Micro-characteristics of a naturally ventilated classroom air quality under varying air purifier placements

A naturally-ventilated operational classroom was instrumented at 18 locations to assess spatial variations of classroom air pollution (CRAP), thermal comfort and ventilation indicators under 10 different scenarios (base scenario without air purifier (AP); three single AP scenarios; three scenarios with two APs at same locations; three scenarios with two APs at different locations). Unlike PM_2.5, monitored PM₁₀ and CO₂ concentrations followed the diurnal occupancy profile. Highest vertical variation (38%) in CO₂ was at the classroom entry zone at 40-300 cm height. CO₂ increased until 225 cm before stratifying further. PM₁₀ increased to highest levels at children sitting height (100 cm) before decreasing to adult breathing height (150 cm). Highest horizontal variations in CO₂ (PM₁₀) were 29% (22%) at 40 cm height between the entry and occupied zones. Teachers' exposure to CO₂ (PM₁₀) in breathing zone varied by up to 6% (3%); the corresponding variations across monitored locations were up to 14% (19%). Teachers' exposure to CO₂ was up to 13% higher than that of children and 18% lower for PM₁₀. Traffic emissions (PM_2.5 and NO_x), secondary pollutants (VOCs and O₃), thermal comfort parameters and noise level in the classroom varied insignificantly among scenarios. PM₁₀ reduction was not doubled by using two air purifiers, which were most effective when placed within the highest PM concentration zone. Cross-comparisons of scenarios showed: use of AP reduced classroom's spatial average PM₁₀ up to 14%; PM₁₀ was reduced by increasing the AP's filtration capacity; and AP had insignificant impact on spatial average CO₂. PM₁₀ showed a maximum reduction of 46% (teacher zone), 62% (occupied zone) and 50% (entry zone) at children's breathing height, depending on usage scenario. This study produced high-resolution data for validating the detailed numerical models for classrooms and informing decision-making on AP's placement to minimise children's exposure to CRAP and re-breathed CO₂.