UBC Faculty Research and Publications

Creating National Air Pollution Models for Population Exposure Assessment in Canada Hystad, Perry; Setton, Eleanor; Cervantes, Alejandro; Poplawski, Karla; Deschenes, Steeve; Brauer, Michael; van Donkelaar, Aaron; Lamsal, Lok; Martin, Randall; Jerrett, Michael; Demers, Paul Aug 31, 2011

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Environmental Health Perspectives • volume 119 | number 8 | August 2011 1123ResearchPredicting air pollution concentrations at reso-lutions capable of capturing local-scale pollut-ant gradients over large geographical areas is becoming increasingly important in multicity and national health studies; in population expo-sure assessment; and in support of policy, sur-veillance, and regulatory initiatives. Currently, fixed-site government monitors are the foun-dation of these activities; however, because of siting criteria, such monitors may fail to fully capture local-scale pollutant variability. In addi-tion, the number of monitors and their spa-tial distribution may be limited, as is the case in Canada. At present, few methodologies are available that adequately capture local-scale pol-lutant variability at a national scale when moni-tor density, distribution, or siting is suboptimal.A number of approaches may be used to model air pollution over large areas, includ-ing interpolation of fixed-site government monitoring data, dispersion modeling, satellite remote sensing, land use regression (LUR), and proximity and deterministic methods. Each approach, however, has inherent limita-tions that restrict its use for producing local-scale pollution estimates. Interpolation of fixed-site air pollution monitoring data has typically been used to predict pollution con-centrations across large areas (Beelen et al. 2009), with recent interest directed towards kriging methods and spatial smoothing with geographic covariates (Beelen et al. 2009; Hart et al. 2009; Yanosky et al. 2008). Fixed-site monitors may not capture entire populations, and measurements typically represent regional and between-city pollution differences due to monitor siting criteria, which prevent monitors from being placed in proximity to major roads and other pollution sources. Dispersion mod-els also exist for large geographical areas and have been incorporated into regulatory and epidemiological studies of air pollution (Cyrys et al. 2005; Nafstad et al. 2003). Importantly, the resolutions of pollutant estimates from dispersion models over large geographical areas are typically restricted, for example, to 1 or 3 km2 (Jerrett et al. 2005). Satellite remote sens-ing is a new methodology available to predict air pollution concentrations over large geo-graphic areas, and a number of studies have evaluated different remotely sensed concen-trations of fine particulate matter [PM with aerodynamic diameter ≤ 2.5 μm (PM2.5)] (e.g., van Donkelaar et al. 2010) and gaseous pol-lutants (Martin 2008) and found moderate to good associations with ground-level monitor-ing data. Currently, the resolution of satellite data limits their use to representing regional pollution concentrations, but indicators of local air pollution may be used in concert to improve the spatial resolution of predictions (Liu et al. 2009). LUR approaches have been used extensively to predict within-city pollut-ant concentrations of nitrogen dioxide (NO2) and PM2.5 (for review, see Hoek et al. 2008), but to a lesser extent for volatile organic com-pounds (VOCs). However, the approach is well suited to modeling pollutants that exhibit significant spatial variation, especially traffic-related VOCs (Atari and Luginaah 2009; Mukerjee et al. 2009; Smith et al. 2006; Su et al. 2010; Wheeler et al. 2008). The city-by-city approach in which LUR models are created is costly, and integration and interpre-tation across multiple city models is difficult. Simple proximity and deterministic approaches have also been widely used as surrogates for exposure to vehicle and industrial sources, Address correspondence to P. Hystad, School of Population and Public Health, University of British Columbia, 2206 East Mall, Vancouver, BC, V6T 1Z3 Canada. Telephone: (604) 312-4768. Fax: (604) 822-9588. E-mail: phystad@gmail.comSupplemental Material is available online (doi:10. 1289/ehp.1002976 via http://dx.doi.org/).We thank R. Allen for providing the Edmonton and Winnipeg monitoring data; D. Crouse, M. Goldberg, and N. Ross for the Montreal data; and O. Atari for the Sarnia data.The research was supported by a grant from the Canadian Partnership against Cancer. Health Canada also provided support for the development of the satellite-derived pollution estimates.The authors declare they have no actual or potential competing financial interests.Received 15 September 2010; accepted 31 March 2011.Creating National Air Pollution Models for Population Exposure Assessment in CanadaPerry Hystad,1 Eleanor Setton,2 Alejandro Cervantes,3 Karla Poplawski,4 Steeve Deschenes,2 Michael Brauer,4 Aaron van Donkelaar,5 Lok Lamsal,5 Randall Martin,5,6 Michael Jerrett,7 and Paul Demers4,81School of Population and Public Health, University of British Columbia, Vancouver, British Columbia, Canada; 2Department of Geography, University of Victoria, Victoria, British Columbia, Canada; 3Department of Geography, and 4School of Environmental Health, University of British Columbia, Vancouver, British Columbia, Canada; 5Department of Physics and Atmospheric Science, Dalhousie University, Halifax, Nova Scotia, Canada; 6Harvard-Smithsonian Center for Astrophysics, Cambridge, Massachusetts, USA; 7School of Public Health, Division of Environmental Health Science, University of California–Berkeley, Berkeley, California, USA; 8Occupational Cancer Research Centre, Cancer Care Ontario, Toronto, Ontario, CanadaBackground: Population exposure assessment methods that capture local-scale pollutant  variability are needed for large-scale epidemiological studies and surveillance, policy, and regulatory purposes. Currently, such exposure methods are limited.Methods: We created 2006 national pollutant models for fine particulate matter [PM with aero dynamic diameter ≤ 2.5 μm (PM2.5)], nitrogen dioxide (NO2), benzene, ethylbenzene, and 1,3-butadiene from routinely collected fixed-site monitoring data in Canada. In multiple regression models, we incorporated satellite estimates and geographic predictor variables to capture back-ground and regional pollutant variation and used deterministic gradients to capture local-scale vari-ation. The national NO2 and benzene models are evaluated with independent measurements from previous land use regression models that were conducted in seven Canadian cities. National models are applied to census block-face points, each of which represents the location of approximately 89 individuals, to produce estimates of population exposure.results: The national NO2 model explained 73% of the variability in fixed-site monitor concen-trations, PM2.5 46%, benzene 62%, ethylbenzene 67%, and 1,3-butadiene 68%. The NO2 model predicted, on average, 43% of the within-city variability in the independent NO2 data compared with 18% when using inverse distance weighting of fixed-site monitoring data. Benzene models performed poorly in predicting within-city benzene variability. Based on our national models, we estimated Canadian ambient annual average population-weighted exposures (in micrograms per cubic meter) of 8.39 for PM2.5, 23.37 for NO2, 1.04 for benzene, 0.63 for ethylbenzene, and 0.09 for 1,3-butadiene.conclusions: The national pollutant models created here improve exposure assessment compared with traditional monitor-based approaches by capturing both regional and local-scale pollution variation. Applying national models to routinely collected population location data can extend land use modeling techniques to population exposure assessment and to informing surveillance, policy, and regulation.key words: air pollution, Canada, fixed-site monitors, gradients, land use regression, population exposure assessment, satellite data. Environ Health Perspect 119:1123–1129 (2011). doi:10.1289/ehp.1002976 [Online 31 March 2011]Hystad et al.1124 volume 119 | number 8 | August 2011 • Environmental Health Perspectivesspecifically in epidemiological studies; yet, such measures in isolation are often poor surrogates for exposure. To date, few population exposure assessments have incorporated multiple sources of data, specifically satellite pollutant estimates, LUR modeling of geographic characteristics, and information on proximity and pollution gradients, to estimate local-scale air pollution concentrations at a national scale.Here we report a modeling initiative to produce 2006 national PM2.5, NO2, benzene, ethyl benzene, and 1,3-butadiene models for Canada that capture local-scale pollutant vari-ability and apply these models to routinely collected population location data to calculate population exposures. This research is part of Carex Canada, a national surveillance initiative designed to estimate the number of Canadians potentially exposed to known or suspected envi-ronmental and occupational carcinogens (Carex Canada 2011). This research adds to the lit-erature on air pollution modeling and exposure assessment by creating national LUR models from fixed-site monitoring data; incorporat-ing various predictor data sets and methods to capture the different scales of pollution sources; and extending LUR modeling techniques to population exposure assessment and to inform-ing surveillance, policy, and regulation.Materials and MethodsPollutant modeling approach. Models were developed in two stages using different pre-dictor variables and methodology to capture background, regional, and local-scale pollution variation. First, for each National Air Pollution Surveillance (NAPS) fixed-site monitoring station, we derived satellite-based estimates (PM2.5 and NO2 only) and geographic vari-ables (e.g., road length, population density, proximity to large emitters) using ArcGIS (version 9.3; ESRI, Redland, CA, USA). We used forward stepwise regression to develop LUR models and retained variables that cor-responded to hypothesized effect directions; we maximized the sums of squares explained by Akaike’s information criterion. Spatial autocor-relation was also evaluated using the Moran’s I statistic in ArcGIS. We sought to develop parsimonious models rather than traditional predictive models that maximize prediction but make interpretation of individual variable contributions difficult. Only variables signifi-cant at the p < 0.05 level were included in the final  models. As expected, NAPS monitoring locations in Canada did not display sufficient variability to estimate model coefficients for important local-scale parameters, such as prox-imity to major roadways, because of monitor siting. Thus, local-scale predictors were under-powered in the LUR modeling approach.In the second stage, we conducted compre-hensive literature reviews to identify determin-istic factors to represent local-scale gradients in pollutant concentrations associated with specific sources (i.e., highways, major roads, gas stations). For each pollutant, we identified concentrations near these selected sources in relation to local background levels and devel-oped deterministic multipliers with distance decay rates (together referred to as gradients in this paper) to apply to the background and regional concentrations predicted by our LUR models. All statistical analyses were conducted using SAS (version 9.1; SAS Institute Inc., Cary, NC, USA).Air quality data. Annual average concentra-tions of PM2.5 (177 monitoring stations), NO2 (134 monitors), and benzene, ethyl benzene, and 1,3-butadiene (53 monitors) were calcu-lated using data from unique NAPS monitor-ing sites that were operating during 2006 (see Figure 1). Continuous monitoring data from a given monitor were included if at least 50% of hourly observations were available for a 24-hr period and at least 50% of days were available in a month. Monthly averages from filter-based PM2.5 measurements required a minimum of three of five valid measurements per month. Annual averages for 2006 were not calculated for individual monitors unless there were at least 6 months of complete data with one valid month per quarter.NAPS includes different monitor types for PM2.5, including tapered element oscillating microbalances (TEOMs), dichotomous par-tisol samplers (Thermo Fisher Scientific Inc., Waltham, MA, USA), and beta-attenuation mass moni tors (Met One Instruments Inc., Grants Pass, OR, USA). Multiple monitors are often present at one location, and our compara-tive analysis found differences in levels measured by TEOMs, which are known to underpredict PM2.5 because of nitrate evaporation (Dann T, personal communication). We therefore selected other monitor types when they were available at the same location. Those stations with only TEOMs available were adjusted based on yearly calibration between collocated dichotomous and TEOM monitors during 2006 [n = 14, dichotomous = 1.640 + 1.089 × (TEOM), R2 = 0.89, p < 0.001]. NO2, benzene, ethyl-benzene, and 1,3-butadiene were measured using standard methods (NAPS 2004).Predictor variables. PM2.5 and NO2 sat-ellite data. Canada-wide concentrations of Figure 1. Location of NAPS monitors that were used to create national PM2.5, NO2, benzene, ethylbenzene, and 1,3-butadiene models.0 250 500 1,000Kilometers0 150 300 Kilometers0 50 100 KilometersAll substancesPM2.5 and NO2PM2.5NO2Benzene, ethylbenzene, 1,3-butadieneBlock points (representing Canadianpopulation location)Monitoring stationsCanadian national air pollution modelsEnvironmental Health Perspectives • volume 119 | number 8 | August 2011 1125PM2.5 and NO2 were estimated using satel-lite atmospheric composition data combined with local, coincident scaling factors from a chemical transport model [Goddard Earth Observing System (GEOS)-Chem 2011]. Ground-level PM2.5 estimates were derived from aerosol optical depth data from the Terra satellite [National Aeronautics and Space Administration (NASA) 2011b], in combina-tion with output from GEOS-Chem simu-lations to estimate the relationship between aerosol optical depth over the atmospheric col-umn and ground-level PM2.5 (van Donkelaar et al. 2010). Ground-level NO2 concentra-tions were estimated from tropospheric NO2 columns retrieved from the ozone monitor-ing instrument on the Aura satellite (NASA 2011a); GEOS-Chem was also used to cal-culate the relationship between the NO2 col-umn and ground-level concentration (Lamsal et al. 2008). Both PM2.5 and NO2 were esti-mated at a 0.1 × 0.1° resolution (~ 10 × 10 km). Estimates for PM2.5 were calculated from 2001–2006 data to ensure sufficient observa-tions. For NO2 estimates, we used data from 2005 and 2006, because ozone monitoring instrument measurements began in late 2004.Geographic data. We modeled regional pollutant variation using geographic predic-tor variables potentially relevant to pollutant sources, emissions, and dispersion. To cap-ture varying spatial influences of predictors, all variables were calculated for circular buffer dis-tances ranging from 50 m to 50 km. Classes of variables included population density derived from census block-face points (Statistics Canada 2006); 1-km land use classifications (Global Land Cover Characterization 2008); high-resolution (30 m) land-use classifica-tions (DMTI Spatial Inc., Markham, Ontario, Canada); sources of large industrial emissions from the Canadian National Pollutant Release Inventory (NPRI; Environment Canada 2010); small point source locations extracted from the Dun and Bradstreet (D&B) Selectory database of businesses (Hoovers, Austin, TX, USA) in Canada; length of and distance to specific road classifications using the DMTI Spatial road network, such as freeway, high-way, major road, and minor road (DMTI Spatial Inc.); length and density of railroads; elevation; and meteorological variables (pre-cipitation and temperature). Any geographic variables with > 30% zero values—those with no predictive features in proximity to a mon-itor—were recoded as binary (i.e., present/absent). In total, 10 variable classes and 270 buffer-specific variables were explored in the LUR models.Deterministic gradients. Gradients were developed with a focus on mobile sources and gas stations. We conducted a comprehen-sive literature review of published studies to identify the distance from sources at which pollutant concentrations typically return to background levels, and an expected ratio of near-source pollutant levels compared with background pollutant levels for each source and pollutant. We searched PubMed (2010), Web of Science (Thomson Reuters 2010), and Google Scholar (2010) using a range of key-words to identify studies with measurements of pollutant gradients. Studies varied widely in terms of location, date, methods, duration of measures, number of samples, and definition of near source and background. We devel-oped linear gradients using the steepest por-tion of the exponential decay curves typically found in the literature, as the tails of the decay functions were very sensitive to local param-eters. Gradients were also selected to represent Canadian conditions. Table 1 summarizes the gradients developed for Canada and applied to the LUR models.To identify the distance of each NAPS monitor from the nearest highway, major road, local urban road, and gas station, we used DMTI road network data and D&B com-mercial data for point sources. If a monitor was close enough to one of these features for the source to influence pollutant levels, we modi-fied the corresponding LUR model results (not including point source industrial variables) to account for the deterministic gradients. For example, based on our review of the literature, we assumed that NO2 concentrations at the side of a highway would be 1.65 times higher than LUR-based background concentrations but consistent with background levels 300 m from the highway; this assumption resulted in a distance decay rate of 0.33% per meter that was applied to the model to estimate NO2  levels within the 300-m gradient buffer.Model evaluation. We used three approaches for model evaluation. Due to the small number of NAPS monitoring stations for PM2.5, NO2, benzene, ethyl benzene, and 1,3-butadience, we did not leave out a percentage for independent postmodel evaluation, because we wanted to capture the greatest range of model predictors possible. Therefore, we first evaluated all LUR models using a bootstrap approach to determine the sensitivity of model prediction and parameter estimates to moni-tor sampling. Random selection of monitors was conducted, with replacement, and variable coefficients and model R2 values were recorded from the new full sample. This was repeated for 10,000 iterations to estimate the 95% con-fidence interval (CI) for overall model predic-tion and individual variable coefficients. Next, we conducted a leave-one-out analysis where each LUR model was repeatedly parameterized on n – 1 data points and then used to predict the excluded monitor measurement. The mean differences between the predicted and meas-ured values were used to estimate model error.Finally, we evaluated the NO2 and ben-zene LUR models, with and without gradients, against independent data (35–196 monitor-ing sites per city) previously collected for LUR models in seven Canadian cities (for a full description of data collection and modeling see Allen et al. 2010; Atari and Luginaah 2009; Crouse et al. 2009; Henderson et al. 2007; Jerrett et al. 2007; Su et al. 2010). Briefly, in each city, monitoring took place over a 2-week period; data from fixed-site monitors, monitor-ing during yearly average concentration periods, or multiple measurement periods were used to estimate yearly averages [see Supplemental Material, Table 1 (doi:10.1289/ehp.1002976) for the city-specific data used for model evalu-ation]. These pollution measurements were col-lected at much higher spatial densities than were NAPS and from monitors that were located to specifically capture spatial pollutant gradients. Consequently, these data were reasonable for use as a gold standard to determine how well the two national NO2 and benzene models (the LUR models and the LUR models with gradi-ents) predicted within-city variation. In addi-tion, we compared the city-specific data with estimates based on inverse distance weighting (IDW) of annual average NO2 and benzene concentrations measured at NAPS monitors Table 1. PM2.5, NO2, benzene, ethylbenzene, and 1,3-butadiene gradients determined from the literature and incorporated with national LUR model predictions.Substance Source Increase at source Gradient distance (m)PM2.5 Highway 1.25a 75bMajor road 1.1a 75bNO2 Highway 1.65a 300cMajor road 1.2a 100cBenzene Gas station 6.5d 100dHighway/major road 3.25e 50fLocal road 1.5e 50fEthylbenzene Highway 3.7g 300hMajor road 2.2g 300hLocal road 1.4g 300h1,3-Butadiene Highway 4i 75iaSmargiassi et al. (2005). bBeckerman et al. (2008), Hitchins et al. (2000), Roorda-Knape et al. (1998), Tiitta et al. (2002). cBeckerman et al. (2008), Gilbert et al. (2003, 2007), Roorda-Knape et al. (1998), Su et al. (2009). dKarakitsios et al. (2007). eHellén et al. (2006), Parra et al. (2009), Thorsson and Eliasson (2006), Vardoulakis et al. (2002). fBeckerman et al. (2008), Thorsson and Eliasson (2006), Venkatram et al. (2009). gParra et al. (2009), Roukos et al. (2009), Wang and Zhao (2008). hWang and Zhao (2008). iVenkatram et al. (2009).Hystad et al.1126 volume 119 | number 8 | August 2011 • Environmental Health Perspectives(with and without deterministic gradients). Because of NAPS monitor density in Canada, kriging could not be applied.Population exposure assessment. The national pollutant models were applied to each of the 478,831 Statistics Canada street block-face centroid locations in 2006 to estimate population exposures. First, we applied the LUR models to each block point to derive a unique predicted pollutant concentration for each point, representing the average expo-sure level for 89 and a SD of ± 158 individu-als. We used a GIS to identify the distance of each block centroid to the nearest high-way, major road, local urban road, and gas stations and adjusted the corresponding LUR model estimate when the street block point was located within an associated gradient. We then estimated population-weighted exposures to PM2.5, NO2, benzene, ethyl benzene, and 1,3-butadiene in the Canadian population as a whole, and we estimated uncertainty using the 95% confidence limits for LUR model predic-tions. Because there was insufficient informa-tion in the literature to examine uncertainty for specific gradients, we selected ± 50% for all gradients (values shown in Table 1).ResultsNational LUR model results. Table 2 sum-marizes the national LUR model results. The PM2.5 model predicted 46% of PM2.5 varia-tion and was dominated by satellite predic-tions, which alone explained 41% of PM2.5 variation. The NO2 model predicted 73% of NO2 variation and length of all roads within 10 km was the dominant predictor, explain-ing 55% of NO2 variation. This variable was only moderately correlated (r = 0.56) to NO2 predictions from satellite data, which further explained 4% of NO2 variation in the final model. The models for benzene, ethyl benzene, and 1,3-butadiene had similar predictive results, explaining 62, 67, and 68% of pol-lutant variability, respectively. Data from one monitor were removed as an outlier from the benzene and ethyl benzene models (St. John Baptiste, located in Montreal east city) and from the 1,3-butadiene model (Sarnia, located in southern Ontario near the Detroit–Windsor border), which were associated with the highest pollutant concentration for each substance.Spatial autocorrelation of national LUR models. Spatial autocorrelation of the LUR model residuals was examined using Moran’s I in ArcGIS. Spatial autocorrelation was present in the PM2.5 LUR model residuals (Moran’s I = 0.33, p < 0.001), indicating that a moderate amount of spatial autocorrelation Table 2. National LUR model results for PM2.5, NO2, benzene, ethylbenzene, and 1,3-butadiene.Variable Distancea Value SE p-ValuePM2.5 model (R 2 = 0.46, RMSE = 1.529)Intercept — 2.802 0.497 < 0.0001Satellite PM2.5 (µg/m3) — 2.392 0.263 < 0.0001NPRI emissions (tonnes) 5 km 1.63e–3 5.95e–4 0.007Industrial land use (m2) 1 km 1.03e–6 4.18e–7 0.014NO2 model (R 2 = 0.73, RMSE = 5.470)Intercept — 13.179 1.374 < 0.0001Satellite NO2 (ppb) — 1.4903 0.355 < 0.0001Industrial land use (m2) 2 km 3.21e–6 5.73e–7 < 0.0001Road length (m) 10 km 7.42e–6 9.04e–7 < 0.0001Summer rainfall (mm) — –0.010 0.002 < 0.0001Benzene modelb (R 2 = 0.62, RMSE = 0.298)Intercept — 0.346 0.069 < 0.001Major road length (m) 10 km 1.18e–6 2.56e–7 < 0.001NPRI emissions (present) 10 km 0.526 0.089 < 0.001Ethylbenzene modelc (R 2 = 0.67, RMSE = 0.193)Intercept — 0.152 0.039 < 0.001Population (count) 10 km 6.74e–7 7.25e–8 < 0.001NPRI emissions (present) 2 km 0.272 0.071 < 0.0011,3-Butadiene modeld (R 2 = 0.68, RMSE = 0.034)Intercept — 0.011 0.009 0.208Road length (m) 750 m 3.89e–6 7.93e–7 < 0.001Highway (present) 500 m 0.041 0.012 0.002Commercial land use (m2) 10 km 1.60e–9 5.97e–10 0.010Satellite PM2.5 and NO2 are satellite-derived estimates of PM2.5 and NO2. Land use is the area of specific land-use types (industrial, commercial) within the associated buffer distance. Road length refers to the length of different road classifications (all, major, highways) within the associated buffer distance. Summer rainfall refers to the amount of rainfall recorded from May to September from the nearest meteorological station. NPRI emissions refer to the amount of annual emissions of the model substance released from industries that reported to the NPRI. NPRI emissions (present) refers to the presence of NPRI facilities that have released a model substance into the air. Population (count) refers to the number of individuals who resided within the associated buffer distance. aRadius of cicular buffers used to derive variables. bOne outlier removed with benzene concentration of 3.55 µg/m3. cOne outlier removed with ethylbenzene concentration of 2.57 µg/m3. bOne outlier removed with 1,3-butadiene concentration of 0.82 µg/m3.Figure 2. National annual average models for PM2.5, highlighting southern Ontario and the city of Toronto (A), and for NO2, highlighting southwestern British Columbia and the city of Vancouver (B), that incorporate satellite-derived pollutant estimates, geographic land use variables, and deterministic gradients. The seven cities shown in (B) represent locations of independent monitoring data used to evaluate the national NO2 and benzene models.PM2.5 model (µg/m3)High: 16Low: 5NO2 model (µg/m3)10 meter cell resolutionHigh: 105Low: 00 500 1,000 km10 km cell resolution0 500 1,000 km1 km resolutionCanadian national air pollution modelsEnvironmental Health Perspectives • volume 119 | number 8 | August 2011 1127remained that was not explained by the PM2.5 model predictors. Clustering of positive resid-uals (model underpredicting by an average of 2.57 μg/m3) occurred in the rural interior of British Columbia. An indicator variable for British Columbia substantially reduced the spatial autocorrelation (Moran’s I = 0.03, p = 0.04). Sensitivity analysis using a sum-mer-only PM2.5 model indicated no spatial autocorrelation (Moran’s I = 0.04, p = 0.01), supporting our hypothesis of woodburning as the primary source of model underprediction in this region. No significant spatial autocor-relation existed in LUR model residuals for NO2 (Moran’s I = 0.03, p = 0.44), benzene (Moran’s I = –0.20, p = 0.13), ethyl benzene (Moran’s  I  = –0.00, p  = 0.87),  and 1,3-butadiene (Moran’s I = 0.09, p = 0.32).Incorporating gradients with national LUR models. Deterministic gradients were added to LUR models, because we could not estimate the effects of local-scale pollution sources from NAPS data alone. Figure 2A illustrates the final PM2.5 model (LUR plus gradients) for Canada as a whole and for southern Ontario and the city of Toronto. Figure 2B illustrates the final national NO2 model (LUR plus gradients) for Canada as a whole and for southwestern British Columbia and the city of Vancouver. These maps illus-trate the spatial resolution of the final national pollutant models; however, for population exposure assessment, the LUR model results and deterministic gradients were applied to street block point locations, as shown in Figure 3, which illustrates the final national benzene model (LUR plus gradients) calcu-lated at the block point level.Evaluation of national pollutant models. Evaluation of LUR models using bootstrap and leave-one-out analyses. The distribution of all model coefficients for each pollutant resulting from bootstrap analysis showed normal distributions. The NO2 model was the least sensitive to monitor selection, with a bootstrap R2 95% CI of 65–81. Models for PM2.5, benzene, ethyl benzene, and 1,3-butadiene demonstrated larger uncer-tainty to monitor selection, with R2 95% CIs of 33–59, 44–80, 49–85, and 53–82, respectively. Variable coefficients for indus-trial NPRI proximity variables were extremely sensitive to monitor selection. The leave-one-out analyses indicated no significant bias in any LUR model, as demonstrated by the mean ± SD error: 1.07e–3 ± 5.61 for NO2; –6.35e–3 ± 1.59 for PM2.5; –0.04 ± 0.32 for benzene; –0.01 ± 0.04 for 1,3-butadiene; and –0.04 ± 0.22 for ethyl benzene.Evaluation of NO2 and benzene mod-els using city-specific data. On average, the national NO2 LUR plus gradient model pre-dicted 43% of the within-city NO2 variation (based on the city-specific data evaluation) compared with 22% predicted based on IDW of NAPS monitors plus gradients (Table 3). National LUR, LUR plus gradients, IDW, and IDW plus gradients models overpredicted the city-specific NO2 measurements, with aver-age city-specific intercepts of 4.56, 7.45, 8.51, and 11.56 μg/m3, respectively. City-specific scatter plots of measured and modeled NO2 concentrations are illustrated in Supplemental Material, Figure 1 (doi:10.1289/ehp.1002976).For benzene, all modeling methods per-formed poorly in explaining within-city ben-zene variation. The LUR plus gradients model explained, on average, only 16% of within-city variability in benzene concentrations compared with 11% based on IDW plus gra-dients (Table 3). In the evaluation using the Montreal city-specific benzene concentrations, four outliers were removed (all concentrations > 2 μg/m3), and one outlier (4.10 μg/m3) was removed in the Toronto evaluation. Benzene models also overpredicted city-specific con-centrations, based on city-specific intercepts of modeled versus measured concentrations [see Supplemental Material Figure 2 (doi:10.1289/ehp.1002976)]. Sarnia, a high-density industrial community with 46 NPRI emitters, had poor NO2 and benzene model evaluations.Canadian population exposure assess-ment. The final LUR models and gradients were applied to all 478,831 street block-face centroid locations to conduct population expo-sure assessments. Estimated mean (95% CI) population exposures (micrograms per cubic meter) to ambient PM2.5, NO2, benzene, ethyl-benzene, and 1,3-butadiene in Canada based on the LUR models were 8.10 (5.84–10.43), 22.40 (13.14–33.51), 0.94 (0.57–1.31), 0.38 (0.25–0.52), and 0.086 (0.035–0.138), respectively. Estimates for the same pollutants based on the national LUR plus gradients models were 8.39 (6.00–11.13), 23.37 (14.01–35.73), 1.04 (0.59–1.49), 0.63 (0.35–1.10), and 0.089 (0.036–0.146), respectively. Wide ranges of exposure levels were estimated in Canada for all substances; see Supplemental Material, Figure 3 (doi:10.1289/ehp.1002976) for popu-lation exposure distributions.DiscussionWe created national pollutant models from fixed-site monitoring data that incorporate Figure 3. National benzene LUR model plus gradients (illustrating the city of Toronto) calculated for each street block point in Canada (n = 478,831).Block point benzeneconcentrations (µg/m3)< 0.490.50–0.810.82–1.25> 2.121.26–2.12 00 0.5 1250 500 1,000KilometersKilometers0 25 50KilometersHystad et al.1128 volume 119 | number 8 | August 2011 • Environmental Health Perspectivessatellite, geographic, and deterministic com-ponents and demonstrated that these models can improve exposure assessment over large geographic areas compared with approaches based solely on interpolation of fixed-site monitoring data. We also demonstrated how these models can be used for population expo-sure assessment.The national LUR models explained 73% of pollution variation in NAPS measurements for NO2, and lesser degrees for PM2.5 (46%), benzene (62%), ethylbenzene (67%), and 1,3-butadiene (68%). The NO2 and PM2.5 mod-els were least sensitive to monitor selection, whereas models for VOCs were more sensi-tive—likely because of the smaller number of monitors on which LUR estimates were based (n = 53). The predictive performance of the PM2.5 model [R2 = 0.46, root mean square error (RMSE) = 1.53 μg/m3] was consistent with other large-scale modeling studies based on different monitoring methodologies and data inputs (Beelen et al. 2009; Hart et al. 2009; Liao et al. 2006; Ross et al. 2007).The national LUR models generally cap-tured regional patterns in pollutant concentra-tions, corresponding to NAPS monitor siting criteria, but were less effective at identifying small-scale geographic predictor variables. For example, only 35 NAPS monitors were located within 500 m of a major road and only 7 monitors were within 500 m of a major industrial emission source. Such small sample sizes greatly reduce the power of the models to capture these specific pollutant sources. Some city-specific LUR methods have used location-allocation methods to more fully represent the true spatial variation in pollution levels and to capture the range of predictor variables (Jerrett et al. 2005). Models based on fixed-site monitor data may therefore need additional approaches to represent local-scale pollutant variability not captured by fixed-site monitors. This was indeed the case with the Canadian NAPS network, but larger regulatory networks, such as those in the United States, may bet-ter represent the range of predictor variables needed to build local-scale LUR models.To address the lack of local-scale geo-graphic variability in the NAPS data, we incorporated deterministic gradients based on proximity to specific sources (i.e., vehicles and gas stations). The final NO2 LUR plus gradi-ent model improved prediction of within-city pollutant variation considerably compared with the LUR model alone and interpola-tion methods. On average, the final model predicted 43% of within-city NO2 variation compared with 18% using IDW. Both the national benzene model and IDW predicted within-city benzene poorly, which may be due to the small number of NAPS monitors on which the model was based, the relatively small variation in within-city benzene levels, or the inability of gradients to capture local benzene concentrations. Similar to the NO2 model, the evaluation of the benzene model with Sarnia data was poor, reflecting the diffi-culty in capturing unique high-density indus-trial conditions in a national model.Gradients were based on literature reviews. The lack of methodological consistency among published data of pollutant level increases near specific sources and the distance required for pollutant levels to return to background were clear limitations. To improve reliability of gra-dients, we used linear functions to represent the decreases in pollutant levels found in the initial portions of the exponential decay curves found in the literature. The methodology used here could be augmented as new gradients become available or with other modeled data.Population exposure assessment was conducted using the national models and census street block-face points. The population-weighted average exposures to PM2.5, NO2, benzene, ethyl benzene, and 1,3-butadiene were 8.39, 23.37, 1.04, 0.63, and 0.089 (μg/m3), respectively. The uncertainty of population exposure estimates were driven primarily by LUR model uncertainty. Although the results of the national LUR models are similar to city-specific LUR models in their predictive capacity and error, we are unaware of any LUR mod-els that have been applied to estimate expo-sure uncertainty. Although these exposures are low compared with other developed countries, exposures in particular locations in Canada are relatively high. For example, the 90th per-centiles of exposures (micrograms per cubic meter) are 9.78 for PM2.5, 34.81 for NO2, 1.61 for benzene, 1.01 for ethyl benzene, and 0.14 for 1,3-butadiene. The ability of the national models to capture local-scale pollutant variabil-ity allows for more realistic exposure assessments and assessments that can potentially identify high-risk populations. Future work will refine approaches for using the national models to cal-culate population exposure assessments, incor-porate socioeconomic information from census to examine environment injustice issues, and integrate national models into a risk-assessment framework that incorporates exposures from other sources and microenvironments.This study faced a number of challenges and limitations to creating national pollutant mod-els from fixed-site monitors and applying these models to estimate Canadian population expo-sures. First, the NAPS monitors in Canada are centered in large metropolitan areas, and mod-eled relationships will therefore be weighted toward these areas. This is appropriate for population exposure assessment, because these locations represent the majority of Canadians, but in rural areas the models could be adjusted or a background concentration could be used. This is particularly relevant to the benzene, ethyl benzene, and 1,3-butadiene models, which were based on data from monitors located almost exclusively in large urban areas or sited near large industrial sources. Second, we had limited data on pollutant sources and source strengths such as traffic volumes. In addition, we did not model emissions from woodburning stoves and forest fires, which may have caused us to underpredict PM2.5 concentrations in the interior of British Columbia. Third, parsimo-nious LUR models were created, because the specificity of model variables may be important for informing surveillance and regulation. This, however, leads to models that do not capture the complex interactions between geographic characteristics and pollutant sources, and even the simplest LUR predictors (e.g., major roads or NPRI facilities within 10 km) capture com-plex mixes of geographic characteristics and pollutant sources. Fourth, we compared model estimates with city-specific measurements Table 3. Evaluation of national NO2 and benzene models, as well as IDW estimates from fixed-site moni-tors, against independent city-specific measurement data. R2 (RMSE)Substance na LURb LUR + Gc IDWd IDW + GeNO2Edmonton 50 0.60 (3.67) 0.41 (4.59) 0.10 (5.52) 0.01 (5.92)Montreal 135 0.41 (4.28) 0.48 (4.04) 0.31 (4.63) 0.41 (4.29)Sarnia 34 0.42 (4.21) 0.49 (4.04) 0.12 (5.15) 0.19 (5.12)Toronto 196 0.18 (7.69) 0.36 (6.78) 0.13 (7.93) 0.32 (6.99)Victoria 40 0.19 (3.95) 0.37 (3.70) 0.23 (3.86) 0.26 (3.98)Vancouver 114 0.31 (6.41) 0.42 (5.93) 0.31 (6.43) 0.36 (6.24)Winnipeg 49 0.54 (3.65) 0.51 (3.86) 0.08 (5.17) 0.02 (5.43)Average 618 0.39 (4.84) 0.43 (4.71) 0.18 (5.53) 0.22 (5.42)BenzeneMontrealf 131 0.33 (0.24) 0.26 (0.25) 0.11 (0.28) 0.05 (0.29)Sarnia 37 0.02 (0.57) 0.04 (0.56) 0.00 (0.57) 0.03 (0.56)Torontog 44 0.03 (0.19) 0.22 (0.17) 0.00 (0.19) 0.34 (0.16)Winnipeg 94 0.08 (0.25) 0.10 (0.25) 0.00 (0.26) 0.01 (0.26)Average 306 0.12 (0.31) 0.16 (0.31) 0.03 (0.33) 0.11 (0.32)aNumber of within-city measurement locations. bNational LUR model. cNational LUR model plus gradients (G). dIDW interpolation of NAPS fixed-site monitoring data. eIDW interpolation of NAPS fixed-site monitoring data plus gradients. fFour outliers removed with highest city concentrations (> 2 µg/m3). gOne outlier removed with highest city concentration (4.10 µg/m3). Canadian national air pollution modelsEnvironmental Health Perspectives • volume 119 | number 8 | August 2011 1129for NO2 and benzene collected in different years and using a variety of methodologies. Nevertheless, these measurements represent the best data on within-city pollutant variability available. Fifth, applying LUR model results to approximately half a million block points is currently extremely computationally and time intensive. Finally, the geographic accuracy of street block centroids may introduce errors into the gradient portions of the models and therefore the exposure assessment, particularly between rural and urban areas. These errors, however, are likely spatially random within rural and urban areas across Canada.ConclusionsNational exposure models were required by Carex Canada to produce population expo-sure assessments that captured both between-city and within-city pollution variability. We created national PM2.5, NO2, benzene, ethyl benzene, and 1,3-butadiene models from fixed-site monitoring data and found that a combination of data sources and methods to capture background, regional, and local-scale pollution variation improved exposure assess-ment over traditional IDW interpolation approaches. The national pollutant models were applied to street block-face points, represent-ing the locations of the Canadian population, to determine population exposure estimates. Estimates of average population exposure levels in Canada are PM2.5 8.39, NO2 23.37, benzene 1.04, ethyl benzene 0.63, and 1,3-butadiene 0.09 (μg/m3). The modeling approach devel-oped here uses readily available data and could be reproduced over time, for example, every 5 years with the Canadian census. 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