Kristi A. Gebhart

Examining the relationship between atmospheric aerosols and light extinction at Mount Rainier and North Cascades National Parks

Abstract During the summer of 1990, the National Park Service carried out a study in the state of Washington called the Pacific Northwest Regional Visibility Experiment using Natural Tracers (PREVENT). The goal of the study was to apportion atmospheric aerosols to scattering and extinction and to source types at Mount Rainier and North Cascades National Parks. The study was designed to collect all necessary emissions, meteorology, ambient concentrations, and atmospheric optical data necessary to support a variety of source attribution techniques. This paper will report on the apportionment of various aerosol species to measured fine mass concentrations and ambient scattering coefficients. One highlight of this study was the near-ambient measurement of atmospheric scattering with a modified integrating nephelometer. It is therefore possible to explore the relationship between hygroscopic aerosols and scattering in the ambient atmosphere.

Back-trajectory analyses of fine particulate matter measured at Big Bend National Park in the historical database and the 1996 scoping study.

Analyses of the sources of fine particles associated with visibility reduction at Big Bend National Park during a 10-year period from 1989-1998 and from a regional visibility scoping study conducted during September and October 1996 at 19 sites in Texas and Mexico are summarized and compared. Fine sulfate particles are the largest fraction of the fine mass, and scattering by sulfates is estimated to be nearly half of the non-Rayleigh light extinction at Big Bend. Fine particulate sulfur concentrations are seasonal, with the highest values occurring during the summer and fall when back trajectory analyses show that air masses are most likely to arrive at Big Bend from the southeast after passing over Mexico or from areas to the northeast including east Texas. Episodically, high concentrations of fine mass and high light extinction values can be due to other species such as fine organic carbon or blowing soil dust. Organic carbon concentrations are often extremely high during the spring, especially during May. A combination of back trajectory analyses and the coincidence of high organic carbon and high non-soil potassium concentrations leads to the hypothesis that these concentrations are due to fires, primarily seasonal agricultural burning in Mexico and Central America. Fine soil concentrations often reach values that are twice the annual mean during July. These concentrations also frequently have high Al/Ca ratios, indicative of Saharan dust. Back trajectories associated with these events show air masses arriving from the southeast and are consistent with the hypothesis of transport of air masses from Africa during July. There is a high frequency of transport of air masses from Mexico to Big Bend, especially during the summer when fine mass concentrations and light extinction are highest. Therefore, sources and potential sources of sulfates and other fine particles in Mexico, particularly in areas southeast of the park have a high likelihood of contributing to visibility degradation at the park. Source areas to the northeast of the park, in east Texas and farther upwind also contribute to high fine sulfate concentrations.

Directional Biases in Back Trajectories Caused by Model and Input Data

Abstract Back trajectory analyses are often used for source attribution estimates in visibility and other air quality studies. Several models and gridded meteorological datasets are readily available for generation of trajectories. The Big Bend Regional Aerosol and Visibility Observational (BRAVO) tracer study of July to October 1999 provided an opportunity to evaluate trajectory methods and input data against tracer concentrations, particulate data, and other source attribution techniques. Results showed evidence of systematic biases between the results of different back trajectory model and meteorological input data combinations at Big Bend National Park during the BRAVO. Most of the differences were because of the choice of meteorological data used as input to the trajectory models. Different back trajectories also resulted from the choice of trajectory model, primarily because of the different mechanisms used for vertical placement of the trajectories. No single model or single meteorological data set was found to be superior to the others, although rawinsonde data alone are too sparse in this region to be used as the only input data, and some combinations of model and input data could not be used to reproduce known attributions of tracers and simulated sulfate.

An investigation of the dominant source regions of fine sulfur in the western United States and their areas of influenve

Abstract Two methods for analyzing source-receptor relationships are used to determine the dominant source regions of particulate sulfur in the western U.S. The first method, area of influence analysis, is based on the residence time of back trajectory endpoints. The other method, principal component analysis, involves examination of the gradients of the spatial eigenvectors of fine sulfur concentrations to determine source regions. Results of both methods are similar and show that southern California, northeastern Mexico, and the large coal-fired power plants in the Four Corners region contribute most strongly to the long-range transport of fine sulfur into remote areas of the western U.S. Several sources with smaller areas of influence are also identified.

Examination of the effects of sulfate acidity and relative humidity on light scattering at Shenandoah National Park

Abstract Sulfate aerosols in the eastern United States of America are known to be acidic, at least on an episodic basis. Intensive particle and optical measurements made during a special study at Shenandoah National Park in the summer of 1991 are used to examine the acidity and how it influences light scattering. Reconstructed fine mass, calculated by summing the constituents, matches measured mass quite well if water associated with sulfates is included. However, reconstructed scattering is much lower than measured even when acidity-dependent mass and scattering efficiencies are used for sulfates. It is hypothesized that the mass of organic particles may be underestimated and/or organics may be hygroscopic. Uncertainty in relative humidity is also fairly large. Reduction of this uncertainty may allow closer agreement between reconstructed and measured scattering in future studies. Sulfates were found to be about 1 2 neutralized on average during this study.

Earlier onset of the spring fine dust season in the southwestern United States

Particulate matter (PM)2.5 dust concentrations (mineral particles with aerodynamic diameters less than 2.5 µm) typically peak in spring and early summer at rural and remote sites across the southwestern United States. Trend analyses indicate that springtime regional mean PM2.5 dust concentrations have increased from 1995 to 2014, especially in March (5.4% yr−1, p < 0.01). This increase reflects an earlier onset of the spring dust season across the Southwest by 1 to 2 weeks over the 20 year time period. March dust concentrations were strongly correlated with the Pacific Decadal Oscillation index (r = −0.65, p < 0.01), which was mostly in its negative phase from 2007 to 2014, during which the region was drier, windier, and less vegetated. The positive spring trend and its association with large-scale climate variability have several important implications for visibility, particulate matter, health effects, and the hydrologic cycle in the region.

Diurnal and seasonal patterns in light scattering, extinction, and relative humidity

Abstract Since 1988, several federal and state governmental agencies in the US have coordinated efforts to operate the interagency monitoring of protected visual environments (IMPROVE) network at sites in remote areas. Most IMPROVE sites are equipped with either a transmissometer to measure light extinction (Bext) or a nephelometer to measure particle scattering (Bsp). Optical, temperature, and relative humidity (RH) measurements are made hourly at these sites. The diurnal and seasonal patterns in these data are examined and discussed here. At many IMPROVE sites the diurnal patterns in RH and therefore Bext or Bsp are as expected based on average temperature. On average, RH is higher at night and during the winter than during warmer times of the day and year. Also as expected, based on RH alone, at many sites hourly mean Bext or Bsp values are either in phase with RH or weakly dependent on time of day. Usually, the diurnal differences are not as large as the seasonal differences. Another group of IMPROVE sites have mean RH patterns similar to those described above but have a different diurnal pattern in measured scattering or extinction. At these sites, the highest mean Bsp or Bext occurs during mid-day rather than at night. At several of these sites, especially those on ridge tops, it is hypothesized that this is because the diurnal shifts in mixing height only allow the surface layer of the atmosphere to reach the monitor during mid-day. Several other sites have unique diurnal or seasonal patterns in average Bsp or Bext that can usually be linked to emissions in nearby source regions or local meteorology and terrain.

Source Apportionment of Sulfur and Light Extinction Using Receptor Modeling Techniques

Most visibility impairment is associated with sulfates, carbonaceous material, and soil-related material.1 Therefore, any visibility source apportionment scheme must address both secondary and primary aerosols. The chemical mass balance (CMB) formalism is usually used to apportion primary particles. It relies on known physical and chemical characteristic aerosols, such as ratios of tracer species, natural or man-made, at the receptors and sources to attribute aerosols to single sources or source types. CMB modeling apportions aerosol species on a sampling-period- by-sampling-period basis. However, if the data set contains an adequate number of samples, regressional techniques, along with less restrictive assumptions, can be used to estimate apportionment of secondary as well as primary species. In a regressional approach, the secondary species is the dependent variable, while the independent variables are tracers that are unique to a single source or group of sources. A key assumption associated with this approach is that the chemical species used as tracers must be uniquely emitted by non-overlapping groups of sources. These techniques were successfully used to develop a semiquantitative apportionment of particulate sulfur, total sulfur (particle plus gaseous sulfur), absorption, and extinction to source categories at receptor sites near the Grand Canyon using data gathered in a special study called Project MOHAVE (Measurement of Haze and Visual Effects). Regression models were used to develop links between trace elements and visibility variables and then to link the trace elements to source categories using CMB analysis. As part of the CMB analysis, a new technique was developed for verifying and/or extracting source profiles from the ambient data set. About 50% of the measured particle sulfur is attributable to coal-fired power plants during summer and winter months, while in the winter months, about 50% of the particle sulfur may be associated with primary sulfur emissions from burning activity and urban emissions during the summer. A variable that is responsible for over 30% of the extinction, babs, is predominately associated with burning activity during the winter and to burning, transportation, and suspended soil during the summer months.

Aerosol species concentrations and source apportionment of ammonia at Rocky Mountain National Park

Changes in ecosystem function at Rocky Mountain National Park (RMNP) are occurring because of emissions of nitrogen and sulfate species along the Front Range of the Colorado Rocky Mountains, as well as sources farther east and west. The nitrogen compounds include both oxidized and reduced nitrogen. A year-long monitoring program of various oxidized and reduced nitrogen species was initiated to better understand their origins as well as the complex chemistry occurring during transport from source to receptor. Specifically, the goals of the study were to characterize the atmospheric concentrations of nitrogen species in gaseous, particulate, and aqueous phases (precipitation and clouds) along the east and west sides of the Continental Divide; identify the relative contributions to atmospheric nitrogen species in RMNP from within and outside of the state of Colorado; identify the relative contributions to atmospheric nitrogen species in RMNP from emission sources along the Colorado Front Range versus other areas within Colorado; and identify the relative contributions to atmospheric nitrogen species from mobile sources, agricultural activities, and large and small point sources within the state of Colorado. Measured ammonia concentrations are combined with modeled releases of conservative tracers from ammonia source regions around the United States to apportion ammonia to its respective sources, using receptor modeling tools. Implications: Increased deposition of nitrogen in RMNP has been demonstrated to contribute to a number of important ecosystem changes. The rate of deposition of nitrogen compounds in RMNP has crossed a crucial threshold called the “critical load.” This means that changes are occurring to park ecosystems and that these changes may soon reach a point where they are difficult or impossible to reverse. Several key issues need attention to develop an effective strategy for protecting park resources from adverse impacts of elevated nitrogen deposition. These include determining the importance of previously unquantified nitrogen inputs within the park and identification of important nitrogen sources and transport pathways.

Reconciliation and Interpretation of Big Bend National Park Particulate Sulfur Source Apportionment: Results from the Big Bend Regional Aerosol and Visibility Observational Study—Part I

Abstract The Big Bend Regional Aerosol and Visibility Observational (BRAVO) study was an intensive monitoring study from July through October 1999 followed by extensive assessments to determine the causes and sources of haze in Big Bend National Park, located in Southwestern Texas. Particulate sulfate compounds are the largest contributor of haze at Big Bend, and chemical transport models (CTMs) and receptor models were used to apportion the sulfate concentrations at Big Bend to North American source regions and the Carbón power plants, located 225 km southeast of Big Bend in Mexico. Initial source attribution methods had contributions that varied by a factor of ≥2. The evaluation and comparison of methods identified opposing biases between the CTMs and receptor models, indicating that the ensemble of results bounds the true source attribution results. The reconciliation of these differences led to the development of a hybrid receptor model merging the CTM results and air quality data, which allowed a nearly daily source apportionment of the sulfate at Big Bend during the BRAVO study. The best estimates from the reconciliation process resulted in sulfur dioxide (SO2) emissions from U.S. and Mexican sources contributing ~55% and 38%, respectively, of sulfate at Big Bend. The distribution among U.S. source regions was Texas, 16%; the Eastern United States, 30%; and the Western United States, 9%. The Carbón facilities contributed 19%, making them the largest single contributing facility. Sources in Mexico contributed to the sulfate at Big Bend on most days, whereas contributions from Texas and Eastern U.S. sources were episodic, with their largest contributions during Big Bend sulfate episodes. On the 20% of the days with the highest sulfate concentrations, U.S. and Mexican sources contributed ˜71% and 26% of the sulfate, respectively. However, on the 20% of days with the lowest sulfate concentrations, Mexico contributed 48% compared with 40% for the United States.