal dispersion, plants at increasing distances from a release point were infested. These eggs were renewed at days 2 and 4 to evaluate persistence. The amount of parasitized patches was registered. Logistic regression analysis was used. The position of the eggs in the plant did not affect the DE (discovery efficiency: proportion of parasitized patches). Time since release negatively affected the DE, while distance affected it only when plants were higher. These results could be used to adjust the release methodology of T. bactrae.

The age–velocity dispersion relation of the Galactic discs from LAMOST–Gaia data

We present the age-velocity dispersion relation (AVR) in three dimensions in the solar neighbourhood using 3,564 commonly observed sub-giant/red-giant branch stars selected from LAMOST, which gives the age and radial velocity, and \emph{Gaia}, which measures the distance and proper motion. The stars are separated into metal-poor (${\rm [Fe/H]<-0.2}$\,dex and metal-rich (${\rm [Fe/H]>-0.2}$\,dex) groups, so that the metal-rich stars are mostly $\alpha$-poor, while the metal-poor group are mostly contributed by $\alpha$-enhanced stars. Thus, the old and metal-poor stars likely belong to the chemically defined thick disc population, while the metal-rich sample is dominated by the thin disc. The AVR for the metal-poor sample shows an abrupt increase at $\gtrsim7$\,Gyr, which is contributed by the thick disc component. On the other hand, most of the thin disc stars with ${\rm [Fe/H]>-0.2}$\,dex display a power-law like AVR with indices of about 0.3--0.4 and 0.5 for the in-plane and vertical dispersions, respectively. This is consistent with the scenario that the disc is gradually heated by the spiral arms and/or the giant molecular clouds. Moreover, the older thin disc stars ($>7$\,Gyr) have a rounder velocity ellipsoid, i.e. $\sigma_\phi/\sigma_{\rm z}$ is close to 1.0, probably due to the more efficient heating in vertical direction. Particularly for the old metal-poor sample located with $|z|>270$\,pc, the vertical dispersion is even larger than its azimuthal counterpart. Finally, the vertex deviations and the tilt angles are plausibly around zero with large uncertainties.

The AMBRE project: The thick thin disk and thin thick disk of the Milky Way

We analyze 494 main sequence turnoff and subgiant stars from the AMBRE:HARPS survey. These stars have accurate astrometric information from Gaia DR1, providing reliable age estimates with relative uncertainties of ±1 or 2 Gyr and allowing precise orbital determinations. The sample is split based on chemistry into a low-[Mg/Fe] sequence, which are often identified as thin disk stellar populations, and high-[Mg/Fe] sequence, which are often associated with thick disk stellar populations. We find that the high-[Mg/Fe] chemical sequence has extended star formation for several Gyr and is coeval with the oldest stars of the low-[Mg/Fe] chemical sequence: both the low- and high-[Mg/Fe] sequences were forming stars at the same time. We find that the high-[Mg/Fe] stellar populations are only vertically extended for the oldest, most-metal poor and highest [Mg/Fe] stars. When comparing vertical velocity dispersion for the low- and high-[Mg/Fe] sequences, the high-[Mg/Fe] sequence has lower vertical velocity dispersion than the low-[Mg/Fe] sequence for stars of similar age. This means that identifying either group as thin or thick disk based on chemistry is misleading. The stars belonging to the high-[Mg/Fe] sequence have perigalacticons that originate in the inner disk, while the perigalacticons of stars on the low-[Mg/Fe] sequence are generally around the solar neighborhood. From the orbital properties of the stars, the high-[Mg/Fe] and low-[Mg/Fe] sequences are most likely a reflection of the chemical enrichment history of the inner and outer disk populations, respectively; radial mixing causes both populations to be observed in situ at the solar position. Based on these results, we emphasize that it is important to be clear in defining what populations are being referenced when using the terms thin and thick disk, and that ideally the term thick disk should be reserved for purely geometric definitions to avoid confusion and be consistent with definitions in external galaxies.

Wildfire smoke plumes transport under a subsidence inversion: Climate and health implications in a distant urban area

This study shows the influence of two large wildfires (one of which was the largest wildfire ever recorded in the region of Castilla y León) in the north-west of the Iberian Peninsula upon the atmospheric air quality of the city of León, Spain, at approximately 70km from the fires, on days with a strong subsidence inversion associated with high pressures. The vertical dispersion of the smoke plume was inhibited and this caused an increase in the particulate matter (PM) in the atmosphere. During this event, average values of up to 1700±600particlescm−3 were registered, most of which corresponding to the smallest fraction of the fine mode. On the other hand, the count median diameter of the fine mode (CMDf) increased gradually from 0.09 to 0.14μm. The PM10 and PM2.5 reached hourly values of 89 and 36μgm−3, respectively. This study also estimates the changes in the optical properties of the particles as well as the associated radiative forcing. The presence of an important load of absorbing aerosols was detected, with instantaneous radiative atmospheric forcing up to +134.6Wm−2. The estimations of the respirable fractions showed in healthy adults high levels of mass concentration of the aerosol that reaches the bronchioles and alveoli (up to 43μgm−3).

Inertial Effects on the Vertical Transport of Suspended Particles in a Turbulent Boundary Layer

In many atmospheric flows, a dispersed phase is actively suspended by turbulence, whose competition with gravitational settling ultimately dictates its vertical distribution. Examples of dispersed phases include snow, sea-spray droplets, dust, or sand, where individual elements of much larger density than the surrounding air are carried by turbulent motions after emission from the surface. In cases where the particle is assumed to deviate from local fluid motions only by its gravitational settling (i.e., they are inertialess), traditional flux balances predict a power-law dependence of particle concentration with height. It is unclear, however, how particle inertia influences this relationship, and this question is the focus of this work. Direct numerical simulations are conducted of turbulent open-channel flow, laden with Lagrangian particles of specified inertia; in this way the study focuses on the turbulent transport which occurs in the lowest few meters of the planetary boundary layer, in regions critical for connecting emission fluxes to the fluxes felt by the full-scale boundary layer. Simulations over a wide range of particle Stokes number, while holding the dimensionless settling velocity constant, are performed to understand the role of particle inertia on vertical dispersion. It is found that particles deviate from their inertialess behaviour in ways that are not easily captured by traditional theory; concentrations are reduced with increasing Stokes number. Furthermore, a similarity-based eddy diffusivity for particle concentration fails as particles experience inertial acceleration, precluding a closed-form solution for particle concentration as in the case of inertialess particles. The primary consequence of this result is that typical flux parametrizations connecting surface emission models (e.g., saltation models or sea-spray generation functions) to elevated boundary conditions may overestimate particle concentrations due to the reduced vertical transport caused by inertia in between; likewise particle emission may be underestimated if inferred from concentration measurements aloft.

Dynamical Effects of the Scale Invariance of the Empty Space: The Fall of Dark Matter?

Abstract The hypothesis of the scale invariance of the macroscopic empty space, which intervenes through the cosmological constant, has led to new cosmological models. They show an accelerated cosmic expansion after the initial stages and satisfy several major cosmological tests. No unknown particles are needed. Developing the weak-field approximation, we find that the here-derived equation of motion corresponding to Newton’s equation also contains a small outward acceleration term. Its order of magnitude is about Newton’s gravity (ϱ being the mean density of the system and the usual critical density). The new term is thus particularly significant for very low density systems. A modified virial theorem is derived and applied to clusters of galaxies. For the Coma Cluster and Abell 2029, the dynamical masses are about a factor of 5–10 smaller than in the standard case. This tends to leave no room for dark matter in these clusters. Then, the two-body problem is studied and an equation corresponding to the Binet equation is obtained. It implies some secular variations of the orbital parameters. The results are applied to the rotation curve of the outer layers of the Milky Way. Starting backward from the present rotation curve, we calculate the past evolution of the Galactic rotation and find that, in the early stages, it was steep and Keplerian. Thus, the flat rotation curves of galaxies appear as an age effect, a result consistent with recent observations of distant galaxies by Genzel et al. and Lang et al. Finally, in an appendix we also study the long-standing problem of the increase with age of the vertical velocity dispersion in the Galaxy. The observed increase appears to result from the new small acceleration term in the equation of the harmonic oscillator describing stellar motions around the Galactic plane. Thus, we tend to conclude that neither dark energy nor dark matter seems to be needed in the proposed theoretical context.

Vertical Pay Dispersion and Innovation Spirit of New Ventures

How differentiated salary management policies affect the innovation spirit of new ventures is an important academic issue. Considering that there is no much comparability of the pay between corporate executives and grassroots employees, this paper subdivides the pay dispersion of new ventures into two levels:pay dispersion between top managers and middle managers (PayDHM), and pay dispersion between middle managers and grassroots workers (PayDML). And it explores the effects of vertical pay dispersion at different levels on innovation activities of new ventures. The results indicate that there exists a significant inverted U-shaped relationship between vertical pay dispersions and the innovation spirit of new ventures, which means that the innovation spirit of new ventures increases first and then decreases as the vertical pay dispersion widens. In addition, the role of pay dispersion between middle managers and grassroots workers in the innovation spirit of new ventures is also affected by board governance and external environment inclusiveness, that is to say at the same level of pay dispersion between middle managers and grassroots workers, new ventures with a board of directors and in a strong inclusive environment have stronger innovation spirit. It extends the achievements of pay dispersion theory research and has important significance to help new ventures to formulate and perfect pay incentives policies.

Deep oxygen penetration drives nitrification in intertidal beach sands

AbstractWe discovered a hotspot of elevated nitrate concentration (median = 431 μM) in shallow beach pore water that extended across the entire length of two barrier islands in the southeastern United States of America. We investigated this feature by surveying groundwater geochemistry, measuring fluctuations in in situ dissolved oxygen (DO) concentrations, modeling groundwater flow, and quantifying nitrification rates. Nitrification of groundwater ammonium was the only possible nitrate source, with a measured potential rate of 0.84 μmol m−2 h−1. However, the observed nitrate concentrations were far in excess of the predicted maximum achievable by aerobic nitrification assuming a 2 : 1 ratio of O : N and around 200 μM DO in air‐saturated seawater. Groundwater DO concentrations within the hotspot (65 cm depth) were consistently 20–50 μM. The nitrate hotspot was located at the top of the water table beneath dry, undersaturated sand that allowed the penetration of air and the dissolution of excess oxygen into the pore fluids. The total dissolved nitrogen concentration of the hotspot was higher than anywhere else on the island, indicating nitrogen accumulation within the hotspot, most likely via ammonium adsorption. Vertical dispersion was the dominant pathway for nitrate loss from the hotspot. This nitrate was consumed in underlying anoxic sand, coupling microbial pathways of nitrogen oxidation and reduction and removing bioavailable nitrogen from the beach ecosystem.

Composition and diel vertical distribution of euphausiid larvae (calyptopis stage) in the deep southern Adriatic

Summary Diel changes in the vertical distribution and abundance of calyptopes were studied in the deepest area of the southern Adriatic over four seasons (July 2003, February 2004, October 2004, April 2009). Temperature variations were limited to the upper 100 m and salinity variations were small. Of previously known adult euphausiid species – 12 for the Adriatic ( Gangai et al., 2012 ) and 13 for the Mediterranean ( Mavidis et al., 2005 ) – calyptopes of 11 species of euphausiids were recorded. Abundance of calyptopes of all species was the highest in spring. Species were characterized according to their mean depth: surface (0–50 m), sub-surface (50–200 m), mesopelagic (200–800 m), or bathypelagic (800–1200 m) and vertical dispersion (scattered or non-scattered). Four diel patterns emerged: (i) nocturnal ascent to upper layers (Euphausia brevis, E. hemigibba, E. krohnii, Nematoscelis megalops, N. couchii), (ii) migration to upper layers at middle of the day and at night, and descent during the morning and evening (Stylocheiron maximum – only winter), (iii) weakly-migrating or non-migrating (S. longicorne), (iv) irregular migration independent of the day/night cycle (S. abbreviatum, S. maximum – during spring, summer and autumn, T. aequalis).

Experimental evidence for concentration-dependence and intraspecific variation of movement behaviour in American lobster (Homarus americanus) larvae

Predicting dispersal paths of marine larvae with extended pelagic durations, such as American lobster (Homarus americanus H. Milne Edwards, 1837), requires understanding the cues to which larvae respond, and how that response reflects changes in larval behaviour. If larvae respond to conspecific presence by varying their movement, then this behaviour can bias laboratory estimates of environmental responses. We tested whether larvae actively decreased their local intraspecific density by measuring how the vertical distribution of larvae changed under high versus low concentrations of conspecifics. We observed weak increases in vertical dispersion at higher concentrations both in newly hatched larvae and in postlarvae, but not in intermediate larval stages. We also tested for differences in horizontal swimming behaviour in high and low concentrations, by fitting a novel random walk model that allowed us to model both larval interactions and persistent turning behaviours. We showed substantial reduction in diffusive behaviour under high concentration conditions resulting from more frequent turns by each larva, but no evidence for consistent avoidance of conspecifics. Our study is the first to demonstrate concentration-dependent behaviours in lobster larvae.

Role of atmospheric circulations in haze pollution in December 2016

Abstract. In the east of China, recent haze pollution has been severe and damaging. In addition to anthropogenic emissions, atmospheric circulations and local meteorological conditions were conducive factors. The number of December haze days over North China and the Huanghuai area has increased sharply since 2010 and was greatest in 2016. During 2016, the most aggressive control measures for anthropogenic emissions were implemented from 16 to 21 December, but the most severe haze pollution still occurred, covering approximately 25 % of the land area of China and lasting for 6 days. The atmospheric circulations must play critical roles in the sub-seasonal haze events. Actually, the positive phase of the East Atlantic–West Russia pattern in the middle troposphere strengthened the anomalous anti-cyclone over the NH area that confined vertical motion below. The associated southerly anomalies made the cold air and surface wind speed weaker, but enhanced the humid flow. Thus, the horizontal and vertical dispersion of atmospheric particulates was suppressed and the pollutants gathered within a narrow space. In December 2016, these key indices were strongly beneficial for haze occurrence and combined to result in the severest haze pollution. The influences of the preceding autumn sea surface temperature near the Gulf of Alaska and the subtropical eastern Pacific, October–November snow cover in western Siberia, and associated physical processes on haze pollution are also discussed.

The Effect of Atmospheric Cooling on Vertical Velocity Dispersion and Density Distribution of Brown Dwarfs∗

Abstract We present a Monte Carlo simulation designed to predict the vertical velocity dispersion of brown dwarfs in the Milky Way. We show that since these stars are constantly cooling, the velocity dispersion has a noticeable trend with the spectral type. With realistic assumptions for the initial mass function, star formation history, and the cooling models, we show that the velocity dispersion is roughly consistent with what is observed for M dwarfs, decreases to cooler spectral types, and increases again for the coolest types in our study (∼T9). We predict a minimum in the velocity dispersions for L/T transition objects, however, the detailed properties of the minimum predominately depend on the star formation history. Since this trend is due to brown dwarf cooling, we expect that the velocity dispersion as a function of spectral type should deviate from the constancy around the hydrogen-burning limit. We convert from velocity dispersion to vertical scale height using standard disk models and present similar trends in disk thickness as a function of spectral type. We suggest that future, wide-field photometric and/or spectroscopic missions may collect sizable samples of distant ( kpc) dwarfs that span the hydrogen-burning limit. As such, we speculate that such observations may provide a unique way of constraining the average spectral type of hydrogen burning.

Evaluation of high-resolution GRAMM–GRAL (v15.12/v14.8) NO&amp;lt;sub&amp;gt;&amp;lt;i&amp;gt;x&amp;lt;/i&amp;gt;&amp;lt;/sub&amp;gt; simulations over the city of Zürich, Switzerland

Abstract. Hourly NOx concentrations were simulated for the city of Zürich, Switzerland, at 10 m resolution for the years 2013–2014. The simulations were generated with the nested mesoscale meteorology and micro-scale dispersion model system GRAMM–GRAL (versions v15.12 and v14.8) by applying a catalogue-based approach. This approach was specifically designed to enable long-term city-wide building-resolving simulations with affordable computation costs. It relies on a discrete set of possible weather situations and corresponding steady-state flow and dispersion patterns that are pre-computed and then matched hourly with actual meteorological observations. The modelling system was comprehensively evaluated using eight sites continuously monitoring NOx concentrations and 65 passive samplers measuring NO2 concentrations on a 2-weekly basis all over the city. The system was demonstrated to fulfil the European Commission standards for air pollution modelling at nearly all sites. The average spatial distribution was very well represented, despite a general tendency to overestimate the observed concentrations, possibly due to a crude representation of traffic-induced turbulence and to underestimated dispersion in the vicinity of buildings. The temporal variability of concentrations explained by varying emissions and weather situations was accurately reproduced on different timescales. The seasonal cycle of concentrations, mostly driven by stronger vertical dispersion in summer than in winter, was very well captured in the 2-year simulation period. Short-term events, such as episodes of particularly high and low concentrations, were detected in most cases by the system, although some unrealistic pollution peaks were occasionally generated, pointing at some limitations of the steady-state approximation. The different patterns of the diurnal cycle of concentrations observed in the city were generally well captured as well. The evaluation confirmed the adequacy of the catalogue-based approach in the context of city-scale air pollution modelling. The ability to reproduce not only the spatial gradients but also the hourly temporal variability over multiple years makes the model system particularly suitable for investigating individualized air pollution exposure in the city.

Temporal and spatial analyses of particulate matter (PM10 and PM2.5) and its relationship with meteorological parameters over an urban city in northeast China

Temporal and spatial characteristics of atmospheric particulate matter (PM10 and PM2.5) and its relationship with meteorology over Shenyang, a city in northeast China, were statistically analyzed using hourly and daily averaged PM mass concentrations measured at 11 locations and surface meteorological parameters, from January 2014 to May 2016. Using averaged data from 11 stations in Shenyang, it was found that the monthly mean PM2.5 mass concentrations were higher in winter (97.2±11.2μgm−3) and autumn (85.5±42.9μgm−3), and lower in spring (62.0±14.0μgm−3) and summer (42.5±8.4μgm−3), similar to the seasonal variation in PM10 concentrations. The monthly ratios of PM2.5/PM10 ranged from 0.41 to 0.87, and were larger in autumn and winter but lowest in spring due to dust activities. PM pollution was concentrated mainly in the central, northern, and western areas of Shenyang in most seasons mainly due to anthropogenic activities such as traffic and residential emission and construction activity as well as natural dust emission. PM concentrations observed over different areas in all seasons generally exhibited two peaks, at 08:00–10:00 local time (LT) and 21:00–23:00 LT, with the exception of PM2.5 in summer, which showed only one peak during the daytime. In addition, PM10 concentrations peaked around 14:00 LT during spring in the western area of Shenyang because of strong thermal and dynamic turbulence, resulting in elevated dust emissions from adjacent dust sources. The relationship between daily PM concentrations and meteorological parameters showed both seasonal and annual variation. Overall, both PM2.5 and PM10 concentrations were negatively correlated with atmospheric visibility, with correlation coefficients (R) of 0.71 and 0.56, respectively. In most seasons, PM concentrations also exhibited negative correlations with wind speed, but showed positive correlations with air pressure, air temperature, and relative humidity. Strong wind speed favored the dispersion of PM pollution, but also aided the release of coarse dust particles in spring. High air pressure and downdrafts restrained the upward movement of atmospheric PM, resulting in an accumulation of particles in the boundary layer. High air temperature favored the transformation of secondary particles through photochemical processes in summer, but also resulted in efficient vertical dispersion of pollutants in autumn and winter, leading to an inverse relationship between temperature and PM concentrations. Large relative humidity usually caused increases in PM concentrations due to the hygroscopic effect of aerosols, but not for PM10 in spring and summer, mainly due to the suppression of dust emissions under wet air conditions in spring and the effects of wet scavenging under high summer rainfall.