In order to investigate associations between air pollution and adverse health effects consistent fine spatial air pollution surfaces are needed across large areas to provide cohorts with comparable ...exposures. The aim of this paper is to develop and evaluate fine spatial scale land use regression models for four major health relevant air pollutants (PM2.5, NO2, BC, O3) across Europe.
We developed West-European land use regression models (LUR) for 2010 estimating annual mean PM2.5, NO2, BC and O3 concentrations (including cold and warm season estimates for O3). The models were based on AirBase routine monitoring data (PM2.5, NO2 and O3) and ESCAPE monitoring data (BC), and incorporated satellite observations, dispersion model estimates, land use and traffic data. Kriging was performed on the residual spatial variation from the LUR models and added to the exposure estimates. One model was developed using all sites (100%). Robustness of the models was evaluated by performing a five-fold hold-out validation and for PM2.5 and NO2 additionally with independent comparison at ESCAPE measurements. To evaluate the stability of each model's spatial structure over time, separate models were developed for different years (NO2 and O3: 2000 and 2005; PM2.5: 2013).
The PM2.5, BC, NO2, O3 annual, O3 warm season and O3 cold season models explained respectively 72%, 54%, 59%, 65%, 69% and 83% of spatial variation in the measured concentrations. Kriging proved an efficient technique to explain a part of residual spatial variation for the pollutants with a strong regional component explaining respectively 10%, 24% and 16% of the R2 in the PM2.5, O3 warm and O3 cold models. Explained variance at fully independent sites vs the internal hold-out validation was slightly lower for PM2.5 (65% vs 66%) and lower for NO2 (49% vs 57%). Predictions from the 2010 model correlated highly with models developed in other years at the overall European scale.
We developed robust PM2.5, NO2, O3 and BC hybrid LUR models. At the West-European scale models were robust in time, becoming less robust at smaller spatial scales. Models were applied to 100 × 100 m surfaces across Western Europe to allow for exposure assignment for 35 million participants from 18 European cohorts participating in the ELAPSE study.
•Robust PM2.5, NO2, BC and O3 hybrid LUR models at a 100x100 m resolution for Western Europe were developed•Models included large scale SAT and CTM estimates and fine scale traffic and land use and were further improved with kriging•Models were robust in time at European scale, becoming less robust at smaller spatial scales.
Empirical spatial air pollution models have been applied extensively to assess exposure in epidemiological studies with increasingly sophisticated and complex statistical algorithms beyond ordinary ...linear regression. However, different algorithms have rarely been compared in terms of their predictive ability.
This study compared 16 algorithms to predict annual average fine particle (PM2.5) and nitrogen dioxide (NO2) concentrations across Europe. The evaluated algorithms included linear stepwise regression, regularization techniques and machine learning methods. Air pollution models were developed based on the 2010 routine monitoring data from the AIRBASE dataset maintained by the European Environmental Agency (543 sites for PM2.5 and 2399 sites for NO2), using satellite observations, dispersion model estimates and land use variables as predictors. We compared the models by performing five-fold cross-validation (CV) and by external validation (EV) using annual average concentrations measured at 416 (PM2.5) and 1396 sites (NO2) from the ESCAPE study. We further assessed the correlations between predictions by each pair of algorithms at the ESCAPE sites.
For PM2.5, the models performed similarly across algorithms with a mean CV R2 of 0.59 and a mean EV R2 of 0.53. Generalized boosted machine, random forest and bagging performed best (CV R2~0.63; EV R2 0.58–0.61), while backward stepwise linear regression, support vector regression and artificial neural network performed less well (CV R2 0.48–0.57; EV R2 0.39–0.46). Most of the PM2.5 model predictions at ESCAPE sites were highly correlated (R2 > 0.85, with the exception of predictions from the artificial neural network). For NO2, the models performed even more similarly across different algorithms, with CV R2s ranging from 0.57 to 0.62, and EV R2s ranging from 0.49 to 0.51. The predicted concentrations from all algorithms at ESCAPE sites were highly correlated (R2 > 0.9). For both pollutants, biases were low for all models except the artificial neural network. Dispersion model estimates and satellite observations were two of the most important predictors for PM2.5 models whilst dispersion model estimates and traffic variables were most important for NO2 models in all algorithms that allow assessment of the importance of variables.
Different statistical algorithms performed similarly when modelling spatial variation in annual average air pollution concentrations using a large number of training sites.
•Multiple statistical algorithms with very different assumptions were compared.•Despite the difference in modeling frameworks, predictions among the models exhibit generally good agreement.•The use of an external evaluation dataset strengthens evaluation by cross-validation.
In order to investigate associations between air pollution and adverse health effects consistent fine spatial air pollution surfaces are needed across large areas to provide cohorts with comparable ...exposures. The aim of this paper is to develop and evaluate fine spatial scale land use regression models for four major health relevant air pollutants (PM
, NO
, BC, O
) across Europe.
We developed West-European land use regression models (LUR) for 2010 estimating annual mean PM
, NO
, BC and O
concentrations (including cold and warm season estimates for O
). The models were based on AirBase routine monitoring data (PM
, NO
and O
) and ESCAPE monitoring data (BC), and incorporated satellite observations, dispersion model estimates, land use and traffic data. Kriging was performed on the residual spatial variation from the LUR models and added to the exposure estimates. One model was developed using all sites (100%). Robustness of the models was evaluated by performing a five-fold hold-out validation and for PM
and NO
additionally with independent comparison at ESCAPE measurements. To evaluate the stability of each model's spatial structure over time, separate models were developed for different years (NO
and O
: 2000 and 2005; PM
: 2013).
The PM
, BC, NO
, O
annual, O
warm season and O
cold season models explained respectively 72%, 54%, 59%, 65%, 69% and 83% of spatial variation in the measured concentrations. Kriging proved an efficient technique to explain a part of residual spatial variation for the pollutants with a strong regional component explaining respectively 10%, 24% and 16% of the R
in the PM
, O
warm and O
cold models. Explained variance at fully independent sites vs the internal hold-out validation was slightly lower for PM
(65% vs 66%) and lower for NO
(49% vs 57%). Predictions from the 2010 model correlated highly with models developed in other years at the overall European scale.
We developed robust PM
, NO
, O
and BC hybrid LUR models. At the West-European scale models were robust in time, becoming less robust at smaller spatial scales. Models were applied to 100 × 100 m surfaces across Western Europe to allow for exposure assignment for 35 million participants from 18 European cohorts participating in the ELAPSE study.
Shock parameters at Earth’s bow shock, in rare instances, can approach the Mach numbers predicted at astrophysical shocks and supernova remnants. We present our analysis of a high Alfv ́en Mach ...number (MA= 27) shock, by utilizing multipoint measurements from the Magnetospheric Multiscale (MMS) spacecraft during a crossing of Earth’s quasi-perpendicular bow shock. We find that the shock dynamics are mostly driven by reflected ions, perturbations that they generate, and nonlinear amplification of the perturbations. Our analyses indicate that reflected ions create modest magnetic enhancements upstream of the shock front which evolve in a nonlinear manner as they traverse the shock foot. They can transform into proto-shocks that propagate at small angles to the magnetic field and towards the bow shock. The nonstationary bow shock shows signatures of both reformation and surface ripples. Our observations indicate that although shock reformation occurs, the main shock layer never disappears. These observations are at high plasmaβ, a parameter regime which has not been well explored by numerical models.
Collisionless shock nonstationarity arising from microscale physics influences shock structure and particle acceleration mechanisms. Nonstationarity has been difficult to quantify due to the small ...spatial and temporal scales. We use the closely spaced (subgyroscale), high-time-resolution measurements from one rapid crossing of Earth's quasiperpendicular bow shock by the Magnetospheric Multiscale (MMS) spacecraft to compare competing nonstationarity processes. Using MMS's high-cadence kinetic plasma measurements, we show that the shock exhibits nonstationarity in the form of ripples.
Simulations and observations of collisionless shocks have shown that deviations of the nominal local shock normal orientation, that is, surface waves or ripples, are expected to propagate in the ramp ...and overshoot of quasi‐perpendicular shocks. Here we identify signatures of a surface ripple propagating during a crossing of Earth's marginally quasi‐parallel (θBn∼45∘) or quasi‐parallel bow shock on 27 November 2015 06:01:44 UTC by the Magnetospheric Multiscale (MMS) mission and determine the ripple's properties using multispacecraft methods. Using two‐dimensional hybrid simulations, we confirm that surface ripples are a feature of marginally quasi‐parallel and quasi‐parallel shocks under the observed solar wind conditions. In addition, since these marginally quasi‐parallel and quasi‐parallel shocks are expected to undergo a cyclic reformation of the shock front, we discuss the impact of multiple sources of nonstationarity on shock structure. Importantly, ripples are shown to be transient phenomena, developing faster than an ion gyroperiod and only during the period of the reformation cycle when a newly developed shock ramp is unaffected by turbulence in the foot. We conclude that the change in properties of the ripple observed by MMS is consistent with the reformation of the shock front over a time scale of an ion gyroperiod.
Key Points
A surface ripple has been observed by MMS at Earth's quasi‐parallel bow shock
Hybrid simulations show that these ripples are transients modulated by shock reformation
Changes in the observed ripple are consistent with reformation processes
Using observations of Earth's bow shock by the Magnetospheric Multiscale mission, we show for the first time that active magnetic reconnection is occurring at current sheets embedded within the ...quasi‐parallel shock's transition layer. We observe an electron jet and heating but no ion response, suggesting we have observed an electron‐only mode. The lack of ion response is consistent with simulations showing reconnection onset on sub‐ion time scales. We also discuss the impact of electron heating in shocks via reconnection.
Plain Language Summary
For the first time, we document an observation of magnetic reconnection occurring at Earth's bow shock. The observations have been made by NASA's Magnetospheric Multiscale mission while the bow shock is under a “quasi‐parallel” geometry, which typically results in a highly disordered structure. Models of shock waves in space plasmas do not currently account for reconnection. This therefore introduces a new avenue of research into how shocks can repartition energy when slowing the solar wind from supersonic to subsonic flow. The observations also introduce a new regime for magnetic reconnection, for which we observe only an electron response at an ion scale reconnecting structure. This work will also attract interest from the broader astrophysics community, as reconnection at shocks may influence cosmic ray generation.
Key Points
Reconnecting current sheets have been observed at a quasi‐parallel bow shock
The ion‐scale current sheet exhibits only an electron jet and heating, with no ion response
Consistent with kinetic simulations, reconnection relaxes complexity in the shock transition region
We present a model of electromagnetic drift waves in the current sheet adjacent to magnetic reconnection at the subsolar magnetopause. These drift waves are potentially important in governing 3‐D ...structure of subsolar magnetic reconnection and in generating turbulence. The drift waves propagate nearly parallel to the X line and are confined to a thin current sheet. The scale size normal to the current sheet is significantly less than the ion gyroradius and can be less than or on the order of the wavelength. The waves also have a limited extent along the magnetic field (B), making them a three‐dimensional eigenmode structure. In the current sheet, the background magnitudes of B and plasma density change significantly, calling for a treatment that incorporates an inhomogeneous plasma environment. Using detailed examination of Magnetospheric Multiscale observations, we find that the waves are best represented by series of electron vortices, superimposed on a primary electron drift, that propagate along the current sheet (parallel to the X line). The waves displace or corrugate the current sheet, which also potentially displaces the electron diffusion region. The model is based on fluid behavior of electrons, but ion motion must be treated kinetically. The strong electron drift along the X line is likely responsible for wave growth, similar to a lower hybrid drift instability. Contrary to a classical lower hybrid drift instability, however, the strong changes in the background B and no, the normal confinement to the current sheet, and the confinement along B are critical to the wave description.
Key Points
Drift waves are potentially important in governing 3D structure of subsolar magnetic reconnection and in generating turbulence
Drift waves displace or corrugate the current sheet and potentially displace the electron diffusion region of magnetic reconnection
Parallel electric fields arise in the drift waves
Hot Flow Anomalies (HFAs) are transients observed at planetary bow shocks, formed by the shock interaction with a convected interplanetary current sheet. The primary interpretation relies on ...reflected ions channeled upstream along the current sheet. The short duration of HFAs has made direct observations of this process difficult. We employ high resolution measurements by NASA's Magnetospheric Multiscale Mission to probe the ion microphysics within a HFA. Magnetospheric Multiscale Mission data reveal a smoothly varying internal density and pressure, which increase toward the trailing edge of the HFA, sweeping up particles trapped within the current sheet. We find remnants of reflected or other backstreaming ions traveling along the current sheet, but most of these are not fast enough to out‐run the incident current sheet convection. Despite the high level of internal turbulence, incident and backstreaming ions appear to couple gyro‐kinetically in a coherent manner.
Plain Language Summary
Shock waves in space are responsible for energizing particles and diverting supersonic flows around planets and other obstacles. Explosive events known as Hot Flow Anomalies (HFAs) arise when a rapid change in the interplanetary magnetic field arrives at the bow shock formed by, for example, the supersonic solar wind plasma flow from the Sun impinging on the Earth's magnetic environment. HFAs are known to produce impacts all the way to ground level, but the physics responsible for their formation occur too rapidly to be resolved by previous satellite missions. This paper employs NASA's fleet of four Magnetospheric Multiscale satellites to reveal for the first time clear, discreet populations of ions that interact coherently to produce the extreme heating and deflection.
Key Points
MMS observations reveal distinct ion velocity‐space populations within a Hot Flow Anomaly (HFA)
The HFA interior varies smoothly in density with a swept‐up pressure excess toward the trailing edge
The HFA interior displays coherent kinematic coupling between antisunward and sunward backstreaming ions
The Magnetospheric Multiscale Mission observes, in detail, charged particle heating and substantial nonthermal acceleration in a region of strong turbulence ( , where is the magnetic field) that ...surrounds a magnetic reconnection X-line. Magnetic reconnection enables magnetic field annihilation in a volume that far exceeds that of the diffusion region. The formidable magnetic field annihilation breaks into strong, intermittent turbulence with magnetic field energy as the driver. The strong, intermittent turbulence appears to generate the necessary conditions for nonthermal acceleration. It creates intense, localized currents ( ) and unusually large-amplitude electric fields ( ). The combination of turbulence-generated and results in a significant net positive mean of , which signifies particle energization. Ion and electron heating rates are such that they experience a fourfold increase from their initial temperature. Importantly, the strong turbulence also generates magnetic holes or depletions in that can trap particles. Trapping considerably increases the dwell time of a subset of particles in the turbulent region, which results in significant nonthermal particle acceleration. The direct observation of strong turbulence that is enabled by magnetic reconnection with nonthermal particle acceleration has far-reaching implications, since turbulence in plasmas is pervasive and may occupy significant volumes of the interstellar medium and intergalactic space. For example, strong turbulence from magnetic field annihilation in the supernova nebulae may dominate large volumes. As such, this observed energization process could plausibly contribute to the supply and development of the cosmic-ray spectrum.
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