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.

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.