Exploring spatial patterns of particulate sulfur and OMH from the Project MOHAVE summer intensive regional network using analyses of variance techniques and meteorological parameters as sort determinants.

Project MOHAVE (Measurements of Haze and Visual Effects) encompassed a 1-yr field study in the southwestern United States from September 1991 through August 1992. The congressionally mandated study was a joint partnership between the U.S. Environmental Protection Agency, Southern California Edison, and the National Park Service. A major objective of this study was to quantify the potential haze impacts on the nearby Grand Canyon National Park from the 1580 MW coal-fired MOHAVE Power Project (MPP). Any regional impacts from MPP were from secondary fine sulfate. In this paper, we explore the temporal and spatial patterns of particulate sulfur (Sp) and "organic mass by hydrogen" (OMH) during the summer intensive, conducted from mid-July through the end of August 1992. Using an innovative hierarchical pattern recognition classification scheme, we developed 6 groups of Sp and 8 groups of OMH temporally similar behaving patterns in the sampling region. From a regional understanding of synoptic meteorology, these Sp patterns were explainable. We observed two regional gradients. One gradient was a west-to-east decreasing gradient, most likely the result of major sources from urban southern California, including the San Joaquin Valley. The other decreasing gradient was from south-to-north, perhaps the result of emissions emanating from the large urban centers in northern Mexico. The patterns for OMH were not as regionally homogeneous as the patterns for Sp. A west-to-east decreasing gradient was observed for OMH, along with reduced values in the lower Colorado River Valley and some higher values in central and eastern Arizona. The west-to-east decreasing gradient suggests the presence of the Los Angeles urban plume, while the higher values in central and eastern Arizona may be due to biogenic emissions and increased seasonal fires.

Characterization of regional transport and dispersion using Project MOHAVE tracer data.

Perfluorocarbon tracers were released continuously from several surface locations and one power plant stack location during the winter (30 days) and summer (50 days) intensive studies as part of Project MOHAVE. Tracers were released in winter from the Mohave Power Plant (MPP) and Dangling Rope, UT, located on the shore of Lake Powell near Page, AZ; and in summer from MPP, the Tehachapi Pass between the Mojave Desert and the Central Valley in California, and El Centro, CA, on the California-Mexico border. At the Tehachapi tracer release site six-hour pulses of a separately identifiable perfluorocarbon tracer were released every four days in order to assess the time for the tracer to clear the monitoring network. Daily 24-hr integrated samples were collected at about 30 sites in four states. Limited tracer concentration data with higher time resolution is also available. Graphical displays and analyses identify several regional transport paths, including a convergence zone in the Mojave Desert, the importance of terrain channeling, especially in winter, and a relationship between 24-hr maximum influence function and distance that may prove useful as a scoping tool and to test regional scale air quality models. In winter, Dangling Rope tracer was routinely transported through the entire length of the Grand Canyon, while in summer, MPP tracer was routinely transported over most of Lake Mead.

Relating summer ambient particulate sulfur, sulfur dioxide, and light scattering to gaseous tracer emissions from the MOHAVE Power Project.

Project MOHAVE was initiated in 1992 to examine the role of emissions from the 1580 MW coal-fired MOHAVE Power Project (MPP) on haze at the Grand Canyon National Park (GCNP), located about 130 km north-north-east of the power plant. Statistical relationships were analyzed between summertime ambient concentrations of a gaseous perfluorocarbon tracer released from MPP and ambient SO2, particulate sulfur, and light scattering to evaluate whether MPP's emissions could be transported to the GCNP and then impact haze levels there. Spatial analyses indicated that particulate sulfur levels were strongly correlated across the monitoring network, regardless of whether the monitoring stations were upwind or downwind of MPP. This indicates that particulate sulfur levels in this region were influenced by distant regional emission sources. A significant particulate sulfur contribution from a point source such as MPP would result in a non-uniform pattern downwind. There was no suggestion of this in the data. Furthermore, correlations between the MPP tracer and ambient particulate sulfur and light scattering at locations in the park were virtually zero for averaging times ranging from 24 hr to 1 hr. Hour-by-hour MPP tracer levels and light scattering were individually examined, and still no positive correlations were detected. Finally, agreement between tracer and particulate sulfur did not improve as a function of meteorological regime, implying that, even during cloudy monsoon days when more rapid conversion of SO2 to particulate sulfur would be expected, there was no evidence for downwind particulate sulfur impacts. Despite the fact that MPP was a large source of SO2 and tracer, neither time series nor correlation analyses were able to detect any meaningful relationship between MPP's SO2 and tracer emission "signals" to particulate sulfur or light scattering.

Second generation chemical mass balance source apportionment of sulfur oxides and sulfate at the Grand Canyon during the Project MOHAVE summer intensive.

Receptor-based chemical mass balance (CMB) analysis techniques are designed to apportion species that are conserved during pollutant transport using conserved source profiles. The techniques will fail if non-conservative species (or profiles) are not properly accounted for in the CMB model. The straightforward application of the CMB model developed for Project MOHAVE using regional profiles resulted in a significant under-prediction of total sulfate oxides (SOx, SO2 plus fine particulate sulfate) for many samples at Meadview, AZ. In addition, for these samples the concentration of the inert tracer emitted from the MOHAVE Power Project (MPP), ocPDCH, was also under-predicted. A second-generation model has been developed which assumes that separation of particles and SO2 can occur in the MPP plume during nighttime stable plume conditions. This second-generation CMB model accounts for all SOx present at the various receptor sites. In addition, the concentrations of ocPDCH and the presence of other inert tracers of emission from regional sources are accurately predicted. The major source of SOx at Meadview was the MPP, but the major source of sulfate at this site was the Las Vegas urban area. At Hopi Point in the Grand Canyon, the Baja California region (Imperial Valley and northwestern Mexico) was the major source of both SOx and sulfate.

Evaluation of wind fields used in Grand Canyon Visibility Transport Commission analyses.

The Grand Canyon Visibility Transport Commission (GCVTC) was established by the U.S. Congress to assess the potential impacts of projected growth on atmospheric visibility at Grand Canyon National Park and to make recommendations to the U.S. Environmental Protection Agency on what measures could be taken to avoid such adverse impacts. A critical input to the assessment tool used by the commission was three-dimensional model-derived wind fields used to transport the emissions. This paper describes the evaluation of the wind fields used at various stages in the assessment. Wind fields evaluated included those obtained from the Colorado State University Regional Atmospheric Modeling System (RAMS), the National Meteorological Center's Nested Grid Model (NGM), and the National Oceanic and Atmospheric Administration's Atmospheric Transport and Dispersion (ATAD) trajectory model. The model-derived wind fields were evaluated at multiple vertical levels at several locations in the southwestern United States by determining differences between model predicted winds and winds that were measured using radiosonde and radar wind profiler data. Model-derived winds were also evaluated by determining the percent of time that they were within acceptable differences from measured winds. All models had difficulties, generally meeting the acceptable criteria for less than 50% of the predictions. The RAMS model had a persistent bias toward southwesterly winds at the expense of other directions, especially failing to represent channeling by north-south mountain ranges in the lower levels. The NGM model exhibited a substantial bias in the summer months by extending northwesterly winds in the eastern Pacific Ocean well inland, in contrast to the observed southwesterlies at inland locations. The simpler ATAD trajectory model performed somewhat better than the other models, probably because of its use of more upper air sites. The results of the evaluation indicated that these wind fields could not be used to reliably predict source-receptor impacts on a particular day; thus, seasonally averaged impacts were used in the GCVTC assessment.

Assessment of the nested grid model estimates for driving regional visibility models in the southwestern United States.

The Nested Grid Model (NGM) is a primitive-equation meteorological model that is routinely exercised over North America for forecasting purposes by the National Meteorological Center. While prognostic meteorological models are being increasingly used to drive air quality models, their use in conducting annual simulations requires significant resources. NGM estimates of wind fields and other meteorological variables provide an attractive alternative since they are typically archived and readily available for an entire year. Preliminary evaluation of NGM winds during the summer of 1992 for application to the region surrounding the Grand Canyon National Park showed serious shortcomings. The NGM winds along the borders between California, Arizona and Mexico tend to be northwesterly with a speed of about 6 m/sec, while the observed flow is predominantly southerly at about 2-5 m/sec. The mesoscale effect of a thermal low pressure area over the highly heated Southern California and western Arizona deserts does not appear to be represented by the NGM because of its coarse resolution and the use of sparse observations in that region. Tracer simulations and statistical evaluation against special high resolution observations of winds in the southwest United States clearly demonstrate the northwest bias in NGM winds and its adverse effect on predictions of an air quality model. The "enhanced" NGM winds, in which selected wind observations are incorporated in the NGM winds using a diagnostic meteorological model provide additional confirmation on the primary cause of the northwest bias. This study has demonstrated that in situations where limited resources prevent the use of prognostic meteorological models, previously archived coarse resolution wind fields in which additional observations are incorporated to correct known biases provide an attractive option.

Tracking Regional Background in a Haze Attribution Experiment.

Emissions from distant source areas are often imagined to provide a steady background to the emissions of whatever local sources are being studied. As part of Project MOHAVE in summer 1992, several air mass markers and an injected stack tracer were measured hourly near the Grand Canyon. Observed haze events generally coincided with transients in methylchloroform and water vapor, which we interpret as endemic tags for air from southern California and the subtropics. The results depict a dynamic regional background.

Development, Evaluation, and Application of a Primary Aerosol Model.

The Segmented-Plume Primary Aerosol Model (SPPAM) has been developed over the past several years. The earlier model development goals were simply to generalize the widely used Industrial Source Complex Short-Term (ISCST) model to simulate plume transport and dispersion under light wind conditions and to handle a large number of roadway or line sources. The goals have been expanded to include development of improved algorithm for effective plume transport velocity, more accurate and efficient line and area source dispersion algorithms, and recently, a more realistic and computationally efficient algorithm for plume depletion due to particle dry deposition. A performance evaluation of the SPPAM has been carried out using the 1983 PNL dual tracer experimental data. The results show the model predictions to be in good agreement with observations in both plume advection-dispersion and particulate matter (PM) depletion by dry deposition. For PM2.5 impact analysis, the SPPAM has been applied to the Rubidoux area of California. Emission sources included in the modeling analysis are: paved road dust, diesel vehicular exhaust, gasoline vehicular exhaust, and tire wear particles from a large number of roadways in Rubidoux and surrounding areas. For the selected modeling periods, the predicted primary PM2.5 to primary PM10 concentration ratios for the Rubidoux sampling station are in the range of 0.39-0.46. The organic fractions of the primary PM2.5 impacts are estimated to be at least 34-41%. Detailed modeling results indicate that the relatively high organic fractions are primarily due to the proximity of heavily traveled roadways north of the sampling station. The predictions are influenced by a number of factors; principal among them are the receptor locations relative to major roadways, the volume and composition of traffic on these roadways, and the prevailing meteorological conditions.

Background continental ozone levels in the rural U.S. southwest desert.

From 1969 through 1978 an extensive ambient air quality monitoring program, including the measurement of atmospheric concentrations of ozone, has been conducted in a remote section of the desert in the southwestern U.S. Using a Monitor Labs chemiluminescent ozone instrument, these levels were measured atop a small mountain 500 feet above the valley floor of the Colorado River. During the winter months, when the prevailing low level winds are northerly in this river valley, rural continental background levels of ozone are attainable. From this continuous ozone data base, 13 of these representative high pressure periods have been selected for analysis. Results indicate a very distinct diurnal trend with an ozone minimum occurring from 0800-1000 hours Mountain Standard Time (MST) and an ozone maximum from 1600-1900 hours MST. Ozone concentrations range from a low of 19 ppb to a high of 44 ppb with the diurnal pattern exhibiting marked repeatability with respect to time of minima and maxima, seasonal changes and concentration levels.