Indoor and outdoor particles in an air-conditioned building during and after the 2013 haze in Singapore

Particles released from biomass burning can contribute to severe air pollution. We monitored indoor and outdoor particles in a mechanically ventilated and air-conditioned building during and after the 2013 haze event in Singapore. Continuous monitoring of time-and size-resolved particles in the diameter range 0.01-10 μm was conducted for two weeks in each sampling campaign. During the haze event, the averaged size-resolved outdoor particle volume concentrations (dV/d(logDp)) for diameters larger than 0.3 μm were considerably higher than those during the post-haze days (9-185 μm3 cm-3 versus 1-35 μm3 cm-3). However, the average number concentration of particles with diameters in the range 10-200 nm was substantially lower on the hazy days than on the post-haze days (11,400 to 14,300 particles cm-3 for hazy days, versus an average of 23,700 particles cm-3 on post-haze days). The building mechanical ventilation system, equipped with MERV 7 filters, attenuated the penetration and persistence of outdoor particles into the monitored building. Indoor particle concentrations, in the diameter ranges 0.3-1.0 μm and 1.0-2.5 μm, closely tracked the corresponding patterns of outdoor particle concentrations. For particles in the size range 0.01-1.0 μm, the size-resolved mean indoor/outdoor (I/O) ratios were in the range 0.12-0.65 with the highest mean I/O ratio at 0.3 μm (0.59 in AC on mode and 0.64 in AC off mode). The air conditioning and mechanical ventilation system with MERV 7 filters provided low single-pass removal efficiency (less than ~ 30%) for particles with diameters of 0.01-1.0 μm. During the haze, for particles larger than ~0.2 μm, lower I/O ratios and higher removal efficiencies occurred with the air conditioning operating as compared to with mechanical ventilation only. This observation suggests the possibility of particle loss to air conditioning system surfaces, possibly enhanced by thermophoretic or diffusiophoretic effects