A seasonal nitrogen deposition budget for Rocky Mountain National Park.

Nitrogen deposition is a concern in many protected ecosystems around the world, yet few studies have quantified a complete reactive nitrogen deposition budget including all dry and wet, inorganic and organic compounds. Critical loads that identify the level at which nitrogen deposition negatively affects an ecosystem are often defined using incomplete reactive nitrogen budgets. Frequently only wet deposition of ammonium and nitrate are considered, despite the importance of other nitrogen deposition pathways. Recently, dry deposition pathways including particulate ammonium and nitrate and gas phase nitric acid have been added to nitrogen deposition budgets. However, other nitrogen deposition pathways, including dry deposition of ammonia and wet deposition of organic nitrogen, still are rarely included. In this study, a more complete seasonal nitrogen deposition budget was constructed based on observations during a year-long study period from November 2008 to November 2009 at a location on the east side of Rocky Mountain National Park (RMNP), Colorado, USA. Measurements included wet deposition of ammonium, nitrate, and organic nitrogen, PM2.5 (particulate matter with an aerodynamic diameter less than 2.5 microm, nitrate, and ammonium) concentrations of ammonium, nitrate, and organic nitrogen, and atmospheric gas phase concentrations of ammonia, nitric acid, and NO2. Dry deposition fluxes were determined from measured ambient concentrations and modeled deposition velocities. Total reactive nitrogen deposition by all included pathways was found to be 3.65 kg N x ha(-1) yr(-1). Monthly deposition fluxes ranged from 0.06 to 0.54 kg N x ha(-1)yr(-1), with peak deposition in the month of July and the least deposition in December. Wet deposition of ammonium and nitrate were the two largest deposition pathways, together contributing 1.97 kg N x ha(-1)yr(-1) or 54% of the total nitrogen deposition budget for this region. The next two largest deposition pathways were wet deposition of organic nitrogen and dry deposition of ammonia; combined they contributed 1.37 kg N x ha(-1)yr(-1) or 37% of the total nitrogen deposition budget. To better understand the nitrogen cycle and key interactions between the atmosphere and biosphere we need to include as many sources and types of nitrogen as possible and understand their variability throughout the year. Here we examine the components of the nitrogen deposition budget to better understand the factors that influence the different deposition pathways and their seasonal variations.

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.

Optical measurements of aerosol size distributions in Great Smoky Mountains National Park: dry aerosol characterization.

Aerosol size distributions were measured during the summertime 1995 Southeastern Aerosol and Visibility Study (SEAVS) in Great Smoky Mountains National Park using an Active Scattering Aerosol Spectrometer (ASASP-X) optical particle counter. We present an overview of the experimental method, our data inversion technique, timelines of the size distribution parameters, and calculations of dry accumulation mode aerosol density and refractive index. Aerosol size distributions were recorded during daylight hours for aerosol in the size range 0.1 < Dp < 2.5 microns. The particle refractive index used for the data inversion was calculated with the partial molar refractive index approach using 12-hr measured aerosol chemical composition. Aerosol accumulation mode volume concentrations ranging from 1 to 26 micron 3 cm-3 were observed, with an average of 7 +/- 5 micron 3 cm-3. The study average dry accumulation mode geometric volume median diameter was 0.27 +/- 0.03 micron, and the mean geometric standard deviation was 1.45 +/- 0.06. Using an internally mixed aerosol model, and assuming chemical homogeneity across the measured particle distribution, an average accumulation mode dry sulfate ion mass scattering efficiency of 3.8 +/- 0.6 m2 g-1 was calculated.

Estimates of particle hygroscopicity during the Southeastern Aerosol and Visibility Study.

Aerosol water content was determined from relative humidity controlled optical particle counter (ASASP-X) size distribution measurements made during the Southeastern Aerosol and Visibility Study (SEAVS) in the Great Smoky Mountains National Park during summer 1995. Since the scattering response function of the ASASP-X is sensitive to particle refractive index, a technique for calibrating the ASASP-X for any real refractive index was developed. A new iterative process was employed to calculate water mass concentration and wet refractive index as functions of relative humidity. Experimental water mass concentrations were compared to theoretically predicted values assuming only ammonium sulfate compounds were hygroscopic. These comparisons agreed within experimental uncertainty. Estimates of particle hygroscopicity using a rural aerosol model of refractive index as a function of relative humidity demonstrated no significant differences from those made with daily varying refractive index estimates. Although aerosol size parameters were affected by the assumed chemical composition, forming ratios of these parameters nearly canceled these effects.

Light scattering characteristics of aerosols as a function of relative humidity: Part I--A comparison of measured scattering and aerosol concentrations using the theoretical models.

The Southeastern Aerosol and Visibility Study (SEAVS) was undertaken to characterize the size-dependent composition, thermodynamic properties, and optical characteristics of the ambient atmospheric particles in the southeastern United States. The field portion of the study was carried out from July 15 to August 25, 1995. As part of the study a relative humidity controlled inlet was built to raise or lower the relative humidity to predetermined levels before the aerosol was passed into an integrating nephelometer or particle-sizing device. Five other integrating nephelometers were operated in various configurations, two of which were fitted with a 2.5 microns inlet. Fine particle (< 2.5 microns) samplers were operated to measure concentrations of sulfate, nitrate, and ammonium ions, organic and elemental carbon, and fine soil. Mass size distributions were measured with an eight-stage, single orifice cascade impactor. Four different strategies for estimating scattering were used. First, an externally mixed model with constant specific scattering coefficients, sulfate ion mass interpreted as ammonium bisulfate, and ammonium bisulfate growth as a function of relative humidity, is assumed. Second, an externally mixed aerosol model, assuming constant dry specific scattering but with sulfate ammoniation and associated composition-dependent hygroscopicity explicitly accounted for, is used. Third, an externally mixed aerosol model, but with sulfate ammoniation, associated growth as a function of relative humidity, and sulfate size distributions, is applied. Fourth, an internally mixed aerosol model with measured sulfur size distributions and estimated size distributions for other species is used with the growth characteristics of the mixture being estimated using the Zdanovskii-Stokes-Robinson (ZSR) assumptions. Only ionic species were considered to be hygroscopic. The second, third, and fourth approaches yield similar results with reconstructed scattering comparing quite favorably with measured scattering. Accounting for sulfate ammoniation and associated water uptake was the most important detail in achieving closure between measurements and modeled scattering. In general, differences between estimated scattering, assuming internally or externally mixed models, was small. These same models were used to estimate wet to dry scattering ratios. The R2 for an ordinary least-squares regression between measured and predicted ratios was high (0.71-0.92), and in most cases the scattering ratio was insensitive to modeling assumptions. However, during some sample periods differences between predicted scattering ratios for the different modeling assumptions were as high as 30%.

Aerosol light scattering measurements as a function of relative humidity.

The hygroscopic nature of atmospheric fine aerosol was investigated at a rural site in the Great Smoky Mountains National Park during July and August 1995. Passing the sample aerosol through an inlet, which housed an array of Perma Pure diffusion dryers, controlled the sample aerosol's relative humidity (RH). After conditioning the aerosol sample in the inlet, the light scattering coefficient and the aerosol size distribution were simultaneously measured. During this study, the conditioned aerosol's humidity ranged between 5% < RH < 95%. Aerosol response curves were produced using the ratio bspw/bspd; where bspw is the scattering coefficient measured at some RH greater than 20% and bspd is the scattering coefficient of the "dry" aerosol. For this work, any sample RH values below 15% were considered dry. Results of this investigation showed that the light scattering ratio increased continuously and smoothly over the entire range of relative humidity. The magnitude of the ratio at a particular RH value, however, varied considerably in time, particularly for RH values greater than approximately 60%. Curves of the scattering coefficient ratios as a function of RH were generated for each day and compared to the average 12-hour chemical composition of the aerosol. This comparison showed that for any particular RH value the ratio was highest during time periods of high sulfate concentrations and lowest during time periods of high soil or high organic carbon concentrations.

Light scattering characteristics of aerosols at ambient and as a function of relative humidity: Part II--A comparison of measured scattering and aerosol concentrations using statistical models.

The eastern United States national parks experience some of the worst visibility conditions in the nation. To study these conditions, the Southeastern Aerosol and Visibility Study (SEAVS) was undertaken to characterize the size-dependent composition, thermodynamic properties, and optical characteristics of the ambient atmospheric particles. It is a cooperative three-year study that is sponsored by the National Park Service and the Electric Power Research Institute and its member utilities. The field portion of the study was carried out from July 15 to August 25, 1995. The study design, instrumental configuration, and estimation of aerosol types from particle measurements is presented in a companion paper. In the companion paper, we compare measurements of scattering at ambient conditions and as functions of relative humidity to theoretical predictions of scattering. In this paper, we make similar comparisons, but using statistical techniques. Statistically derived specific scattering associated with sulfates suggest that a reasonable estimate of sulfate scattering can be arrived at by assuming nominal dry specific scattering and treating the aerosols as an external mixture with ammoniation of sulfate accounted for and by the use of Tang's growth curves to predict water absorption. However, the regressions suggest that the sulfate scattering may be underestimated by about 10%. Regression coefficients on organics, to within the statistical uncertainty of the model, suggest that a reasonable estimate of organic scattering is about 4.0 m2/g. A new analysis technique is presented, which does not rely on comparing measured to model estimates of scattering to evoke an understanding of ambient aerosol growth properties, but rather relies on measurements of scattering as a function of relative humidity to develop actual estimates of f(RH) curves. The estimates of the study average f(RH) curve for sulfates compares favorably with the theoretical f(RH) curve for ammonium bisulfate, which is in turn consistent with the study average sulfate ammoniation corresponding to a molar ratio of NH4/SO4 of approximately one. The f(RH) curve for organics is not significantly different from one, suggesting that organics are weakly to nonhygroscopic.

Interpretation of trends of PM25 and reconstructed visibility from the IMPROVE network.

Under the IMPROVE visibility monitoring network, federal land managers have monitored visibility and fine particle concentrations at 29 Class I area sites (mostly national parks and wilderness areas) and Washington, DC since 1988. This paper evaluates trends in reconstructed visibility and fine particles for the 10th (best visibility days), 50th (average visibility days), and 90th (worst visibility days) percentiles over the nine-year period from 1988-96. Data from these sites provides an indication of regional trends in air quality and visibility resulting from implementation of various emission reduction strategies.

An analysis of the yearly changes in sulfur concentrations at various national parks in the United States, 1980-1996.

An apparent increasing trend in the summer concentrations of particulate sulfur at Shenandoah (for the time period 1982-1995) and at Great Smoky Mountains (for the time period 1984-1995) has been pointed out by some researchers. Others have suggested that these increasing trends may be an analytical artifact resulting from the switch from the Stacked Filter Units (SFU) measurement system to the IMPROVE (Interagency Monitoring of Protected Visual Environments) measurement system that occurred during the winter of 1987. To obtain a better understanding of the effect of the protocol change, we investigate the changes in the seasonal averages of sulfur concentrations for successive pairs of years for the period 1980-1996 for about 20 national park sites in the United States. For the period 1980-1987, we use sulfur data from the old (SFU) database and for the period 1988-1996, we use the IMPROVE database. Changes from one year to the next similar to that between 1987 and 1988 occurred during other years and seasons suggesting that chance causes alone could perhaps explain it, the degree to which chance could have caused the changes was measured using the permutation test for matched. At the very least, additional information such as side by side readings using SFU and IMPROVE measurement methods, may be needed to better understand any systematic effect in the sulfur measurements that may be ascribable to the protocol change.

Trends in the extremes of sulfur concentration distributions.

Understanding the response of air quality parameters such as visibility to the implementation of new air quality regulations, population growth and redistribution, and federal land managing practices is essential to the evaluation of air quality management plans on air quality in federal Class I areas. For instance, the reduction of SO2 emissions from large single point sources should result in the decrease of extreme sulfate concentrations, while population growth in geographic areas outside of urban centers could cause a slow widespread increase of sulfate and organic concentrations. The change in federal land managing practice of increased prescribed fire on a year-round basis in lieu of large naturally occurring wild fires could have the same effect; that is, the frequency of high sulfur days increase and low sulfur days decrease as the result of the management practice. Therefore, it is of interest to examine the trends associated with the proportion of days during which the concentration of some aerosol species is above or below a certain threshold and decide whether this proportion of days is increasing or decreasing or shows a lack of trend. This is a direct indication of whether the quality of the environment is improving or worsening, or neither.

A preliminary look at source-receptor relationships in the Texas-Mexico border area.

Several factors have recently caused visibility impairment at Big Bend National Park, TX, to be of interest. Analyses of historical data collected there have shown that visibility is poorer and fine particle concentrations are higher at Big Bend than at other monitored Class I areas in the western United States. In addition, air masses frequently arrive there after crossing Mexico, where emissions are not well known. During September and October 1996, a field study was undertaken to begin examining the aerosol, visibility, and meteorology on both sides of the border. Results indicate that, during the study, the largest fractions of fine mass and light extinction at Big Bend were due to sulfates and the trace elements most closely associated with sulfate particles were Na and Se. Based on back trajectory modeling and the spatial, temporal, and inter-species relationships in the fine particle concentrations measured during the study, sulfates arrived at the park from both Mexico and the United States. Se was higher in Texas than in Northern Mexico, while V, Pb, Zn, Ni, and Mn were on average much higher in Mexico.

Sampling duration calculations.

Routine air quality monitoring produces filter samples that, when analyzed, yield the total amount of the aerosol present in the volume of air drawn by the pump in the monitoring device during the given sampling period. From this we obtain an average concentration of the aerosol for the given duration. The samples are therefore really aggregate samples. A natural question then is "what is the effect of the duration of aggregation on the accuracy and precision of the estimate of the quantity of interest?" The answer depends on a number of factors, such as the quantity that is being estimated: a mean, or an extreme value, or some other quantity; the nature of the measurement error--additive versus multiplicative; the costs of laboratory analyses, and so on. In this paper, we investigate these issues when the interest is in estimating the mean concentration of a specified aerosol species over a fixed time period. In particular, we propose a method for determining a sampling duration that will yield the "best estimate" of the mean concentration for a given cost whenever appropriate statistical assumptions hold.

Characteristics of winter and summer aerosol mass and light extinction on the Colorado plateau.

This paper focuses on the spatial variability of fine mass and extinction budgets taking data from the winter and summer months of 1992. The study area included southern California, southern Nevada, southern Utah, Arizona, and parts of New Mexico. Two types of monitoring sites were operated: intensive and secondary or satellite. At the intensive sites, all major aerosol species were measured as well as extinction or scattering. At the satellite sites, trace elements including sulfur and hydrogen, absorption, and gravimetric fine mass were measured. Where all aerosol species are measured, the spatial variability of extinction budgets is examined assuming an externally mixed aerosol. At the satellite sites, an approximated fine mass budget is derived and the variability of these budgets in space and time are examined. This effort was part of a study called Project MOHAVE (Measurement of Haze and Visual Effects) carried out with the principal objective of understanding the relative contribution of regional and local sources to visibility impairment on the Colorado Plateau and specifically, the Grand Canyon. Generally, the contribution of sulfates, organics, and absorption to extinction are about equal at 20-30% with the coarse mass fraction being about 10-20%. The one exception is in southern California where the nitrate contribution tends to be higher in the winter than summer. During the summer, concentration gradients tend to be spread out across the study area, while during the winter months, variability in concentration and budgets tends to occur on a smaller scale.

Human visual sensitivity to plumes with a Gaussian luminance distribution: experiments to develop an empirical probability of detection model.

This paper discusses results of a research project designed to develop an empirical model that could be used as a tool to predict human visual sensitivity to plumes. The resultant probability of detection algorithm (PROBDET) allows one to estimate the probability of a plume of known size, shape and contrast being detected visually. As a basis for the algorithm, a series of laboratory experiments using a high threshold signal detection procedure and computer generated images of plumes with Gaussian luminance distributions was conducted to measure human visual sensitivity to plumes. Results of the laboratory experiments are compared with results of contrast sensitivity experiments that examined visual sensitivity to stimuli with square and sine wave luminance distributions. An example of the PROBDET algorithm is presented to demonstrate its potential usefulness for assessing how probability of detection estimates change as plume size and contrast parameters vary. Since this research was designed to build on existing knowledge, a discussion of that knowledge and how it relates to the research conducted is also presented. The focus of this discussion is on the human visual system (HVS) and on how visual sensitivity is affected by factors such as the luminance of the stimulus and the surround, the luminance distribution of the stimulus, the size of the surround, and the size and spatial frequency characteristics of the stimulus.

Atmospheric haze: Its sources and effects on visibility in rural areas of the continental United States.

Spatial and temporal trends in visibility are examined at a national level. It is shown that visibility is impaired in all antional parks approximately 90% of the time, and that eastern visibilities are about 10 times lower than western visibilities. Measurement of atmospheric particulates that affect visibility shows that sulfates associated with man-made emissions of sulfur dioxide are the single largest contributor to visibility reduction, except in the northwestern United States, where organic aerosols contribute significantly. In the East, coal fired power plants along the Ohio River Vallery contribute the most SO2, while oil refining activities and other industrial sources in southern California, copper smelters in southern Arizona, and industrial activity along the Gulf Coast of Mexico contribute most of the SO2, and thus sulfates, in the West.