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The section considers how you can make sure your ambient air quality monitoring meets appropriate quality assurance requirements.

1. Measurement uncertainties

All measurement results have an associated degree of uncertainty which defines the limits within which the true value lies to a specified level of confidence. This is evaluated by combining several uncertainty components and expressed as a plus or minus quantity (that is an interval about the result).

Measurements are subject to random and systematic uncertainties.

Random uncertainties are those which can be related to a statistical probability function, such as the Gaussian distribution. They give rise to a variation about the mean in which there is a greater probability that the value lies closer to the mean than to the extremes. The more measurements made the greater the confidence that the mean is close to the true value. Random uncertainties may be due to short-term fluctuations in temperature or air pressure or are introduced because of small variations in the in the procedure used. You can control them by making sure you follow all procedures correctly.

Systematic uncertainties are biases, which may be present in the measurement equipment or inherent in the procedure. They have the effect of displacing the results from the true value. The bias does not change when you repeat a measurement under the same conditions no matter how many measurements you make. The mean result will not represent the true value of the parameter being measured although it may have a fixed relationship with it. Examples of systematic uncertainties are:

• an offset in the measuring system
• drift in its response between calibrations
• bias in the interpretation of an analogue scale
• the uncertainty of the value of the reference standard

You can reduce them by making sure that:

• all the items of equipment used meet their specification
• primary standards are properly certified
• secondary standards, such as flow measurement devices or transfer gas standards, are properly calibrated

You should use maintenance and calibration schedules to keep systematic uncertainties under control.

Beyond the uncertainties associated with the sensor itself, mobile measurements introduce uncertainties related to the three-dimensional nature of the sampling techniques. Random uncertainty of a plume quantification caused by attempting to measure a moving target can only be determined by measuring it many times and as quickly as practicable. It is important to minimise systematic uncertainty by making sure that you sample the whole plume.

Wind speed and direction, where factored into measurements, can increase random uncertainty if you only use an average of measurements. Wind measurements may also add an unquantifiable systematic uncertainty if poorly located compared to the location of measurements. Ground-based wind speed measurements should take account of increases in wind speed with height if you use them in drone-based mass balance quantification methods.

Quantifiable uncertainty should be combined in an uncertainty budget for an example of this, see The development and trial of an unmanned aerial system for the measurement of methane flux from landfill and greenhouse gas emission hotspots Research Explorer The University of Manchester.

2. Sampling systems

Where collection or measurement systems involve the use of a sample line, there is the potential for sample loss because of adsorption, diffusion, deposition or reaction. You must consider the composition and length of the sample line to make sure that any losses are at an acceptable limit. For reactive species, the sample line material must be chemically inert, such as Polytetrafluoroethylene (PTFE) or stainless steel. You should keep the lengths of line as short as is practically possible. You must test instantaneous grab sampling systems to make sure that, under the conditions of storage, they maintain the integrity of the sample.

3. Sample volumes and flow rates

Apart from diffusive (passive) monitors, most techniques rely on active sampling of the pollutant into the collection or measurement system using a pump. Of these, many give a response that is directly proportional to the sample volume. Here the accurate quantification of sample volume is a crucial step towards determining the overall limits of uncertainty of the measurement. You should check flowmeters against laboratory reference standards, traceable to national standards. You should verify and adjust, if necessary, flow rates and sample volumes of sampling systems.

Other techniques, such as continuous real-time monitors, may control the sample flow using physical methods, for example critical orifices or mass-flow controllers. This maintains a constant flow to the detector. You normally do not need to measure sample flow rate in these systems because the response of the instrument would routinely be determined using certified calibration standards. Some continuous methods, such as those using infrared or ultraviolet absorption detectors, are independent of flow. It is only necessary to check that the sample flow rate remains within defined limits.

Where you use active sampling by adsorption or absorption, the collection efficiency will depend upon the flow rate. An excessive flow rate may result in lower efficiencies because of sample breakthrough. The causes can be:

• adsorption and desorption effects
• insufficient residence time between the sample and collection medium preventing physical or chemical combination
• overloading of collecting medium

You should carry out validation checks to make sure that the sample flow rate used does not lead to significant losses.

4. Sample storage, transit, and audit trail

Where you take samples for subsequent analysis, you must have procedures to make sure that the sample is identifiable throughout the sampling, sample preparation and analysis. Your procedures should also make sure that you maintain sample integrity. You should give each sample a reference number that identifies it throughout the system. You should mark sample containers so that it is difficult to remove the reference marks accidentally. You should specifically design a log to note, along with any other relevant information, the:

• reference number
• sampling location
• date and time of sample
• purpose of the sample
• sampling method
• method of sample treatment

If another organisation is to analyse the sample, note the date you pass the sample to the second organisation in the log. Preservation of sample integrity depends on the type of sample. You should:

• store particulate samples collected onto filter media individually in sealed containers
• make sure that you minimise losses from the filter surface
• store most other samples under cool conditions in the dark
• seal bubblers and impingers or wash out into suitable bottles marked with the appropriate reference number and store in cool boxes for transport to the laboratory for analysis
• store all samples under conditions where there is minimal risk of contamination

5. Calibration and validation

The calibration of an instrument or method determines the relationship between the result of the measurement and the actual concentration of the pollutant. It is an essential part of the quality control procedure.

Continuous gas analysers

You should carry out the calibration of continuous gas analysers at the following 3 levels:

• a functional zero and span check
• routine 2-point (zero and span) calibration using certified standards
• full validation

You may use sources that are not traceable to national standards when doing functional zero and span checks. This provides a useful check on the basic performance of the instrument. Do not use the instrument responses for data scaling or calibration.

Do the routine 2-point (zero and span) calibration of a continuous gas analyser using a suitable zero source. Alternatively use a source with ambient atmospheric levels, if more relevant, for example for methane or nitrous oxide. Then use a calibration gas having a concentration within the measurement range expected in the surveyed environment. The zero source may be a certified zero grade air or generated in-situ from ambient air using suitable scrubbers. The calibration gas should be of a certified grade to ensure traceability to national standards. You should do this calibration at least every 2 weeks. You can use the results in the data ratification process and for data scaling.

You must carry out full-scale validation to demonstrate the instruments response over its full working range. You should do this:

• when the instrument is first installed
• when it undergoes major maintenance
• when you move it
• every 6 months

The purpose of validation is to relate the response of the instrument to a range of standards that are traceable to national standards. You should also include validation of the standards used for the routine calibrations.

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