Important threshold values need an exact description of the way and method to obtain the corresponding data. With respect to the intense discussion in Germany with prohibitions of driving with diesel automobiles, this question will be important.
In environmental analysis crucial threshold limits need an especially careful analytical planning, validation, and verification to prove the quality of obtained data. Examples are clinical analysis which would result in a critical surgery or in metallurgy to prove that certain constituent is accurately determined. In the latter case any disagreement between two partners (e.g. seller and buyer) leads automatically to a so-called “Schiedsanalyse” where the disagreeing partners perform the chemical analysis according to a special “Schiedsanalysen Handbuch” (with agreed upon SOPs “Standard Operation Procedures”).
With respect to the European limit of 40 µg/m³ air I would be interested in the SOP to guarantee surveillance with a high data quality to justify the dramatic consequences. Especially: Standardization of the measuring locations and validated analytical method (considering main wind direction, wind strengths, high of air intake position, humidity to consider scrubbing effects, dust to consider surface adsorption effects, sampling times and intervals, instrument calibration results or comparison of the obtained results with another method based on a different principle resulting a convincing guess of the reliability).
I think a simple transference of toxicological data obtained with carefully calibrated NOx – air mixtures and convincingly applied safety factors to a threshold limit for general traffic must be accompanied by SOPs defining the measurement details. Especially important will be the location (of air intake); how is the most important factor of comparable ventilation standardized?
The European air quality limits are defined in the directive 2008/50/EC. Macro- and microscale sitings of sampling points are defined in Annexure III of the directive. Reference methods of measurement are defined in Annexure VI. Some parts of directive 2008/80/EC have been amended in directive 2015/1480, expecially Quality Assurance and Documentation, but also the reference method for NO2 evaluation: "The reference method for the measurement of nitrogen dioxide and oxides of nitrogen is that described in EN 14211:2012 “Ambient air — Standard method for the measurement of the concentration of nitrogen dioxide and nitrogen monoxide by chemiluminescence”.", referring to the revised EN standard.
SOP for NOx with dry air calibration standards is acceptable; but how to standardize the location of the measurements (distance, orientation, and height from the street and identical ventilation conditions)? This would be needed for comparability and law enforcement (e.g., the positioning of measuring points in other EU member states).
An incomplete quote of the requirements:
— Sampling points shall in general be sited in such a way as to avoid measuring very small micro-environments in their immediate vicinity, which means that a sampling point must be sited in such a way that the air sampled is representative of air quality for a street segment no less than 100 m length at traffic-orientated sites, where feasible;
— the flow around the inlet sampling probe shall be unrestricted (in general free in an arc of at least 270° or 180° for sampling points at the building line) without any obstructions affecting the airflow in the vicinity of the inlet (normally some metres away from buildings, balconies, trees and other obstacles and at least 0,5 m from the nearest building in the case of sampling points representing air quality at the building line),
— in general, the inlet sampling point shall be between 1,5 m (the breathing zone) and 4 m above the ground. Higher siting may also be appropriate if the station is representative of a large area and any derogations should be fully documented;’,
— the inlet probe shall not be positioned in the immediate vicinity of sources in order to avoid the direct intake of emissions unmixed with ambient air,
— traffic-orientated sampling probes shall be at least 25 m from the edge of major junctions and no more than 10 m from the kerbside.,
I'd say, the requirements are quite detailed. Some of the conditions are difficult to meet in an urban environment. And in critical cases, measurements might be supported by dispersion simulation in order to assess the validity of the site selection for an assessment of a larger area.
Many thanks for all your information. I was a bit lazy to search for those information not familiar for one not working in the environmental area.
I still have problems in accepting a truly representative air sampling of air samples with an analyte not uniformly mixed with air and varying in time. Despite all requirements, how to prevent a personal bias?
Example: Automobile manufacturers would install measuring points preferable at sites known to be locally windier and in our geographical location westerly of a street.
In TV I have seen many measuring location immediately on the kerbside of a street and not 10 m away.
Are the measured data accompanied by the local wind speed and direction and how reliable is any correction algorithm leading to comparable results (standardized ventilation condition) with other measuring points?
Is there a demand to compare the quasi continuous measured NOx data of really representative sampling sites with, e.g., integrating passive sampling using a different analytical method before the decision is taken that the threshold value has been exceeded?
Since we have in the meantime many critical cases in Germany leading to drastically actions, the validity of the data should be (not might) assessed by accepted methods without any unproven assumptions (can dispersion simulations perform this task?).
The statistical variation of the data is not that important, as the most important air quality limit is defined as an annual mean. Weather conditions vary from year to year, which is visible from the data provided from continuous measurements e.g. on www.umweltbundesamt.de . The variation of meteorological conditions makes the prognosis and repeatable measurement of short term air quality data really difficult.
And although we have Dieselgate and cheating car manufacturers, the development still goes in the right direction, as a view on some of the data provided by the Umweltbundesamt demonstrates:
https://www.youtube.com/watch?v=g2n2-beabVg
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