Mapping roadside nitrogen dioxide concentrations using non-stationary kriging

Report of a studyAtmospheric nitrogen dioxide (N02) concentrations around a major road in Alsace (France) are estimated on a fine grid using measurements given by passive samplers and a geostatistical approach. Data are referenced to a local coordinate system where (x, y) are respectively the distance from and along the road. They show a strong non-stationarity which does not allow ordinary kriging to be used in the estimation. Therefore a trend is modelled by a combination of exponential and polynomial functions. Experimental residuals are then computed as the differences between measurements and the trend. The idea is to interpolate the residuals at the nodes of the grid, applying kriging methods, and to add them to the trend estimate. Since their variance is not stationary either, an intermediary step is required. lt consists in modelling the standard deviation of the residuals as a function of the drift and normalizing the residuals by this model. This defines a new regionalized variable which can be estimated in the framework of stationary geostatistics. Two possible kriging systems are tested, depending on the fitted variogram model: in the first one, a pure nugget effect (white noise) is used, in which case the best linear estimator of N02 concentration is the trend model; in the second one, a structured exponential variogram is adjusted. This case study shows that non-stationarity may not only characterize the raw variable but can also affect the variance of a phenomenon. lt illustrates the interest of modelling it so as to improve the experimental variogram, fit an acceptable variogram model and compute the variance of the estimation error even if the estimator is reduced to a simple regression function

Optimal spatial design for air quality measurement surveys: what criteria?

International audienceIn this work, we present a spatial statistical methodology to design benzene air concentration measurement surveys at the urban scale. In a first step, we define an a priori modeling based on an analysis of data coming from previous campaigns on two different agglomerations. More precisely, we retain a modeling with an external drift which consists of a drift plus a spatially correlated residual. The statistical analysis performed leads us to choose the most relevant auxiliary variables and to determine an a priori variogram model for the residual. An a priori distribution is also defined for the variogram parameters, whose values appear to vary from a campaign to another. In a second step, we optimize the positioning of the measuring devices on a third agglomeration according to a Bayesian criterion. Practically, we aim at finding the design that minimizes the mean over the urban domain of the universal kriging variance, whose parameters are based on the a priori modeling, while accounting for the prior distribution over the variogram parameters. Two optimization algorithms are then compared: simulated annealing and a particle filter based algorithm

Using different spatial scale measurements in a geostatistically based approach for mapping atmospheric nitrogen dioxide concentrations. Application to the French Centre region

International audiencePassive sampling surveys followed by geostatistical data analysis have become a common and efficient way of mapping background concentrations at regional and urban scale. Traffic related pollution is also a matter of concern as regards people exposure but since it acts at shorter spatiotemporal scales, it is usually not integrated in the same maps. However, to provide more comprehensive information to the authorities and the public, the organisms responsible for air quality monitoring are searching for innovative ways of representing background and roadside concentrations together. A methodology based on geostatistics and the examination of the relationships between season averaged nitrogen dioxide concentrations and auxiliary variables is proposed in this study. It is applied to data collected in the French Centre region

Spatial representativeness of an air quality monitoring station. Application to NO2 in urban areas

International audienceThe present study aims at setting up a geostatistical methodology that could be implemented in an operational context to assess the spatial representativeness of a measurement station. In the proposed definition, a point is considered as belonging to the area of representativeness of a station if its concentration differs from the station measurement by less than a given threshold. Additional criteria related to distance or environmental characteristics may also be introduced. Concentrations are first estimated at each point of the domain applying kriging techniques to passive sampling data obtained from measurement surveys. The standard deviation of the estimation error is then used, making a hypothesis on the error distribution, to select the points, at a fixed risk, where the difference of concentration with respect to the station is below the threshold. The methodology is then applied to NO2 experimental datasets for different French cities

Vapor emissions from contaminated soils into buildings : comparison between predictions from transport model and field measurements

International audienceSoil vapor migration into house, with subsequent inhalation, is often the main exposure pathway to humans at sites contaminated with Volatile Organic Compounds (VOCs). In the case of VOCs contamination, quantification of indoor gas concentrations is therefore essential while assessing risks for human health. Two approaches are commonly used for quantification of indoor concentrations: indoor gas measurement or transfer modelization from the source (soil, soil vapor phase, groundwater). Model development is relatively well advanced but measurements for model calibration and "validation" hardly exist in the literature. Furthermore, predictions of indoor gas concentrations from different models may vary by several orders of magnitude, depending on the application. Therefore, the validity of the risk calculation obtained through models, and hence of the site management, remains highly uncertain. The research project presented here aims at providing such "validation" data. Long-term goals of the research are improvement of modelization and also development of tools for site-related model-selection. The program consisted in comparing modelization and experimental measurements on a test site. The site was a former factory with a concrete slab, contaminated with chlorinated solvents (trichloroethene and perchloroethylene) in unsaturated soils. Measurements concerned contaminant concentrations and fluxes in different media and at different transfer stages, but also key mode! parameters. The equations of Johnson & Ettinger and VOLASOIL models were used. Air concentrations measured at various times show significant variations, and also differ from model prediction by one or two orders of magnitude. Despite thorough parameter measurements, uncertainty on input values, related to site heterogeneity, induced high uncertainty in the modelization. The preliminary results presented here show intrinsic limitation of some measurements and hence of model validation, but also the need for data on more sites, including very important site instrumentation which would allow to document the impact of site-heterogeneity

Estimation of the areas of air quality limit value exceedances on national and local scales. A geostatistical approach

International audienceEach year Member States have to report to the European Commission on the exceedances of air quality limit values which occurred on their territory. Quantitative information is required about the areas and population exposed to such exceedances. A probabilistic methodology for defining exceedance zones has been developed, based on preliminary air quality mapping. Atmospheric concentration fields estimated by kriging and the corresponding kriging variance are used to identify areas where the exceedance or non-exceedance can be considered as certain and areas where the situation with respect to the limit value is indeterminate. The methodology is applied on national and urban scales focusing on exceedances of PM10 daily limit value and NO2 annual limit value. Results are discussed from operational perspectives

A Highly Instrumented Underground Research Gallery as a Monitoring Concept for Radioactive Waste Cells - Data Measurement Qualification

International audienceThis document presents a dedicated work performed in AndraÕs underground research laboratory (URL) in order to prepare, to test and to qualify the future monitoring system of underground disposal for the French long-lived, intermediate and high level radioactive wastes. Part of the monitoring system qualification process relies on testing sensors on full-scale demonstrators and ensuring that we carefully observe the desired parameters. One of these demonstrators is a concrete liner in a tunnel aiming at support the mechanical pressure of the host rock. A 3.6 meter long section of this gallery has been highly instrumented by various technologies of sensors. This paper describes the monitoring system installed and demonstrates how a numerical model of the demonstrator has been utilized to comfort and validate sensors' measurements

The PREV'AIR system, an operational system for large scale air quality forecasts over Europe : applications at the local scale

International audienceSince Summer 2003, the PREV'AIR system has been delivering through the Internet daily air quality forecasts over Europe. This is the visible part of a wider collaborative project - the PREV'AIR project - launched by the French Ministry for Ecology and Sustainable Development (MEDD), aiming at: (1) Providing technical support on atmospheric pollution management in Europe, in the framework of negotiations on trans-boundary air pollution. (2) Providing large scale national air quality information based on numerical simulations and observations. The PREV'AIR system is a complementary monitoring tool with respect to the local information delivered by the French qualified associations in charge of regional air quality monitoring (AASQA). PREV'AIR relies on a chain of numerical tools: air quality simulation models, modules ensuring the provision of meteorological and air quality input data to these models, modules enabling the extraction and use of the numerical data computed by the system. The outputs of the PREV'AIR system (secondary pollutants forecasts and maps) are archived to build up a large scale air quality simulation data base over Europe

Www.airqualitynow.eu, a common website and air quality indices to compare cities across europe

International audienceAir quality is a public concern. This is partly due to the "right to know" principle embodied in European legislation. Despite this common legislation, the way air quality is being interpreted and communicated differs considerably. For specialists raw monitoring data for Europe are available but these are not usable by the general public. Easy to understand and internationally comparable air quality information from one city to another is scarce: there are almost as many air quality indices as air quality monitoring networks. The CITEAIR II project (Common information to European Air, INTERREG IVc) facilitates the comparison of urban air quality in near real-time by introducing common air quality indices at hourly, daily and annual scales and by developing a forecast for those indices for D+0 and D+1.The implementation was based on a common website www.airqualitynow.eu using readily available simple IT-solutions. This paper describes those tools which both aimed at presenting the air quality of the participating cities in a comparable way and not to replace more targeted local information. Their added value is to provide, for the first time, a European and comparable picture of the air quality in near real-time easily accessible through a common platform and presentation of the results. The website is designed to receive and display data from any city wanting to join. The main part is dedicated to compare the cities index values using different time scales (hourly, daily or annual) and two types of exposure thanks to a background and a traffic index. In addition, space is offered to cities for presenting themselves according to a common template, providing background information on their specific air pollution situation and associated reduction measures. Participating is easy: cities upload their data through ftp and the indices calculations are automatically made. The website provides a dynamic picture of the air quality and is updated each hour enticing viewers to make repeated visits. However, participation with only a daily update or with yearly data is feasible as wel

The PREV’AIR system, an operational system for large scale air quality forecasts over Europe; applications at the local scale

International audienceNumerical simulations of pollution events with deterministic models have become easier for the last decade thanks to increasing computer skills. Hence three-dimensional chemistry-transport-runs can be performed on a single workstation for long-term simulation or real-time forecast over large scale areas. Furthermore, fast Internet download and high file storage capacity in data processing make it possible to use a wide database of meteorological parameters and pollutant concentration measurements. The PREV'AIR System rests on those technological progresses for delivering daily air qualiry forecasts in operational conditions