Fine particle formation in indoor environments: levels, influencing factors and implications
Experiments were conducted in an 11-m 3 stainless steel chamber to investigate secondary particle formation/growth in indoor environments. Experimental results indicate that rapid particle growth occurs due to homogeneous reactions between ozone and terpenes, and subsequent gas-to-particle partitioning of reaction products. Experimental results also indicate that many consumer products can emit significant amounts of terpenes that can serve as precursors to the formation of indoor fine particles. A new Indoor Chemistry and Exposure Model (ICEM) was used to predict dynamic particle mass concentrations based on detailed homogeneous chemical mechanisms and partitioning of semi-volatile products to particles. The ICEM allows for the simulation of air exchange processes, indoor emissions, chemical reactions, deposition, and variations in outdoor air quality. Predicted indoor secondary particle mass concentrations are in good agreement with experimental results. Both experimental and model results suggest that secondary particle mass concentrations increase significantly at lower building air exchange rates. This result is significant given a continuing trend toward building weatherization for purposes of energy conservation. Predicted indoor secondary particle concentrations increase with lower temperatures, higher outdoor particle levels, higher outdoor ozone levels, and higher indoor terpene emission rates. Indoor secondary particle concentrations resulting from reactions between ozone that originates outdoors and terpenes that originate from indoor sources can be higher than indoor particle concentrations resulting from the transport of outdoor particles. If ozone generation air “purifiers” and elevated terpene levels are simultaneously present in indoor environments, the resulting indoor secondary particle mass concentrations can exceed 65 mg/m3 . The implications of this study are significant. It appears that it is now possible to reasonably simulate complex indoor chemistry and particle growth dynamics using a state-of-the-art model (ICEM). More importantly, it appears that under some conditions, indoor air chemistry can lead to significant increases in human exposure to fine particles. Such exposure could be reduced by avoiding indoor sources of ozone, e.g., from ozone generators marketed as air “purifiers”, or by reducing the use of consumer products that contain terpenes, especially during the summer ozone season.