A multi-pollutant methodology to locate a single air quality monitoring station in small and medium-size urban areas.

Air quality management is underpinned by continuous measurements of concentrations of target air pollutants in monitoring stations. Although many approaches for optimizing the number and location of air quality monitoring stations are described in the literature, these are usually focused on dense networks. However, there are small and medium-size urban areas that only require one monitoring station but also suffer from severe air pollution. Given that target pollutants are usually measured at the same sampling points; it is necessary to develop a methodology to determine the optimal location of the single station. In this paper, such a methodology is proposed based on maximizing an objective function, that balances between different pollutants measured in the network. The methodology is applied to a set of data available for the city of Cartagena, in southeast Spain. A sensitivity analysis reveals that 2 small areas of the studied city account for 80% of the optimal potential locations, which makes them ideal candidates for setting up the monitoring station. The methodology is easy to implement, robust and supports the decision-making process regarding the siting of fixed sampling sites.

Study of the effect of sample pressure on in situ BTEX chromatographs.

In this paper, the influence of sample pressure on benzene measurements obtained with two automated in situ gas chromatographs is studied. The analysers were calibrated using a non-linear regression at 293 ± 1 K and 101.3 ± 0.2 kPa. A gas mixture of benzene in air (5 µg/m3) was produced and measured at calibration conditions. Subsequently, the sample pressure was changed to 80 and 110 kPa. Differences in readings were observed even though the pressure compensators were on, indicating incorrect performance of this tool. Further tests with two different benzene in air mixtures (5 and 40 µg/m3) at 80, 90, 105 and 110 kPa were also carried out. Results showed that the analysers take air from one or several unidentified inlets (apart from the sampling port) when the sampling pressure is lower than the atmospheric one. This is usually the case in air monitoring stations, so this phenomenon is particularly important as systematic biases could be affecting air quality data.

Preparation and validation of fully synthetic standard gas mixtures with atmospheric isotopic composition for global CO2 and CH4 monitoring.

We report the preparation and validation of the first fully synthetic gaseous reference standards of CO2 and CH4 in a whole air matrix with an isotopic distribution matching that is in the ambient atmosphere. The mixtures are accurately representative of the ambient atmosphere and were prepared gravimetrically. The isotopic distribution of the CO2 was matched to the abundance in the ambient atmosphere by blending (12)C-enriched CO2 with (13)C-enriched CO2 in order to avoid measurement biases introduced by measurement instrumentation detecting only certain isotopologues. The reference standards developed here have been compared with standards developed by the National Institute of Standards and Technology and standards from the WMO scale. They demonstrate excellent comparability.

Uncertainty models and influence of the calibration span on ambient air measurements of NO2 by chemiluminescence.

An assessment of uncertainty in the hourly and annual limit values of NO 2 measurements by molybdenum NO2-to-NO conversion followed by chemiluminescence detection was carried out at 3 different span concentrations (100, 300 and 700 nmol mol (-1)). The uncertainty of the linearity term was calculated for each span concentration by considering (i) a zero-and-span calibration and (ii) a multipoint calibration. Two uncertainty models were applied for the overall uncertainty estimation: (i) the Standard EN 14211 and (ii) a mechanistic model that considers the NO 2 reduction in the converter. The main difference between these models stems from considering or not the possible covariances derived from interactions between NO x and NO concentrations and the converter's efficiency. For both models, the span determined whether or not it was possible to meet the quality objective requested by the EU Air Quality Directives in the annual limit value when no linearity corrections were performed in environments with NO z/NO2 ratios ≤ 0.04. In environments with significant amounts of NO z species (NOz/NO2≥ 0.12), the expanded uncertainty can easily be higher than the data quality objective if bias' corrections are not or cannot be applied.

The use of experimental data and their uncertainty for assessing ozone photochemistry in the Eastern Iberian Peninsula.

Observation-based methods are useful tools to explore the sensitivity of ozone concentrations to precursor controls. With the aim of assessing the ozone precursor sensitivity in two locations: Paterna (suburban) and Villar del Arzobispo (rural) of the Turia river basin in the east of Spain, the photochemical indicator O(3)/NO(y) and the Extent-of-Reaction (EOR) parameter have been calculated from field measurements. In Paterna, the O(3)/NO(y) ratio varied from 0 to 13 with an average value of 5.1 (SD 3.2), whereas the averaged value for the EOR was 0.43 (SD 0.14). In Villar del Arzobispo, the O(3)/NO(y) ratio changed from 5 to 30 with a mean value of 13.6 (SD 4.7) and the EOR gave an averaged value of 0.72 (SD 0.11). The results show two different patterns of ozone production as a function of the location. The suburban area shows a VOC-sensitive regime whereas the rural one shows a transition regime close to NO(x)-sensitive conditions. No seasonal differences in these regimes are observed along the monitoring campaigns. Finally, an analysis of the influence of the measurement quality of NO(y), NO(x) and O(3) on the uncertainty of the O(3)/NO(y) ratio and the EOR was performed showing that the uncertainty of O(3)/NO(y) is not dependent on either its value or the individual values of O(3) and NO(y) but just on the quality of O(3) and NO(y) measurements. The maximum uncertainty is 26% as long as the combined uncertainties of O(3) and NO(y) remain below the 7.5%. The case of the EOR is different and its uncertainty depends on both the value of the EOR parameter and the individual concentration values of NO(y) and NO(x). The uncertainty of the EOR estimation can be very high (>200%) if the combined uncertainties of both NO(y) and NO(x) are high (>7.5%), or especially, if u(NO(y)) and u(NO(x)) differ considerably from each other (>3.5%).

Vertical concentration gradients of volatile organic compounds in two NS-oriented street canyons.

Diffusive samplers were used to measure the vertical concentrations of benzene, toluene, n-hexane, cyclohexane, ethylbenzene and o-, m- and p-xylenes on both sides of two NS-oriented street canyons in Murcia (Spain) during a 5-day period. Non-dimensional relationships of concentration and height were calculated in order to study the behaviour of their concentration vertical profiles. The results show that the vertical profiles of benzene, toluene, n-hexane and cyclohexane concentrations were similar in both streets and on both sides of each street. Some differences were found in vertical profiles between streets and sides for ethylbenzene and xylenes, probably due to their higher affinity for adsorption into building materials. The similarities found for the first set of VOCs suggest that the dynamics of the dispersion was the same for both streets and was mainly influenced by microscale thermal effects. Finally, the concentration measurements of benzene, toluene, n-hexane, cyclohexane, and ethylbenzene were adjusted to expressions in the form c = c 0(h/h 0) A , and a regression coefficient R 2 = 0.962 (p = 0.0000) was obtained. The decreasing concentration of these compounds with height should be taken into account when assessing population exposure to these pollutants.

Influence of temperature changes on ambient air NOx chemiluminescence measurements.

Users of automatic air pollution monitors are largely unaware of how certain parameters, like temperature, can affect readings. The present work examines the influence of temperature changes on chemiluminescence NO(x) measurements made with a Thermo Scientific 42i analyzer, a model widely used in air monitoring networks and air pollution studies. These changes are grouped into two categories according to European Standard EN 14211: (1) changes in the air surrounding the analyzers and (2) changes in the sampled air. First, the sensitivity tests described in Standard EN 14211 were performed to determine whether the analyzer performance was adapted to the requirements of the standard. The analyzer met the performance criteria of both tests; however, some differences were detected in readings with temperature changes even though the temperature compensator was on. Sample temperature changes were studied more deeply as they were the most critical (they cannot be controlled and differences of several tens of degrees can be present in a single day). Significant differences in readings were obtained when changing sample temperature; however, maximum deviations were around 3% for temperature ranges of 15°C. If other possible uncertainty contributions are controlled and temperature variations with respect to the calibration temperature are not higher than 15°C, the effect of temperature changes could be acceptable and no data correction should have to be applied.

Performance evaluation of two commercial chemiluminescence NOx analysers according to European Standard EN 14211.

According to Directive 2008/50/EC, all equipment used for NO(x) fixed measurements must comply with the reference method or equivalent by 11 June 2013. Up to that date, non-type-approved equipment can continue to be used in air monitoring networks and will probably also be used in air pollution studies even after that date. In this context, it is advisable to know how these "old" measuring instruments respond to the requirements of European Standard EN 14211: 2005: Ambient Air Quality--Standard method for the measurement of the concentration of nitrogen dioxide and nitrogen monoxide by chemiluminescence. In this work, we carry out a thorough performance evaluation of two commercial NO(x) analysers, both purchased before 2010, according to the mentioned Standard. Results show that water vapour interference is one of the most important disconformities with this Standard. The expanded uncertainty estimation of both analysers was below the quality objective of the above-mentioned Directive (15%); however, this calculation was performed by applying some modifications to the proposed model of Standard EN 14211. These modifications are also discussed in the paper.

Study of the uncertainty in NO2 chemiluminescence measurements due to the NO-O3 reaction in sampling lines.

INTRODUCTION: The change in light intensity that takes place when an ambient air sample is drawn into the detection chamber of a chemiluminescence monitor generates changes in the concentrations of several species, such as NO(2), NO and O(3). Although this phenomenon has been known for several decades, there is still no commonly accepted approach on when or how to correct for it in NO(2) and O(3) readings. DISCUSSION: In this work, we have assessed the expanded uncertainty of two chemiluminescence NO( x ) analysers commercially available according to EN 14211:2005, with the aim of establishing the maximum allowable standard uncertainty due to the reaction between NO and O(3) in the sampling system. CONCLUSION: Although this maximum allowable uncertainty cannot be a universal value-as it will depend on the performance of each analyser-our results have led us to propose the conservative value of 2%. We have also proposed a methodology for improving data quality which could be easily implemented by those responsible for air quality data validation.

Assessing the impact of petrol stations on their immediate surroundings.

This paper describes a novel methodology for evaluating the extent to which petrol stations affect their surroundings. The method is based on the fact that the ratio of the concentrations of aliphatic and aromatic hydrocarbon pollutants in the air of the petrol stations and their surroundings (basically determined by vapor emissions from unburned gasoline) differs from the ratio found in urban air, which is mainly influenced by traffic emissions. Bearing this in mind, the spatial limit of influence of petrol stations in any direction would be the first point, moving away from the station, where the ratio becomes equal to the urban background ratio. Application of the methodology involves multipoint measuring campaigns of the air at the studied petrol station and built-up area in general and processing the data with software capable of providing isoconcentration contours. The procedure should help local authorities in terms of land management, so that a "belt" can be established around petrol stations where housing or vulnerable populations and activities such as those in schools, hospitals and community centers should be restricted.

Consequence analysis to determine damage to buildings from vapour cloud explosions using characteristic curves.

The objective of this paper is to propose a methodology to estimate the consequences to buildings from the pressure wave caused by unconfined vapour cloud explosions (VCEs). This methodology is based on the use of characteristic overpressure-impulse-distance curves, shown in a previous paper [F. Diaz Alonso, E. Gonzalez Ferradas, J.F. Sanchez Perez, A. Miñana Aznar, J. Ruiz Gimeno, J. Martinez Alonso, Characteristic overpressure-impulse-distance curves for vapour cloud, explosions using the TNO Multi-Energy model, J. Hazard. Mater. A137 (2006) 734-741]. They allow the overpressure and impulse at each distance from the explosion to be determined. Since they can be combined with damage criteria (such as those shown by the PROBIT equations), they can be used to perform consequence analysis as the damage is shown in the same diagram as the overpressure, impulse and distance. Since damages suffered by buildings usually affect people inside, it is important to take them into account when performing consequence analysis. This is done in this paper, where diagrams and equations are presented to determine minor damage to buildings (broken windows, displacement of doors and window frames, tile displacement, etc.), major structural damage (cracks in walls, collapse of some walls) and collapse (the damage is so extensive that the building is partially or totally demolished). This paper completes the consequence analysis to humans outdoors shown by F.D. Alonso et al. [F. Diaz Alonso, E. Gonzalez Ferradas, T. Jimenez Sanchez, A. Miñana Aznar, J. Ruiz Gimeno, J. Martinez Alonso, Consequence analysis to determine the damage to humans from vapour cloud explosions using characteristic curves, J. Hazard. Mater., in press].