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.

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.

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 PM_2.5 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 PM_2.5) 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, PM_2.5 and NO₂ measurement simultaneously can be both cost-effective and a future-proof investment.

In selecting monitoring locations, key considerations include:

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.

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.