Part 3: Installing NO2 and PM monitors around a construction site – when, where and how many?
09 December 2021
14 September 2017
In this blog series, Part 1 explained how construction activities can generate harmful emissions of nitrogen dioxide (NO2) and particulate matter (PM), and what monitoring regulations apply.
Part 2 discussed principal NO2 and PM monitoring techniques, and guidance for selecting appropriate methods for construction and demolition sites.
In this final post, we look at siting considerations for installing air quality monitors around a construction site such as when, where and how many.
When does monitoring start?
Most local area authorities require all demolition and construction sites to monitor air pollution. On some sites, determining the existing or baseline pollution levels for at least three months prior to starting construction is likely to be a condition of the planning application. This allows existing levels to be defined more accurately, and can assist with setting or interpretation of “trigger thresholds”.
Construction Environmental Management Plans (CEMP) outline how a construction project will avoid, minimise or mitigate effects on the environment and surrounding area. The CEMP will usually include a Dust Management Plan with site-specific methods to be used to monitor and control PM dust emissions. If NO2 monitoring is required the monitoring period and technique(s) will also be outlined in the CEMP. Since development projects have the potential to affect air quality, an air quality assessment may be required.
Air quality assessments are typically undertaken by independent consultants and consider both the construction and operational phases of the project. They may be required as part of a formal Environmental Impact Assessment (EIA) or as a stand-alone report submitted in support of an application for planning permission. A typical EIA will include baseline air quality monitoring for a period of months.
How many monitors are required?
Where monitoring of PM concentrations is to be carried out, best practice guidance recommends a minimum of two monitoring sites, one upwind and one downwind of the construction site, in relation to the prevailing wind. This allows source apportionment to be carried out (e.g. when trigger thresholds are exceeded) particularly if wind speed and direction data are available, and also allows for coverage during variable weather conditions.
If the planning consent requires simultaneous monitoring of PM10 dust (coarse particles) and PM2.5 emissions (fine particles), which is becoming more common, then additional monitors may be needed, especially in the case of single-fraction instruments. Optical PM analysers that measure multiple fractions (TSP, PM10, and PM2.5) simultaneously can offer a significant cost advantage.
For consents that require monitoring baseline NO2 concentrations and/or NO2 emissions during construction an appropriate analyser should be installed or a number of diffusion tube samplers may be distributed around the site. For this monitoring requirement, analysers that combine PM10, PM2.5 and NO2 measurement simultaneously can be both cost-effective and a future-proof investment.
Where should the monitors be located?
In selecting monitoring locations, key considerations include:
the number of sites,
proximity to sensitive receptors,
permanent position or relocation during construction phases,
availability of electrical power,
access to the monitoring sites, and
Monitoring is particularly important close to Sensitive Receptor Locations where the air quality objectives/limit values may apply. These are locations in close proximity which may be sensitive to construction air pollution impacts. Sensitive receptors include hospitals, schools, childcare facilities, elderly housing and residential properties.
Under such circumstances, the monitors should be located close to the sensitive receptors rather than directly at the site boundary. Other non-site sources of NO2 and/or PM such as roads, rail, landfills or other polluting activities in the area should be taken into account as well. It is important that responsibility for any exceedance of trigger values (or air quality objectives/limit values) is allocated appropriately.
For siting in the microenvironment, the monitor inlets should be located in a clear, unobstructed position, and some metres away from any large structures (such as walls of buildings) that might interrupt airflow. They should be open to the sky with no overhanging trees or other structures. Ideally the monitor should be located between 1.5 to 4m above ground level.
When to relocate the monitors?
Monitoring during some stages of the construction works may be quite some distance from the polluting activities and may not provide adequate information on maximum levels of NO2 and/or PM dust at the site boundary. Secondly, as construction phases proceed, new sensitive receptor locations may be introduced within close proximity to the revised site boundary. These situations would require relocating of the monitoring sites.
Construction and demolition sites as hotspots, contribute considerably to high local concentrations of NO2 and PM pollution harmful to health and the environment.
Major construction sites are required to monitor noise, vibration and dust PM10, and implement trigger thresholds and mitigation measures.
Additional requirement to monitor PM2.5 and NO2 in ultra-low emission zones is determined by the local planning authority and may become mandatory at all sites in the future.
Real-time monitoring with trigger ‘alerts’ enables abatement controls, can measure their effectiveness, and provides source apportionment in event of complaints.
Remote optical monitors measure PM10 and other particulate fractions, are easy to deploy around construction sites, relatively low-cost and require minimal service.
Analysers are available to measure PM10, PM2.5 and NO2 simultaneously in real-time, providing alerts and near-reference data and optional integrated wind sensors for source apportionment.
Baseline monitoring of PM10, PM2.5 and NO2 levels may be required for impact assessment in the planning application.
A minimum of two monitoring stations are normally required at each construction site.
Monitors should be located on site boundaries, close to sensitive local receptors, and be relocated as required during construction phases.
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