Salt Lake Valley Research Articles

Dynamically Induced Displacements of a Persistent Cold-Air Pool

We examine the influence of a passing weather system on a persistent cold-air pool (CAP) during the Persistent Cold-Air Pool Study in the Salt Lake Valley, Utah, USA. The CAP experiences a sequence of along-valley displacements that temporarily and partially remove the cold air in response to increasing along-valley winds aloft. The displacements are due to the formation of a mountain wave over the upstream topography as well as adjustments to the regional horizontal pressure gradient and wind-stress divergence acting on the CAP. These processes appear to help establish a balance wherein the depth of the CAP increases to the north. When that balance is disrupted, the CAP tilt collapses, which sends a gravity current of cold air travelling upstream and thereby restores CAP conditions throughout the valley. Intra-valley mixing of momentum, heat, and pollution within the CAP by Kelvin–Helmholtz waves and seiching is also examined.

Relationship between particulate air pollution and meteorological variables in Utah's Salt Lake Valley

Critical meteorological factors affecting daily particulate concentrations during winter for Utah's urbanized Salt Lake Valley are examined on the basis of forty years of data. In a typical winter, the National Ambient Air Quality Standard for particulate matter with aerodynamic diameter less than 2.5 microns (PM2.5) is exceeded during 6 multi-day events comprising 18 winter days. Multi-day episodes of high stability produce these events, as synoptic-scale high-pressure ridges transit across Utah. The valley heat deficit, a bulk measure of atmospheric stability, exhibits large winter-to-winter variations that are highly related to similar variations in PM2.5. While control strategies have led to downward trends in concentrations for some primary pollutants, no long-term trends in valley heat deficit are evident over the 40 years. PM2.5 concentrations rise gradually over a period of days after a heat deficit threshold is exceeded as the air within the valley becomes decoupled from generally stronger winds aloft. Concentrations climb at a typical rate of about 10 μg m−3 d−1 over a four-day period to about 60 μg m−3 during these episodes. During episodes when PM2.5 concentrations exceed 35 μg m−3, the atmospheric column in the valley is characterized by: temperature below 0 °C; relative humidity in excess of 50%; and light wind speeds. PM2.5 concentrations in excess of 35 μg m−3 are four times more likely when the valley is snow covered than when it is not. A stepwise multiple linear regression based upon selected meteorological variables is used to estimate daily values of PM2.5 during two independent winters. The correlation between observed and estimated PM2.5 for these winters reaches 0.81.

A numerical study of a persistent cold air pool episode in the Salt Lake Valley, Utah

The Weather Research and Forecasting (WRF) model is used to simulate a persistent cold air pool (CAP) episode observed in December 2010 during the Persistent Cold Air Pool Study (PCAPS) field campaign in Salt Lake Valley, Utah. The availability of intensive observations from PCAPS, especially the added vertical profiles, allows for an in-depth analysis of CAP structure and evolution and model evaluation beyond what was done in similar work before. Comparisons of the WRF simulation with surface observations and upper air soundings indicate that WRF is capable of simulating the observed spatial and temporal variation of the valley atmosphere during the CAP episode. The model also successfully captures the formation and removal of the persistent temperature inversion. Process analysis helps quantify the contribution of various physical processes to the buildup and breakup of the inversion. The inversion developed due primarily to a rapid warming above the valley by synoptic subsidence and warm advection and to strong radiative cooling within the valley under clear sky conditions. The inversion was lifted when strong southerly winds entered the valley from the gap mixing out the cold air in the valley. Synoptic-scale cold advection above the valley contributed to the inversion removal by weakening the inversion from aloft. Sensitivity experiments suggested that the WRF simulation of the CAP episode is more sensitive to large-scale initialization fields with North American Mesoscale Model outperforming North American Regional Reanalysis and National Centers for Environmental Prediction (NCEP) Eta analysis than to the choice of boundary layer parameterizations and land surface models.

The Persistent Cold-Air Pool Study

The Persistent Cold-Air Pool Study (PCAPS) was conducted in Utah's Salt Lake valley from 1 December 2010 to 7 February 2011. The field campaign's primary goal was to improve understanding of the physical processes governing the evolution of multiday cold-air pools (CAPs) that are common in mountain basins during the winter. Meteorological instrumentation deployed throughout the Salt Lake valley provided observations of the processes contributing to the formation, maintenance, and destruction of 10 persistent CAP episodes. The close proximity of PCAPS field sites to residences and the University of Utah campus allowed many undergraduate and graduate students to participate in the study. Ongoing research, supported by the National Science Foundation, is using the PCAPS dataset to examine CAP evolution. Preliminary analyses reveal that variations in CAP thermodynamic structure are attributable to a multitude of physical processes affecting local static stability: for example, synoptic-scale processes impact changes in temperatures and cloudiness aloft while variations in boundary layer forcing modulate the lower levels of CAPs. During episodes of strong winds, complex interactions between the synoptic and mesoscale f lows, local thermodynamic structure, and terrain lead to both partial and complete removal of CAPs. In addition, the strength and duration of CAP events affect the local concentrations of pollutants such as PM2.5.

Downslope Flows on a Low-Angle Slope and Their Interactions with Valley Inversions. Part I: Observations

AbstractThermally driven downslope flows were investigated on a low-angle (1.6°) slope on the west side of the floor of Utah’s Salt Lake Valley below the Oquirrh Mountains using data from a line of four tethered balloons running down the topographic gradient and separated by about 1 km. The study focused on the evolution of the temperature and wind structure within and above the slope flow layer and its variation with downslope distance. In a typical situation, on clear, undisturbed October nights a 25-m-deep temperature deficit of 7°C and a 100–150-m-deep downslope flow with a jet maximum speed of 5–6 m s−1 at 10–15 m AGL developed over the slope during the first 2 h following sunset. The jet maximum speed and the downslope volume flux increased with downslope distance. The downslope flows weakened in the late evening as the stronger down-valley flows expanded to take up more of the valley atmosphere and as ambient stability increased in the lower valley with the buildup of a nocturnal temperature inversion. Downslope flows over this low-angle slope were deeper and stronger than has been reported previously by other investigators, who generally investigated steeper slopes and, in many cases, slopes on the sidewalls of isolated mountains where the downslope flows are not subject to the influence of nighttime buildup of ambient stability within valleys.

Downslope Flows on a Low-Angle Slope and Their Interactions with Valley Inversions. Part II: Numerical Modeling

Abstract The characteristics of well-developed downslope winds observed by tethered balloon soundings at multiple locations over a low-angle slope in the Salt Lake Valley are studied using the Regional Atmospheric Modeling System (RAMS). The model successfully simulated the key properties of the observed wind and temperature structure and evolution and provided insight into the forcing mechanisms. The results show that, although the slope angle is only 1.6°, the buoyancy force associated with the local temperature perturbation caused by nocturnal cooling of the slope surface is capable of producing the unusually strong and deep downslope winds observed by the tethersondes. The hypothesis that the flow is produced locally by the temperature deficit is further confirmed by analysis of the momentum budget that indicates a very small contribution from advection to the downslope mass flux. The analysis also reveals the importance of the along-slope pressure gradient force, which has been neglected by some previous investigators. On an isolated slope, the pressure gradient force, which develops as the downslope-flow layer deepens with downslope distance, is important mostly in the upper part of the downslope wind layer where it counterbalances the buoyancy force. On a slope in a valley, the pressure gradient force interacts with the valley inversion to produce intermittency in the downslope jet and may also significantly slow the flow as the inversion strengthens during the night. The simulations for two different observational nights indicate that the maximum downslope wind speed is sensitive to ambient stability, with near-neutral ambient stability yielding a stronger downslope jet than does a more stable ambient atmosphere. Sensitivity studies suggest that an increase in down-valley winds leads to a decrease in the maximum downslope wind speed and an increase in the thickness of the downslope wind layer. An increase in slope roughness, on the other hand, increases the height of the downslope jet but has little effect on other properties. The downslope wind is stronger over a gentle 1.6° slope than over a much steeper slope of 11°, mainly because of the combination of the stronger buoyancy and weaker pressure gradient over the gentle slope.

Inferring biogenic and anthropogenic carbon dioxide sources across an urban to rural gradient

We continuously monitored CO(2) concentrations at three locations along an urban-to-rural gradient in the Salt Lake Valley, Utah from 2004 to 2006. The results showed a range of CO(2) concentrations from daily averages exceeding 500 p.p.m. at the city center to much lower concentrations in a non-urbanized, rural region of the valley. The highest values were measured in the wintertime and under stable atmospheric conditions. At all three sites, we utilized weekly measurements of the C and O isotope composition of CO(2) for a 1-year period to evaluate the CO(2) sources underlying spatial and temporal variability in CO(2) concentrations. The results of an inverse analysis of CO(2) sources and the O isotope composition of ecosystem respiration (delta(18) O (R)) showed large contributions (>50%) of natural gas combustion to atmospheric CO(2) in the wintertime, particularly at the city center, and large contributions (>60%) of biogenic respiration to atmospheric CO(2) during the growing season, particularly at the rural site. delta(18) O (R) was most enriched at the rural site and more isotopically depleted at the urban sites due to the effects of irrigation on ecosystem water pools at the urban sites. The results also suggested differences in the role of leaf versus soil respiration between the two urban sites, with seasonal variation in the contribution of leaf respiration at a residential site and relatively constant contributions of leaf respiration at the city center. These results illustrate that spatial and temporal patterns of urban CO(2) concentrations and isotopic composition can be used to infer patterns of energy use by urban residents as well as plant and soil processes in urban areas.

Lake-Breeze Fronts in the Salt Lake Valley

AbstractWinds at the Salt Lake City International Airport (SLC) during the April–October period from 1948 to 2003 have been observed to shift to the north (up-valley direction) between late morning and afternoon on over 70% of the days without precipitation. Lake-breeze fronts that develop as a result of the differential heating between the air over the nearby Great Salt Lake and that over the lake’s surroundings are observed at SLC only a few times each month. Fewer lake-breeze fronts are observed during late July–early September than before or after that period. Interannual fluctuations in the areal extent of the shallow Great Salt Lake contribute to year-to-year variations in the number of lake-breeze frontal passages at SLC. Data collected during the Vertical Transport and Mixing Experiment (VTMX) of October 2000 are used to examine the structure and evolution of a lake-breeze front that moved through the Salt Lake Valley on 17 October. The onset of upslope and up-valley winds occurred within the valley prior to the passage of the lake-breeze front. The lake-breeze front moved at roughly 3 m s−1 up the valley and was characterized near the surface by an abrupt increase in wind speed and dewpoint temperature over a distance of 3–4 km. Rapid vertical mixing of aerosols at the top of the 600–800-m-deep boundary layer was evident as the front passed.

Dispersion of Perfluorocarbon Tracers within the Salt Lake Valley during VTMX 2000

Abstract Six perfluorocarbon tracer experiments were conducted in Salt Lake City, Utah, during October 2000 as part of the Vertical Transport and Mixing (VTMX) field campaign. Four tracers were released at different sites to obtain information on dispersion during stable conditions within down-valley flow, canyon outflow, and interacting circulations in the downtown area. Some of the extensive tracer data that were collected are presented in the context of the meteorological field campaign measurements. Tracer measurements at building-top sites in the downtown area and along the lower slopes of the Wasatch Front indicated that vertical mixing processes transported material up to at least 180 m above the valley floor, although model simulations suggest that tracers were transported upward to much higher elevations. Tracer data provided evidence of downward mixing of canyon outflow, upward mixing within down-valley flow, horizontal transport above the surface stable layer, and transport within horizontal eddies produced by the interaction of canyon and down-valley flows. Although point meteorological measurements are useful in evaluating the forecasts produced by mesoscale models, the tracer data provide valuable information on how the time-varying three-dimensional mean and turbulent motions over urban and valley spatial scales affect dispersion. Although the mean tracer transport predicted by the modeling system employed in this study was qualitatively similar to the measurements, improvements are needed in the treatment of turbulent vertical mixing.

Katabatic Flow Mechanisms on a Low-Angle Slope

Abstract Momentum and heat budget equations for katabatic flows on sloping surfaces are revisited. Terms in these equations are evaluated using wind and potential temperature data from four tethered-balloon data collection systems on a 3-km line running down a 1.6° slope at the foot of the Oquirrh Mountains in Utah’s Great Salt Lake valley. The analyses focus on the development with downslope distance of the katabatic flow and the associated negatively buoyant layer under synoptically undisturbed conditions. With strong ambient stratification, the katabatic flow shows little variation between sites, suggesting a state close to local equilibrium. When the ambient stratification is weaker, the acceleration of the katabatic flow between two tethersonde sites is systematically larger than what would be predicted based on observed buoyancy. Comparison of observed flow direction with the local topographic gradient indicates that slope curvature, associated with small deviations from the basically planar slope, may be responsible for the anomalous increase. It is concluded that the cross-slope homogeneity of the flow, which is assumed in simplified katabatic flow models, may be significantly disturbed even on slopes that appear to be planar to the observer.

Predictability of Low-Level Winds by Mesoscale Meteorological Models

Abstract This study describes the verification of model-based, low-level wind forecasts for the area of the Salt Lake valley and surrounding mountains during the 2002 Salt Lake City, Utah, Winter Olympics. Standard verification statistics (such as bias and mean absolute error) for wind direction and speed were compared for four models: the Eta, Rapid Update Cycle (RUC-2), and Global Forecast System of the National Centers for Environmental Prediction, and the fifth-generation Pennsylvania State University–NCAR Mesoscale Model (MM5). Even though these models had horizontal grid increments that ranged over almost two orders of magnitude, the highest-resolution MM5 with a 1.33-km grid increment exhibited a forecast performance similar to that of the other models in terms of grid-average, conventional verification metrics. This is in spite of the fact that the MM5 is the only model capable of reasonably representing the complex terrain of the Salt Lake City region that exerts a strong influence on the local circulation patterns. The purpose of this study is to investigate why the standard verification measures did not better discriminate among the models and to describe alternative measures that might better represent the ability of high-horizontal-resolution models to forecast locally forced mesogamma-scale circulations. The spatial variability of the strength of the diurnal forcing was quantified by spectrally transforming the time series of wind-component data for each observation location. The amount of spectral power in the band with approximately a diurnal period varied greatly from place to place, as did the amount of power in the bands with periods longer (superdiurnal) and shorter (subdiurnal) than the diurnal. It is reasonable that the superdiurnal power is largely in the synoptic-scale motions, and thus can be reasonably predicted by all the models. In contrast, the subdiurnal power is mainly in nondiurnally forced small-scale fluctuations that are generally unpredictable with any horizontal resolution because they are unobserved in three dimensions by the observation network. A strong positive relationship is demonstrated between the strength of the local forcing at each observation location, as measured by the spectral power in the diurnal band of the wind component time series, and forecast skill, as reflected by an alternative verification metric, a measure of anomaly correlation. However, the mean-absolute error showed no relationship to the power in the diurnal band. Two other measures of comparison among the low-level wind forecasts, the direction climatology and the spatial variance, showed a positive correlation between forecast quality and horizontal resolution.

Stably Stratified Flows near a Notched Transverse Ridge across the Salt Lake Valley

This paper describes observed and simulated interactions among atmospheric forcing, cold-pool development, and complex mountain terrain at the south end of the Salt Lake valley, near the Jordan Narrows and the Traverse Range. The Advanced Regional Prediction System (ARPS), a three-dimensional, nonhydrostatic compressible new-generation large-eddy simulation code in generalized terrain-following coordinates with advanced model parameterizations, was used. Past studies showed that a finer resolution produces more accurate simulations, and so this study used six one-way nested grids to resolve the complex topography. Horizontal grid spacing ranged from 20 km (initialized by Eta 40-km operational analyses) to 250 m; the finest grid had 200 vertically stretched levels between 5 m and 20 km above the surface. Two intense operating periods with weak synoptic forcing, stable stratification, and pronounced nighttime drainage were selected for simulation from the October 2000 Vertical Transport and Mixing (VTMX) experiment. Qualitative agreement between simulations and observations at four stations was good. Usually, the quantitative agreement was also good. Finer horizontal and vertical resolution improved agreement, capturing daytime and nighttime temperature structures, including inversion-layer strength. The simulations showed a complex flow near the Jordan Narrows, with hydraulic jumps and internal waves initiated by the Traverse Range to either side.

An Evaluation of Mesoscale Model Predictions of Down-Valley and Canyon Flows and Their Consequences Using Doppler Lidar Measurements during VTMX 2000

Abstract A mesoscale model, a Lagrangian particle dispersion model, and extensive Doppler lidar wind measurements during the Vertical Transport and Mixing (VTMX) 2000 field campaign were used to examine converging flows over the Salt Lake valley in Utah and their effect on vertical mixing at night and during the morning transition period. The simulated wind components were transformed into radial velocities to make a direct comparison with about 1.3 million Doppler lidar data points and to evaluate critically the spatial variations in the simulated wind fields aloft. The mesoscale model captured reasonably well the general features of the observed circulations, including the daytime up-valley flow; the nighttime slope, canyon, and down-valley flows; and the convergence of the flows over the valley. When there were errors in the simulated wind fields, they were usually associated with the timing, structure, or strength of specific flows. The simulated flow reversal during the evening transition period prod...

Forecasts of Valley Circulations Using the Terrain-Following and Step-Mountain Vertical Coordinates in the Meso-Eta Model

Abstract The nonhydrostatic version of the NCEP Meso-Eta Model is used to perform simulations that differ by only the vertical coordinate to determine the differences in forecasted valley circulations associated with the step-mountain and terrain-following vertical coordinates and whether one coordinate produces consistently superior forecasts at meso-γ and micro-α scales. A horizontal grid spacing of 850 m is used. The model forecasts are evaluated using data from the October 2000 Vertical Transport and Mixing (VTMX) field campaign in the Salt Lake valley. The forecasts of the diurnal evolution of the dominant circulations in the Salt Lake valley, including valley, slope, and canyon flows, and their modification by synoptic forcing during five intensive observation periods, were qualitatively similar to the measurements. Forecasts produced by the step-mountain and terrain-following vertical coordinates each have their own advantages and disadvantages and neither vertical coordinate outperformed the other...

An Evaluation of the MM5, RAMS, and Meso-Eta Models at Subkilometer Resolution Using VTMX Field Campaign Data in the Salt Lake Valley

Abstract This study presents what is, to the authors' knowledge, the first intercomparison and evaluation of three state-of-the-art mesoscale numerical models, the fifth-generation Pennsylvania State University–NCAR Mesoscale Model (MMS), the Regional Atmospheric Modeling System (RAMS), and the NCEP Meso-Eta, at horizontal resolution finer than 1 km. Simulations were carried out for both weak and strong synoptic forcing cases during the Vertical Transport and Mixing (VTMX) field campaign conducted in the Salt Lake valley in October of 2000. Both upper-air and surface observations at high spatial and temporal resolution were used to evaluate the simulations with a focus on boundary layer structures and thermally driven circulations that developed in the valley. Despite differences in the coordinate systems, numerical algorithms, and physical parameterizations used by the three models, the types of forecast errors were surprisingly similar. The common errors in predicted valley temperature structure include...

Using noble gases to investigate mountain-front recharge

Mountain-front recharge is a major component of recharge to inter-mountain basin-fill aquifers. The two components of mountain-front recharge are (1) subsurface inflow from the mountain block (subsurface inflow), and (2) infiltration from perennial and ephemeral streams near the mountain front (stream seepage). The magnitude of subsurface inflow is of central importance in source protection planning for basin-fill aquifers and in some water rights disputes, yet existing estimates carry large uncertainties. Stable isotope ratios can indicate the magnitude of mountain-front recharge relative to other components, but are generally incapable of distinguishing subsurface inflow from stream seepage. Noble gases provide an effective tool for determining the relative significance of subsurface inflow, specifically. Dissolved noble gas concentrations allow for the determination of recharge temperature, which is correlated with recharge elevation. The nature of this correlation cannot be assumed, however, and must be derived for the study area. The method is applied to the Salt Lake Valley Principal Aquifer in northern Utah to demonstrate its utility. Samples from 16 springs and mine tunnels in the adjacent Wasatch Mountains indicate that recharge temperature decreases with elevation at about the same rate as the mean annual air temperature, but is on average about 2 °C cooler. Samples from 27 valley production wells yield recharge elevations ranging from the valley elevation (about 1500 m) to mid-mountain elevation (about 2500 m). Only six of the wells have recharge elevations less than 1800 m. Recharge elevations consistently greater than 2000 m in the southeastern part of the basin indicate that subsurface inflow constitutes most of the total recharge in this area.

Potential effects of ozone, climate, and spruce budworm on Douglas-fir growth in the Wasatch Mountains

This study assessed the potential for ozone injury to vegetation in the central Wasatch Mountains of Utah by determining if Douglas-fir (Pseudotsuga menziesii var. glauca (Beissn.) Franco) experienced reduced growth since the onset of high ozone concentrations. Dendrochronological techniques were used to model Douglas-fir growth in six central Wasatch Mountain stands, where ozone concentrations suggest the possibility of injury to vegetation, and in four Bear River stands, where ozone concentrations are lower than injury thresholds. Previous-year growth, temperature, and Palmer's Z index variables explained 53–70% of the variance in annual growth. Reduced Douglas-fir growth, not explained by the model or insect and disease records, occurred after 1970 in the central Wasatch Mountains but not in the Bear River Mountains. Douglas-fir growth in the central Wasatch Mountains was slightly negatively correlated with the previous-year ozone concentrations in nearby Salt Lake valley. Between 1962 and 1973, a fluctuating budworm population due to varying spring precipitation was suggested to have replaced precipitation as the factor limiting tree growth, causing an anomalous negative relationship between precipitation and Douglas-fir growth. Because of a suite of possible factors (e.g., undetected climatic variables, fungal pathogens, competitive interactions) influencing Douglas-fir growth, ozone may be one factor in a larger stressor complex reducing growth.

THE VTMX 2000 CAMPAIGN

A month-long meteorological field campaign sponsored by the Department of Energy's Environmental Meteorology Program was conducted during October 2000 in the Salt Lake Valley to study vertical transport and mixing (VTMX) processes. The goals of the program are to increase our understanding of these processes, to improve our ability to measure and characterize them, and to incorporate that improved knowledge into conceptual and numerical models that can be used to describe and predict them. The program is currently concentrating on nocturnal stable periods and morning and evening transition periods, and it is further focused on urban areas located in valleys, basins, or other settings affected by nearby elevated terrain. Approximately 75 people participated in the campaign. The campaign featured a wide range of remote sensing and in situ measurements, including those from six radar wind profilers, six sodars, five radio acoustic sounding systems, a Doppler lidar, two aerosol lidars, and a water vapor lidar, as many as 22 rawinsonde soundings per Intensive Observing Period (IOP), and the simultaneous release of up to seven perfluorocarbon tracers. Preliminary results show the existence of strong cold pools forming over the valley center with significant wind shear aloft and intermittent turbulence close to themore » surface, a heat island over the downtown area at night and areas with substantially cooler temperatures nearby, regions of strong convergence and divergence affected by a narrow jet through a gap in the mountains to the south and flows out of the canyons to the east, and extensive wave activity.« less

Evaluation of Pile Capacity Prediction Methods Based on Cone Penetration Testing Using Results from I-15 Load Tests

Nine axial pile load tests were performed in connection with the reconstruction of I-15 through Salt Lake Valley, Utah. In addition, load test data were collected for seven other piles in the valley. The soil profile at these test sites generally consisted of saturated soft to medium stiff silts and clays underlain by a dense sand bearing stratum at 25 to 30 m. Test piles consisted of closed-end steel pipe piles 300 to 600 mm in diameter with a concrete fill. Because cone penetration soundings were available for 15 of these test piles, these data were used to evaluate the accuracy of five available design methods that are based on cone penetration testing. Methods evaluated were the Dutch, Schmertmann, Tumay, Laboratoire des Ponts et Chaussées (LPC), and Fellenius methods. The Davisson method was used to define ultimate capacity. For most design methods, the estimated skin friction was on average only 40 to 60 percent of the measured resistance. The Dutch method predicted 82 percent of measured skin friction, on average, but still exhibited significant scatter for individual tests. Underprediction of skin friction by the LPC method was caused by unnecessarily severe restrictions for steel piles. Without these restrictions, the standard error was cut in half and estimated capacity was 96 percent of the measured capacity, on average. The LPC method also provided the best match with measured end bearing resistance (within 11 percent on average), whereas other methods overestimated capacity by 60 to 80 percent. Overall, the LPC method without side friction limitations gave 97 percent of the measured capacity on average and had the lowest standard error of any of the methods.

A Comparison of Indoor/Outdoor PM10Concentrations Measured at Three Hospitals and a Centrally Located Monitor in Utah

Abstract This research measured daily 24-hour PM10 concentrations at various locations in Salt Lake City, Utah, from November 1994 to May 1995. Between four and six indoor locations were sampled at each of three hospitals. Indoor data were compared with outdoor roof data from each respective hospital and with the city's central monitoring location. The study goals were to: (1) evaluate variation of PM10 concentrations at four different outdoor sampling locations across the Salt Lake Valley; (2) determine if a centrally located monitor can predict PM10 concentrations across the valley; (3) compare indoor/outdoor roof concentrations at three hospitals to determine if an outdoor roof sampler can be used to predict indoor concentrations; (4) evaluate variation in PM10 concentrations inside hospitals to see if a single indoor sampler can estimate exposure for an entire hospital; and (5) determine if a centrally located monitor can predict indoor concentrations in the three hospitals. Results from outdoor sampl...