Indoor/outdoor air pollution relationships in ten commercial buildings: PM2.5 and NO2

• NO2 & PM2.5 air quality monitored outside and inside commercial buildings in Dublin.
• Indoor/outdoor PM2.5 ratios were all found to be close to or above 1.
• Naturally ventilated buildings had lowest NO2 concentrations indoors.
• Strong diurnal NO2 relationships between outdoor and indoor air quality.
• Indoor/outdoor ratios increased overnight as outdoor concentrations reduced.

PM2.5 and NO2 concentrations were measured simultaneously indoors and outdoors for ten different city centre buildings (shops and offices) in Dublin, Ireland. Outdoor concentrations were measured in two locations either at ground level outside the building or at the air intake of the building's ventilation system. The ratio of indoor to outdoor (I/O) PM2.5 concentrations were all found to be close to or above 1, indicating that either the fabric and/or operational environment of the buildings, whether naturally or mechanically ventilated, was not performing any significant function to reduce particulate concentrations from outdoors or that indoor sources were present. Indoor PM2.5 concentrations during working hours were generally below 25 μg m−3 PM2.5, with naturally ventilated shops showing the highest concentrations. Lower indoor NO2 concentrations were measured during working hours in naturally ventilated buildings compared to the mechanically ventilated buildings, although most buildings showed strong diurnal relationships between outdoor NO2 concentrations and indoor air quality. Indeed, street level concentrations of NO2 seemed to have a stronger influence on the indoor air quality of the mechanically ventilated buildings, than with the corresponding air quality measured at their roof level ventilation intakes. The buildings with the greatest reduction for NO2 were older naturally ventilated offices. I/O ratios of both pollutants, but particularly NO2, increased significantly overnight as outdoor concentrations reduced to a much greater extent than indoors. This would indicate a benefit in promoting increased air exchange between the outdoors and indoors during night–time periods in order to flush out air pollutants.