Role of aerosols in enhancing SVOC flux between air and indoor surfaces and its influence on exposure

Indoor surfaces play an important role in the transport of, and exposure to, semi-volatile organic compounds (SVOCs) in buildings. In this study, we develop a model that accounts for SVOC transport mediated by particles and find that, due to large gas-particle partition coefficients along with large differences in Brownian and gas diffusivities, SVOC transport across concentration boundary layers is significantly enhanced in the presence of particles. Two important dimensionless parameters, Bi-m,Bi-g and Bi-m,Bi-g/(m,p), were identified: Bi-m,Bi-g is the ratio of 1) the characteristic time for the SVOC to transport across the concentration boundary layer to 2) the characteristic time for boundary layer to either be swept of SVOCs by particles or saturated by release of SVOCs from particles. This parameter can be regarded as a dimensionless mass transfer coefficient. Bi-m,Bi-g/Bi-m,Bi-p characterizes the SVOC mass associated with particles, relative to SVOCs in the gas-phase. Analysis on monodisperse particles shows that flux can be enhanced by as much as a factor of 5 over transport in the absence of particles, for a large particle/gas partition coefficient (log K-part = 13), small particles (d(p) similar to 0.1 mu m) and a small free stream velocity (U-infinity = 0.01 m s(-1)). As particle diameter decreases, flux enhancement tends to increase. However, as particles become very small (e.g., d(p) < 0.05 mu m), flux enhancement for SVOCs with log K-part = 13 decreases slightly. Particles larger than 2 mu m do not significantly influence the flux. An exponential correlation is found to fit the results for polydisperse particles associated with typical indoor environments, cooking and smoking. Two illustrative examples are used to show that, 1) the timescale for di(2-ethylhexyl) phthalate (DEHP) to approach equilibrium between the gas and a surface is shortened from 3.0 years to 0.45 years; and 2) in the presence of particles, the gas-phase DEHP concentration and emission rate are predicted to be as much as 4 times higher by our model than that by prior model estimates. Particle mediated gas-phase transport of SVOCs can result an increase in occupant exposure by a factor of 4-10. (C) 2012 Published by Elsevier Ltd.