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Guidance: Monitoring ambient air: choosing a monitoring technique and method

Environment Agency

September 10
08:36 2024

1. Decide the sampling approach

1.1. Omnidirectional sampling or directional sampling

Often, ambient air quality surveys are designed to assess the impact of a particular regulated process, works or site. You can determine the source contribution by omnidirectional sampling or directional sampling.

You can carry out omnidirectional sampling from all directions or under all wind directions. When you assess the data, consider the frequency and occurrence of winds from the direction of the suspected source.

Directional sampling is when you link an active sampling system to a wind vane and anemometer. The system allows sampling to occur only when the wind is blowing from a specified direction and is above a minimum wind speed.

The most common approach is omnidirectional sampling.

Directional samplers are most useful when there is a clearly defined suspected source of the pollutant and the background air concentration is low in comparison. In this case you would locate a single directional sampler downwind of the source for an appropriate duration. This should give a good estimate of that sources contribution.

It can be an advantage to use a directional monitoring unit that has a two-component wind-vane-operated sampler. One component collects the sample when the wind arrives from the direction of the source. The other collects a sample arriving on winds from the remaining directions.

Most active directional apparatus collects the sample from a pre-selected wind direction arc of say 30 to 70 degrees that has the axis of interest as its centre line. You should choose the distance from the source to allows the use of an acceptance arc in this range. At the same time, you must take account of the type of source. For example, the plume from an elevated source, such as a chimney stack, will usually reach ground level after a downwind distance equivalent to between 10 and 20 times the stack height. But you could get a more accurate estimate from atmospheric dispersion modelling studies.

Emissions from other interfering pollution sources within the acceptance arc will lead to an overestimation of the contribution of the targeted source of interest. If there are other sources that lie outside the acceptance arc, then consider their contribution as part of the background concentration. You can minimise these situations by carefully choosing the most suitable sampling distance from the suspected source, in conjunction with an appropriate acceptance arc. If this is not sufficient, you can position several directional samplers in different directions around the works. This will obtain an integrated value of the contribution of the targeted source relative to that of other sources and the general background. Alternatively, you can set up directionally resolving equipment to sample from 3 or more wind sectors.

1.2. Fixed-point sampling

The most common sampling system is that of a network of monitoring points at fixed locations. Each point provides concentration measurements. They could be instantaneous spot-concentration values or time-averaged concentrations, from a fixed point in space. The success of a fixed network of on site samplers will be dependent on the care with which you choose the locations. Section 8 of monitoring ambient air: monitoring strategy covers the principles applying to the location of fixed monitoring points in detail.

1.3. Open-path methods

An alternative, less-common approach is open-path measurement made directly in the atmosphere. No extraction or measurement of a sample of air takes place within the instrument at a localised point. Instead, you determine the average concentration of a specifically targeted pollutant over a long measurement path. Some methods allow the concentration to be spatially resolved. Most, however, give the average concentration over the whole path length. This is useful for assessing the transfer of pollutants across site boundaries and along roads and runways. But it may be difficult to interpret integrated-path data.

Another open-path technique, laser interferometry detection and ranging (LIDAR), can measure aerosol particles. Differential absorption LIDAR (DIAL) can carry out range-resolved measurements of specific pollutants over several kilometres by analysing backscattered laser radiation.

LIDAR and DIAL are particularly suitable for producing two-dimensional or three-dimensional maps of pollutant concentrations over large areas such as industrial complexes.

Measurement by open path techniques tends to be expensive because of the complexity of the equipment and data handling facilities.

You can use open path methods for mobile sampling. This may be vehicle-mounted instruments for carrying out measurements from many locations, or for measuring the pollution concentration profile along a given route. Systematic traversing of a plume emitted from an elevated point source is an application well suited to mobile monitoring systems. But because meandering of the plume can distort the pattern, each traverse gives only an approximation to the instantaneous cross wind spread of the plume. You need several hours of sampling to define the plume envelope in a way that could relate to patterns observed from fixed networks.

1.4. Mobile point-concentration measurements

You can measure point-concentrations continuously from a mobile platform, usually a vehicle or a drone. You can deploy these systems to precisely quantify emissions of one or several pollutants.

Concentration measurements taken from a drone have the advantage of sampling plumes above the ground. When sources are elevated above ground it can be difficult to investigate or even detect pollution if you measure at ground level. Particularly if you have limited access downwind of sources.

In drone-based sampling, you may collect concentration measurements in a series of horizontal transects at various altitudes downwind of sources. Combined this data with meteorological (wind) data and measurements of background concentrations, to produce an estimate of pollutant emissions rates using the principle of mass-balancing. Take care to sample beyond the sides and the top of any expected plume and as close to the ground as is practicable. If obstructions or flight restrictions prevent this then other methods may produce better results. It may also be desirable to conduct measurements upwind of a source. This will characterise the local atmospheric background and account for potential contributions from sources upwind.

Each individual flux estimate is subject to high uncertainty because of the variability of plume morphology and advection. Averaging over multiple measurements may reduce and better quantify uncertainty. You should do a survey of gas concentration samples on a predefined flight path. You should do the survey as quickly as practicable to limit measurement uncertainty and bias. Also, you should characterise the relationship between wind speed and altitude in the planetary boundary layer. This will avoid overestimating emissions from measurements taken near to ground level. For more information, see Methods for quantifying methane emissions using unmanned aerial vehicles: a review.

Another mobile method of quantification using instrumentation installed on ground-based vehicles is to release a known rate of tracer gas such as acetylene. The tracer release should be co-located with the source of interest. You take continuous concentration measurements of the tracer and target gas (pollutant) in a series of transects downwind of the source. You can then use the ratio of these concentrations to derive an emission rate of the pollutant. It may be necessary to have several tracer release locations on large sites (such as a landfill). A limitation to this method is that it may not be possible to sample a plume because of a lack of access or the plume being high above ground level. For more information on this method, see Guidelines for landfill gas emission monitoring using the tracer gas dispersion method ScienceDirect.

2. Choice of measuring method

For many pollutants, you must use a reference method to comply with standards. But, where no reference method is applicable, there are general principles to consider.

2.1. Method performance

The monitoring certification scheme (MCERTS) is the Environment Agencys monitoring certification scheme for environmental permit holders. We use MCERTS to approve people, instruments, and laboratories. See

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