Air Monitoring 23


- compliance assessment procedures - reporting requirements - quality assurance and control requirements


- arrangements for the assessment and reporting of exceptional emissions.


As mentioned at the beginning of this article, it is necessary to make a distinction between the uncertainty of a method or an instrument, which has to be assessed in order to decide if it is good enough to be used to monitor emissions, and the uncertainty associated to a measured value, that has to be set in order to allow for proper reporting and ensure compliance with emission limit values. Evaluating the uncertainty of a specifi c method/instrument is not suffi cient in itself to provide a value that can be associated with a measurement in order to prove compliance. The measured value can be compared with the limit, taking account of the associated uncertainty in measurements. When monitoring is applied for compliance assessment it is particularly important to be aware of measurement uncertainties during the whole monitoring process. This is a complex exercise, even more so if one considers the complex relationship between an emission limit value and its monitoring requirements: especially given that in the quality assurance procedures, many target values are defi ned with respect to the ELV (for example as a given percentage of the ELV).


The uncertainty of a measurement is a parameter, associated with the measurement result, that characterises the dispersion of the values that could reasonably be attributed to the measurand (i.e. the extent to which measured values can actually differ from the real value). In general, the uncertainty is expressed as a plus or minus interval around the measurement result with a 95% statistical confi dence.


When the permit explicitly specifi es an applicable standard method for the regulated parameter, as in the case of a periodic measurement, the means to determine the measurement uncertainty should be fully described in the standard method. When the permit leaves open the choice of a standard method for the regulated parameter, the external dispersion corresponds to the uncertainty of a measurement result. This includes the systematic differences (i.e. bias) that may exist between the results obtained with different applicable standard measurement methods for the same regulated parameter. However, for continuously monitored parameters, as mentioned above, it is necessary for compliance assessment to evaluate the uncertainty of a measured value, which for a continuously measured parameter will include uncertainty of the Standard Reference Method (SRM), uncertainty of the Automated Measuring System (AMS) and uncertainty of the quality assurance procedures such as the ones laid down in EN14181. It comes as an obvious conclusion that using for example parameters from the QAL2 procedure such as its standard deviation will not provide a technically sound approach to evaluate the overall measurement uncertainty for purposes of compliance with an ELV.


Identifi cation of the uncertainty sources can be useful to calculate the total uncertainty. This is a diffi cult but important exercise, especially if we consider the direction that the current review of the Industrial Emissions Directive is taking. It is also especially important in those cases when the measurement results are close to the ELV. The Commission published its proposal for IED review in April 2022. It includes several changes including for Article 15.3 that should state:


“The competent authority shall set the strictest possible emission limit values that are consistent with the lowest emissions achievable by applying BAT in the installation, and that ensure that, under normal operating conditions, emissions do not exceed the emission levels associated with the best available techniques (BAT-AELs) as laid down in the decisions on BAT conclusions referred to in Article 13(5). The emission limit values shall be based on an assessment by the operator analysing the feasibility of meeting the strictest end of the BAT-AEL range and demonstrating the best performance the installation can achieve by applying BAT as described in BAT conclusions.”


We can read in this new wording of Article 15.3 that the Commission is explicitly supporting competent authorities to set ELVs at values that are below the upper end of BAT-AELs. Should


this proposal go through the co-decision process as it is it would require a much higher ambition than what is the status quo. That said, this proposal is also complemented by the addition of Article 15a on compliance assessment, stating:


“1. For the purpose of assessing compliance with emission limit values in accordance with Article 14(1), point (h), the correction made to measurements to determine the validated average emission values shall not exceed the measurement uncertainty of the measuring method. [Note of the authors: strictly speaking this should be the measurement uncertainty of the measured results]


2. The Commission shall by [the fi rst day of the month following 24 months after the date of entry into force of this Directive] adopt an implementing act establishing the measuring method for assessing compliance with emission limit values set out in the permit with regard to emissions to air and water. This implementing act shall be adopted in accordance with the examination procedure referred to in Article 75(2). The method referred to in the fi rst subparagraph shall address, as a minimum, the determination of validated average emission values and shall set out how measurement uncertainty and the frequency of exceedance of emission limit values are to be taken into account in the compliance assessment.”


If the future scenario for IED installations is to strive to achieve the most ambitious ELV based on BAT-AEL ranges while at the same time having to respect compliance rules set directly in the IED, it is necessary that the approach towards “how measurement uncertainty” is “to be taken into account in the compliance assessment” should be technically sound and guarantee the legal certainty for both operators and competent authorities.


Conclusions


The challenges of monitoring, including how to determine measurement uncertainty, are no secret to the metrology community. However, the world outside this bubble discovered, only a few years ago, that measurement uncertainty has a signifi cant impact on compliance with the legislation and that it responds to technical constraints and cannot be bent to human willpower. Even now, the discussion among policy makers seems to revolve around “we have to be more ambitious on uncertainty than just the values mentioned in the IED”. As any expert in monitoring will know very well, monitoring aspects such as uncertainty are determined by the actual capabilities of instruments and methods and it is not possible to arbitrarily set a requirement on uncertainty without considering what can practically be done (taking into account technological progress).


Given the review of the IED and the new proposals on the table for compliance assessment, all IED installations will eventually face the challenge of coping with the linkages between ELVs and their monitoring requirements. Developing a common and sensible approach will provide a sound basis for legal certainty when it comes to compliance with ELVs, especially if in the future ELVs will be set closer and closer to operating values.


One step that would be of great help industry and regulatory authorities would be to develop common approaches to determine the uncertainties in measured values, approaches which are pragmatic and practical to apply, while taking into account the complex issues we have introduced, albeit very superfi cially, in this article.


The main sources of uncertainties are those associated with the measurement steps of the monitoring data production chain, such as: SRM


sampling plan


taking of the sample sample pre-treatment, transport/storage/preservation of the sample, sample treatment, analysis/quantifi cation.


AMS


System uncertainties from QAL1 Repeatability Linearity


Sampling (parts not controlled by QAL3) Sample fl ow


External sources uncertainties in fl ow measurements when loads are calculated


uncertainties in data handling, e.g. the uncertainties related to missing values when calculating a daily or other average


uncertainties due to the dispersion of results associated with systematic differences (“bias”) that may exist between results obtained with different applicable standard measurement methods for the same regulated parameter


uncertainties due to the use of secondary method or of surrogates uncertainties due to inherent variability (e.g. of a process or weather conditions).


Quality assurance process


(EN 14181)


Calibration to a ‘worse’ performing SRM (15 repeats locked into a calibration function for 3 yrs) Drift (QAL3) AST not detecting change in calibration Change in emission matrix (interferent compounds) Different measured components (HCl vs chlorides) Inhomogeneity in sample plane (larger stack – power sector)


Additional uncertainty sources if reporting mass emission


Additional uncertainty in emission rates (load) Flow


Representativeness of fl ow/concentration Conversion to common basis Missing data


Author Contact Details Lighea Speziale - Environmental advisor - Seeds Consulting • Via degli Artisti, 8b, 50132 Florence, Italy • Email: lighea.speziale@seedsconsulting.eu • Web: www.seedsconsulting.eu


Rod Robinson - Principal Research Scientist at National Physical Laboratory Lighea Speziale Rod Robinson WWW.ENVIROTECH-ONLINE.COM


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