Jack D. Shannon

A model of regional long-term average sulfur atmospheric pollution, surface removal, and net horizontal flux

Abstract The Advanced Statistical Trajectory Regional Air Pollution (ASTRAP) model combines efficient calculation of long-term regional-scale concentrations and fluxes of pollutant sulfur with improved parameterizations of boundary-layer processes. The parameterizations include diurnal and seasonal variations of dry deposition velocities for SO2 and sulfate, rate of transformation from SO2 to sulfate, vertical structure of the planetary boundary layer, and emission rates. The deposition velocity variations simulate some recent experimental results indicating that the deposition velocity of sulfate particles may be of the same order as that for gaseous SO2, not an order of magnitude less as often modeled. The transformation rate variation simulates a proposed photochemical effect. The stability variations simulate the cycle of nocturnal inversion formation, intensification, rise, and dissipation, as frequently observed in field investigations of the planetary boundary layer. The emission rate variation simulates the effects of such factors as daily patterns of electrical demand. An additional improved parameterization is the treatment of wet deposition as a function of the half power of the six-hourly precipitation amount, an empirical result. Budget studies of emissions from the eastern U.S. and Canada indicate that wet removal, dry removal, and horizontal transport out of the area are of similar magnitude during the summer months; during winter dry removal is only about one half as large as the other terms.

Regional trends in wet deposition of sulfate in the United States and SO2 emissions from 1980 through 1995

Abstract Regional trends of seasonal and annual wet deposition and precipitation-weighted concentrations (PWCs) of sulfate in the United States over the period 1980–1995 were developed from monitoring data and scaled to a mean of unity. To reduce some effects of year to year climatological variability, the unitless regional deposition and PWC trends were averaged (hereafter termed CONCDEP). The SO2 emissions data over the same period from the United States, Canada, and northern Mexico, aggregated by state and province, were weighted appropriately for each deposition region in turn to produce scaled trends of the emissions affecting each region. The emission-weighting factors, which were held constant year to year, were estimated by exercise of a regional transport model. The sulfate CONCDEP regional trends are generally similar to those of regionally weighted SO2 emissions, although the latter trends are less steep and the former trends have more year to year variability. In eastern regions, sulfate CONCDEPs and SO2 emissions patterns both generally show an initial decrease, an essentially trendless middle period, and a final decrease as reductions mandated by the Acid Rain Provisions of the 1990 Clean Air Act Amendments began. Linear regressions of regional sulfate CONCDEPs on corresponding regionally weighted SO2 emissions produced statistically significant relationships in all regions. The analysis indicated that although regional sulfate CONCDEPs decreased relatively faster than did SO2 emissions during the period in all regions except the Great Plains, in general the slopes were not significantly different from unity.

Estimation of S and NOx-N deposition budgets for the United States and Canada

Extensive wet deposition monitoring data, along with much sparser dry deposition monitoring data and isolated field characterizations of droplet deposition, are used to produce an annual deposition budget for S and NOx-N for each contiguous U.S. state and Canadian province. The contribution to wet deposition from natural sources is estimated, and wet deposition is adjusted accordingly, to focus the analysis on deposition associated with anthropogenic emissions. The budgets, which are appropriate for 1985–1987, include adjustments for increased local wet and dry deposition near point sources and urban areas, which are generally avoided in selecting monitoring sites. When aggregated over the United States and Canada, estimated wet, dry, and droplet deposition account for about 30, 29, and 4% of anthropogenic S emissions, respectively, implying that 37% is exported. For NOx-N, estimated wet, dry, and droplet deposition account for 30, 43, and 4% of anthropogenic emissions, with 23% exported. Results are in general agreement with a variety of other observational analyses and regional modeling simulations, particularly for eastern North America, and are consistent with estimates of net horizontal mass fluxes of S and NOx-N across the Atlantic coast of North America.

Indications of nonlinearities in processes of wet deposition

Abstract Numerical simulations have been carried out with a model consisting of clear-air chemistry, in- cloud chemical reactions, and dynamic processes of cloud development in order to examine the time history of cloudwater pH and sulfate production in a cumulus cloud and the relationship between pollutant precursors and corresponding acidic chemical species in wet deposition. Preliminary results indicate that the molar ratio SO42−/NC3− in cloud water increases as the ratio SO2/NO2 increases, that the relationship between the increase of precursor SO2/NO2 and the increase of SO42−/NO3− in cloud water is nonlinear, and that the degree of this nonlinearity becomes more significant for cases when the cloud condensation nuclei content in air is assumed to be invariant.

Estimation of Wet and Dry Deposition of Pollutant Sulfur in Eastern Canada as a Function of Major Source Regions

The contributions of major anthropogenic source regions to wet and dry deposition of total S in eastern Canada are estimated for a winter month and a summer month with the ASTRAP model. Results indicate that the U.S. and Canada contribute approximately equal amounts to total S deposition in Canada; Canadian sources contribute more than one half of dry deposition and less than one half of wet deposition.

Modeling visibility for assessment

Abstract Thevisibility assessment scoping model (VASM), a Monte Carlo technique for simulatinga and assessing regional-scale impairment of visibility, is described. The input requirements of the method for each receptor are modest: geometric means and standard deviations describing the within-season distributions of daily concentration of each of the six principal particle species; matrices of interspecies partial correlations of daily concentrations; relative humidity (RH) climatology; and seasonal mean concentrations for each particle species. The lognormal distribution parameters and the correlation matrices are abstracted from past observations. For those particle species with emission precursors that are varied in the policy scenarios, the concentration means are obtained from a regional atmospheric transport model; the means of the other particle species are assumed to be statistically similar to previous observations. Seasonal distributions of haze intensity are produced from sets of short-term Monte Carlo realizations of concentrations and RH and appropriate formulae for light extinction. VASM simulations for rural sites in both the relatively hazy eastern United States and the more pristine western United States compare favorably with transmissometer observations. Application of VASM, in concert with utility SO2 emission projections and atmospheric transport modeling, indicates that the median seasonal reduction of haziness expected at Shenandoah by the year 2010, relative to 1990 conditions, will range from 1.3 deciviews in winter to 2.2 deciviews (dv) in summer. If calculated as visual range, the corresponding improvements are 14 and 24%, respectively.

Evaluation of three regional air quality models

Abstract This paper summarizes the results of a thorough assessment of existing regional air quality models. Forty-two candidate models were reviewed and three repsesentative models were selected for rigorous and comprehensive assessment. The underlying scientific theories used in the models were evaluated, revealing many limitations. For example, the techniques used in the preparation of meteorological fields that drive the models give insufficient consideration to the physical basis of the relevant atmospheric processes. The primary operational evaluation of each of the models was performed by comparing calculated values with observations from the EPRI Sulfate Regional Experiment (SURE). Both short-term (6-h averages) and long-term (annual averages) comparisons reveal poor correlations for both SO2 and SO2−4 for the three models evaluated ranging from 0.05 to 0.32 for 3- to 6-h SO2 concentration to 0.03 to 0.59 for 24-4 and monthly averages; in some cases, the correlations are negative. The results also show that calculated concentrations are generally characterized by high biases for 3- to 6-h concentration predictions. Biases tend to be somewhat smaller for monthly averages. All three models underpredicted wet deposition with average normalized residuals of approximately 0.2 for ENAMAP-2, and 0.5 for RTM-II and ACID.

Selected strategies to reduce acidic deposition in the U.S.

Although the research needed to fully characterize acidic deposition and its range of effects is many years from completion, political initiatives are being taken today to formulate control programs aimed at reducing the emissions of atmospheric pollutants believed to be responsible for environmental damage. This article is an attempt to shed light on this controversial policy issue by examining the attributes of several plausible acid deposition control strategies

Scales of sulfur concentrations and deposition from the perspective of the receptor

Abstract Median source-receptor separation distances and atmospheric transport times for atmospheric [SO 2 ] and [SO 4 [ 2− ] and wet and dry deposition of S are calculated from the perspective of the receptor. The temporal and spatial separations are weighted according to the relative contribution of the source to the total atmospheric concentration or deposition at the receptor as calculated with the Advanced Statistical Trajectory Regional Air Pollution (ASTRAP) model; thus, the scales are functions not just of climatological conditions and processes of atmospheric chemistry (as resolved and treated by the regional model) but also of the emission distribution. Calculations are made for a grid of receptor points covering the United States, Canada, and northern Mexico. The minimum spatial and temporal scale lengths occur in the principal emission region, the Ohio Valley, and near isolated western sources; maximum scale lengths occur in remote regions without significant local emissions. The spatial scale of [SO 4 2− ] is typically almost twice as large as that of [SO 2 ] or dry deposition of S in the eastern United States, and the spatial scale of wet deposition of S is slightly greater than that for [SO 4 2− ]. The temporal scales of [SO 4 2− ] and wet deposition of S are two to three times those of [SO 2 ] and dry deposition of S in the eastern United States. Weighted-mean spatial separations are larger than median separations by roughly 25% except in areas remote from sources where they are about equal. Over most of the region scales for wet deposition of S are shortest in summer and longest in winter.

Modeled Sulfur Deposition Trends Since 1900 in North America

Inventories of historical emissions of acidic deposition precursors in the U.S. have been assembled by Gschwandtner et al. (1985), referred to hereafter as G85, and by Husar (1986). One might expect that such data would be widely applied in studies of possible long-term effects associated with acid deposition, such as lake acidification or forest damage. Use of the inventories in such investigations, however, has been surprisingly limited. This may be because atmospheric deposition rather than precursor emissions is the direct input to ecological systems. Although some researchers have used emission trends as surrogates for deposition trends, the relationships between aggregated emissions and resulting patterns of atmospheric concentrations or deposition have changed over the years as the seasonal, horizontal, and vertical distributions of emissions have changed. In the hope of providing additional information useful to researchers investigating deposition effects, we have simulated wet and dry S deposition and atmospheric concentrations of SO2 and sulfate over the United States and Canada since 1900 with the ASTRAP model (Shannon, 1985). Source-receptor matrices aggregated to the state or province level have been calculated as well. Simulations are seasonal, at five-year intervals from 1900 through 1960 and at one-year intervals from 1960 through 1985.