Evaluation of Micro-Sensors to Monitor Ozone in Ambient Air

Micro-sensors are very small sensors with physical dimensions in the sub-micrometer to millimetre range that are used to monitor ozone (O3) in ambient air. They are either based on the variation of the resistance of a semi-conductor or on miniaturized electrochemical cells able to deliver a current varying with the level of the pollutant of interest. In the last years, some technological progress took place and a few commercial sensors are now available in the market. In fact, micro-sensors represent a promising technology in several fields like: monitoring of O3 in ambient air to survey of the limit/target values of the Air Quality Directive, rapid mapping of air pollution over small area, validation of dispersion models, evaluation of exposure of population, emissions monitoring and forest monitoring. However, due to reliability problems there is a hesitancy to apply these sensors for air pollution monitoring. The suitability of these sensors is evaluated in this report. In this study, the response time of micro-sensors is investigated. Moreover, warming time after a cold start, linearity, drift over time and the effect of NO2 interference, wind velocity, temperature and humidity on the response of sensors are presented. The comparison of the response of these samplers versus UV photometry is investigated both under controlled conditions using exposure chamber and under field conditions. All sensors used in the study are commercially available. O3 is determined according to the specifications of the manufacturers, without modification of the model equation proposed by the manufacturers.JRC.H.4-Transport and air qualit

Estimation of the Measurement Uncertainty of Ambient Air Pollution Datasets Using Geostatistical Analysis

We developed a methodology able to automatically estimate of measurement uncertainty in the air pollution data sets of AIRBase. The figures produced with this method were consistent with expectations from laboratory and field estimation of uncertainty and with the Data Quality Objectives of the European Directives. The proposed method based on geostatistical analysis is not able to estimate directly the measurement uncertainty. It estimates the nugget effect together with a micro-scale variability that must be minimized by accurate selection of the type of station. Based on the results obtained so far, it is likely that measurement uncertainty is best estimated using all background stations of whatever area type. So far the methodology has been used to estimate uncertainty in 4 different countries independently. This work should be continued for the whole Europe or for background station without national borders. The method has been shown to be also useful to compare the spatial continuity of air pollution in different countries that seems to be influenced by the topography of each country. Moreover, it may be used to quantify the trend of measurement uncertainty over long periods like decade with the possibility to evidence improvement in the data quality of AIRBase datasets. Thanks to the implemented outlier detection module that would also be of interest as the warning system when Member States report they measurement to the European Environment Agency, we have proposed an easy solution to investigate wrong classified stations in AIRBase.JRC.DDG.H.4-Transport and air qualit

Point Centred Variography to Assess the Spatial Representativeness of Air Quality Monitoring Sites: Application to the Datasets of the FAIRMODE Intercomparison Exercise of Spatial Representativeness

Common definitions for the spatial representativeness of air quality monitoring sites are based on the evaluation of the similarity of pollutant concentrations, in which the representativeness area of a monitoring site is basically described by the set of all locations where the concentration of a pollutant does not differ from the measurements at the central point by more than a certain threshold. Classical geostatistical analysis describes the spatial correlation structure of a concentration field in terms of the variogram. In contrary, the point centred variography is based on the average of squared concentration differences observed in pairs formed between a particular central point and the set of all other points in the domain. It thereby places a monitoring station in the context of the local or regional air quality pattern. In this report we demonstrate how a mathematical inversion of the point centred variogram can provide information about the extent of the spatial representativeness area of a monitoring site. The application of this approach is tested on a set of modelling data from the city of Antwerp. This dataset contains information at a very high spatial (street level) and temporal resolution for three main pollutants (PM10, NO2 and Ozone), over the whole city. Furthermore, FAIRMODE (Forum for Air Quality Modeling in Europe) is currently concluding an intercomparison exercise on spatial representativeness methods, which is also based on sharing this same dataset.JRC.C.5-Air and Climat

Measuring air pollution with low-cost sensors

Low-cost air quality sensors are attracting more and more attention. They offer air pollution monitoring at a lower cost than conventional methods, making air quality monitoring possible in many more locations. Too good to be true? At the current stage of development, unfortunately yes. Measurements by low-cost sensors are often of minor and questionable data quality than the results from official monitoring stations as carried out by EU Member States in accordance with European legislation and International standards. Sensors may become a game changer in monitoring air pollution, traffic-management, personal exposure and health assessment, citizen science and air quality assessment in developing countries. This brochure explains our current understanding of the advantages and disadvantages of sensors. Technological progress will hopefully change the picture of this summary of sensor performance in the next few years. It is also a plea to evaluate and validate sensors with field and laboratory tests in order to understand the meaning of and uncertainties in their signals.JRC.C.5-Air and Climat

Protocol of evaluation and calibration of low-cost gas sensors for the monitoring of air pollution

Metrology for Chemical Pollutants in Air (MACPoll) is a joint research project of the European Metrology Research Programme of EURAMET that among other activities deals with low-cost gas sensors for air quality monitoring. Gas sensors are identified as emerging measuring devices for “indicative measurements” regulated in the Air Quality Directive. Compared to reference measurements, gas sensors would allow air pollution monitoring at a lower cost. The Directive allows using indicative measurements without restriction in the zones where the upper assessment threshold (UAT) is not exceeded while they permit a reduction of 50 % of the minimum reference measurements where the UAT is exceeded. The Directive does not specify any indicative method but it requires to demonstrate that they can meet a data quality objective (DQO) that is about twice less stringent than the one of reference methods. The DQO is defined as a relative expanded uncertainty. Since the Directive does not give any guidance for this demonstration, within MACPoll it has been decided to draft a protocol for the evaluation of gas sensors.JRC.H.2-Air and Climat

Screening tools for data quality and outlier detection applied to the Airbase ambient air pollution database

In order to provide scientifically sound information for regulatory purposes and environmental impact assessment, long term meso- to large-scale datasets of ambient air quality provide an indispensible means for model calibration, evaluation and validation. However, the collection of high quality datasets with suitable spatial coverage for air pollution management and decision support poses many challenges. It is thus critical to establish expedient tools for the efficient assessment and data quality control of air pollution measurements in large scale national and international monitoring networks. The European Environmental Agency collects, in the Air Quality Database named AirBase, measurements of ambient air pollution at more than 6000 monitoring stations from over 30 countries. The quality of these data depends on the chosen method of measurements and QA/QC procedures applied by each country. We present a methodology to automatically screen the AirBase records for internal consistency and to detect spatio-temporal outliers nested in the data. We implemented a spatial-set outlier detection method, which considers both attribute values and spatial relationships. Specifically, we adapted the “Smooth Spatial Attribute method” that was developed for the identification of outliers in traffic sensors. The method relies on the definition of a neighbourhood for each air pollutant measurement, corresponding to a spatio-temporal domain limited in time (+/- 1 day) and distance (+/- 1 degree) around location x. It is assumed that within a given spatio-temporal domain in which the attribute values of neighbours have a relationship due to the emission, transport and reaction of air pollutants, outliers will be detected by extreme values of their attributes compared to the attribute values of their neighbours. The implemented method can be of interest as a data quality screening system when countries report their measurements to the European Environment Agency. Beyond this, it could also provide a simple solution to investigate the accuracy of station classification in AirBase.JRC.H.2-Air and Climat

Evaluation of low-cost sensors for air pollution monitoring: Effect of gaseous interfering compounds and meteorological conditions

In this report the performances of low-cost sensors for air pollution monitoring are evaluated in order to give guidance to users on which parameters to take into account when performing field calibration of these sensors prior to monitor air pollution with those. In particular, the effect of gaseous interfering compounds and meteorological conditions on four low cost sensors selected to be mounted on the AirSensEUR platform is characterised. The selected sensors are of the electrochemical type, as they are less power consuming and they have been shown, in previous studies, to lead to fastest response time and to suffer less from gaseous interferences than metal-oxide sensors. Ten set of four sensors for ozone (Membrapor O3/M-5), nitrogen dioxide (Alphasense NO2-B43F), carbon monoxide (Membrapor CO/MF-200) and nitrogen monoxide (Alphasense NO-B4) were evaluated under controlled conditions in a laboratory exposure chamber. The tests allow the evaluation of the interference of the gaseous compounds together with the effect of relative humidity, temperature and pressure variations. In general, each sensor was found to be highly linear when measuring its target gaseous species. Concerning the gaseous interferences, it appears that only the ozone sensors showed a high interference (> 75%) to nitrogen dioxide. The ozone filter of the NO2-B43F appears to be effective. The sensitivity of the CO and NO sensors was sufficient to be able to detect concentration levels expected at ambient gaseous concentrations. However, the interference of NO on the CO/MF-200 was found relevant at high NO and low CO values. The cross sensitivity (CO and NO on O3 and NO2 sensors and vice-versa) was found low or not significant for each type of sensor. The meteorological evaluation showed that the four types of sensor behave similarly concerning the temperature interference. In fact both sensors showed a quadratic response with the increase of the temperature. Relative humidity was only found relevant for the two Alphasense sensors (NO2-B43F and NO-B4) with a linear type of sensitivity associated with a clear hysteresis effect. Ambient pressure, however, was found relevant only for the two Membrapor sensors (O3/M-5 and CO/MF-200). Finally, the good reproducibility between sensors for the majority of effects including sensitivity to gas concentration and to meteorological variables with relative standard deviation within less than 10 % suggests that satisfactory calibration of sensors could be achieved without the need of a full characterisation of each sensor. By using calibration coefficient equal to the averages of the effects given in this report, reasonable calibration function could be established. This result is promising, allowing for future increase of the use of sensors for low cost for air pollution monitoring both by expert institutes and citizen science projects.JRC.C.5-Air and Climat

Field Evaluation of NanoEnvi Microsensors for O3 Monitoring

Previous studies have showed that microsensors can successfully measure ozone in ambient air for a limited period of time after on-site calibration by comparison to ultraviolet photometry. This method is generally more successful than the calibration in exposure chambers under controlled conditions because of the difference between laboratory and fields air matrixes. To expand this result, we carried out an experiment at two sampling sites. At the first site, the microsensors were calibrated during a few days. Subsequently, the calibrated microsensors were taken to another sampling site where the effectiveness of the calibration function of the first site was evaluated. The trend of the differences between UV photometry and microsensors were analyzed to evidence possible drift of the microsensors over time. The correlation of these differences with meteorological data was investigated to evidence possible interference and to propose new calibration methods.JRC.H.2-Air and Climat

Review of low-cost sensors for the ambient air monitoring of benzene and other volatile organic compounds

This report presents a literature review of the state of the art of sensor based monitoring of air quality of benzene and other volatile organic compounds. Combined with information provided by stakeholders, manufacturers and literature, the review considered commercially available sensors, including, PID based sensors, semiconductor (resistive gas sensor) and portable on-line measuring devices (sensor arrays). The bibliographic collection includes the following topics: sensor description, field of application in fixed, mobile, indoor and ambient air monitoring, range of concentration levels and limit of detection in air, model descriptions of the phenomena involved in the sensor detection process, gaseous interference selectivity of sensors in complex VOC matrix, validation data in lab experiments and under field conditions.JRC.C.5-Air and Climat

The Evaluation of the Intercomparison Exercise for SO2, CO, O3, NO and NO2 - June 2007

In June 2007 in Ispra (IT), 9 AQUILA (Network of European Air Quality Reference Laboratories) laboratories and one laboratory of the World Health Organisations (WHO) Euro-Region met at an intercomparison exercise to evaluate their proficiency in the analysis of inorganic gaseous pollutants covered by European Air Quality Directives (SO2, CO, NO, NO2 and O3). The proficiency evaluation, where each participant¿s bias was compared to two criteria, provides information on the current situation and capabilities to the European Commission and can be used by participants in their quality control system. In terms of criteria imposed by the European Commission, 60% of the results reported by AQUILA laboratories were good both in terms of measured values and reported uncertainties. Another 37% of the results had good measured values, but the reported uncertainties were either too small (4%) or too high (33%). The comparability of results among AQUILA participants is satisfactory for O3, SO2, CO and NO measurement method, but the pollutant NO2 needs further improvements and harmonization programmes.JRC.H.4-Transport and air qualit