Indoor particle number and PM2.5 concentrations were investigated in a radio station surrounded by busy roads. Two extensive field measurement campaigns were conducted to determine the critical parameters affecting indoor air quality. The results indicated that indoor particle number and PM2.5 concentrations were governed by outdoor air, and significantly affected by the location of air intake and design of HVAC system. Prior to the upgrade of the HVAC system and relocation of the air intake, the indoor median particle number concentration was 7.4 ×103 particle/cm3 and the median PM2.5 concentration was 7µg/m3. After the relocation of air intake and the redesign of the HVAC system, the indoor particle number concentration was between 2.3×103 and 3.4×103 particle/cm3, with a median value of 2.7×103 particle/cm3; and the indoor PM2.5 concentration was in the range of 3 - 5µg/m3, with a median value of 4µg/m3. By relocating the air intake of the HVAC, the outdoor particle number and PM2.5 concentrations near the air intake were reduced by 35% and 55%, respectively. In addition, with the relocation of air intake and the redesign of the HVAC system, the particle number penetration rate was reduced from 42% to 14%, and the overall filtration efficiency of the HVAC system (relocation of air intake, pre-filter, AHU and particle losses in the air duct) increased from 58% to 86%. For PM2.5, the penetration rate after the upgrade was approximately 18% and the overall filtration efficiency was 82%. This study demonstrates that by using a comprehensive approach, including the assessment of outdoor conditions and characterisation of ventilation and filtration parameters, satisfactory indoor air quality can be achieved, even for those indoor environments facing challenging outdoor air conditions
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