Article pubs.acs.org/est
Size-Resolved Deposition Rates for Ultrafine and Submicrometer Particles in a Residential Housing Unit Wan-Chen Lee,*,† Jack M. Wolfson,† Paul J. Catalano,‡ Stephen N. Rudnick,§ and Petros Koutrakis† †
Department of Environmental Health, Harvard School of Public Health, 401 Park Drive, Landmark Center West, Boston, Massachusetts 02215, United States ‡ Department of Biostatistics and Computational Biology, Dana-Farber Cancer Institute and Department of Biostatistics, Harvard School of Public Health, 44 Binney Street, Dana-Farber Cancer Institute CLSB 11015, Boston, Massachusetts 02115, United States § Department of Environmental Health, Harvard School of Public Health, 665 Huntington Avenue, Room 1-B21, Boston, Massachusetts 02115, United States S Supporting Information *
ABSTRACT: We estimated the size-resolved particle deposition rates for the ultrafine and submicrometer particles using a nonlinear regression method with unknown particle background concentrations during nonsourced period following a controlled sourced period in a well-mixed residential environment. A dynamic adjustment method in conjunction with the constant injection of tracer gas was used to maintain the air exchange rate at three target levels across the range of 0.61−1.24 air change per hour (ACH). Particle deposition was found to be highly size dependent with rates ranging from 0.68 ± 0.10 to 5.03 ± 0.20 h−1 (mean ± s.e.). Our findings also suggest that the effect of air exchange on the particle deposition under enhanced air mixing was relatively small when compared to both the strong influence of size-dependent deposition mechanisms and the effects of mechanical air mixing by fans. Nonetheless, the significant association between air exchange and particle deposition rates for a few size categories indicated potential influence of air exchange on particle deposition. In the future, the proposed approach can be used to explore the separate or composite effects between air exchange and air mixing on particle deposition rates, which will contribute to improved assessment of human exposure to ultrafine and submicrometer particles.
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INTRODUCTION Epidemiological studies have shown association between exposure to ambient fine particulate matters and adverse health effects such as mortality and onset of cardiovascular events.1−3 Exposure to indoor fine particles is of particular concern, as people spend more than 85% of their time in enclosed buildings with the majority of that time in residences.4 An important parameter in assessing residential particle exposures is indoor particle deposition rate, a natural removal mechanism that contributes to the reduction of airborne particle levels indoors. Studies conducted in occupied houses have demonstrated size-specific characteristics for deposition rates: elevated levels for larger (>1 μm) and ultrafine particles ( 3) in the laboratory. It included water evaporation rates of 3.57 ± 1.33, 8.02 ± 1.49, and 13.19 ± 1.54 g/h for one, two, and three nebulizers combined, respectively. bC(0) is the measured total initial particle concentrations during decay tests.
Figure 1. Comparison of the predicted and measured particle concentrations during the decay periods for the 11 particle size categories, using data from one sampling day (0.61 ACH) as an example. The solid markers represent the actual measurements whereas the solid lines are the predicted decay curves from the NLIN procedure.
for ki,A and constant across all ACH for ki,all. Uncertainties for the estimates from the NLIN procedure were reported as standard errors. To evaluate the effect of air exchange rate on ki, the estimates determined from the second level (ki,A=0.60, ki,A=0.90, ki,A=1.20) were first examined using a global F test with a significance level of 0.05 for each particle size category, to see whether the ki from at least one target ACH were different from the ki from the others. Subsequently, pairwise comparisons (n = 3) of the ki,A values were made with the level of significance of 0.0167. The F statistics used for the pairwise comparison procedure were based on the paired data, except that we used the same
mean squared error (MSE) from the global test. By doing so, we were able to make three comparisons on the same basis (using the same denominator for the F statistics), and the higher degrees of freedom from the MSE would contribute to more stable results in the analysis.
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RESULTS Measured Parameters. The measured parameters and indoor conditions in the apartment for the three target air exchange rates are shown in Table 1. The relative humidity (RH) was below the deliquescence point of NaCl of 75.3% (at 25 °C); thus, the particle-associated water was expected to 10285
dx.doi.org/10.1021/es502278k | Environ. Sci. Technol. 2014, 48, 10282−10290
Environmental Science & Technology
Article
evaporate completely, leaving cubic crystals of NaCl as the aerosol.22 The variability of indoor temperature and RH across the test days was small with coefficients of variation of less than 5%, except for the RH at A = 0.90 ACH (coefficient of variation = 10.16%). Estimated Size-Resolved Deposition Rate. Comparisons between the measured particle concentrations by particle size versus time and the fitted (predicted) values from the NLIN procedure were made for all nine test days to evaluate the goodness of fit of the model. Figure 1 shows an example of the fitted plot using measurements from one test day under 0.61 ACH. There was good agreement between measured and model-predicted particle concentrations for all particle sizes across all air exchange rates. The steepness of descent for the decay curves increased with increase in deposition rates, which varied substantially by particle size. The estimated ki values from the NLIN procedure are presented in Table 2 for each of three target air exchange rates. The results showed strong size dependence of the estimated deposition rates (Figures 1 and S4, Supporting Information). As a general trend, the deposition rate decreased as the size increased for the ultrafine particles (
