**2. Methods**

### *2.1. Case Study—A Sub-Urban Air Quality Monitoring Station in Dublin, Ireland*

Ireland has a range of air quality monitoring stations across the country, which are part of the national ambient air quality monitoring programme (AAMP). The data collated from these monitoring sites are used to inform on air quality at the local and national levels, and are being used for forecast modelling. The Blanchardstown sub-urban site is one of the 17 national sites, managed by the Environmental Protection Agency (EPA), which monitors NO2 and is classified as a suburban monitoring site. It is located to the west of Dublin city centre in Ireland [30]. The Blanchardstown air quality monitoring station was selected, as it provided continuous, high-resolution NO2 data emissions over a 1-year period. Its location is adjacent to the major arterial carriageway around Dublin city centre, and as a monitoring location, is therefore affected by traffic emissions. In this manner, NO2 hourly data was collected throughout 2013, with 96% of data capture and availability from [31], and the information needed about weather conditions in Dublin in 2013 was obtained on [32].

EU legislation for NO2 limit values (2008 CAFE Directive and S.I. number 180 of 2011) align with the World Health Organisation (WHO) guidelines, with 1-hour and 1-year limit values of 200 μg/m<sup>3</sup>

and 40 μg/m<sup>3</sup> respectively. In circumstances in which the hourly value is exceeded on three consecutive hours, short-term action plans must be implemented by local authorities to mitigate against continued pollution events (limiting traffic flows, restricting construction work, industrial processes, etc.). Despite there being no daily NO2 limit value in the EU and WHO, some countries have set an average daily limit of 100 μg/m<sup>3</sup> (range from 80–150 μg/m3) [31].

The sources and trends of NO2 emissions over the last 20 years has seen a recent increasing trend due to growth in the transport sector and a recovery since the 2008 economic downturn. As such, Ireland's air quality in relation to NO2 is considered to be deteriorating, as measurement data suggests it may reach limit values and the national emissions ceiling in the coming years.
