Abstract

It is shown that Urban Airshed Model (UAM-IV) calculated air pollutant concentrations during photochemical smog episodes in Atlanta, Georgia, depend strongly on the numerical parameterization of the daytime vertical diffusivity. Results found suggest that vertical mixing is overestimated by the UAM-IV during unstable daytime conditions, as calculated vertical diffusivity values exceed measured and comparable literature values. Although deviations between measured and UAM-IV calculated air pollutant concentrations may only in part be due to the UAM-IV diffusivity parameterization, results indicate the large error potential in vertical diffusivity parameterization. Easily implemented enhancements to UAM-IV algorithms are proposed, thus improving UAM-IV modeling performance during unstable stratification.

Abstract

A method for deriving estimates of long-term acidic deposition over eastern North America based on a limited number of Regional Acid Deposition Model runs has been developed. The main components of this method are the identification of a representative sample of events for model simulation and the aggregation of the deposition totals associated with the events. Meteorological categories, defined according to 3-day progressions of 850-mb wind flow over eastern North America, were used to guide the selection of events. This paper describes how events were selected from the categories and how they were combined (aggregated) to estimate long-term deposition. The effectiveness of the category-based approach was compared against alternate aggregation approaches and it was found to provide the best sample-based estimates of long-term wet sulfate deposition across eastern North America.

Thirty events from the 1982–85 time period were selected using a set of predetermined criteria and aggregated to estimate seasonal and annual SO²⁻ ₄, NO⁻ ₃, and H⁺ deposition at 20 Utility Acid Precipitation Study Program sites. The accuracy of the estimates varied geographically and depending upon whether they were for the annual or seasonal time periods. Over the main area of interest (a smaller 13-site region), the mean rms errors for annual deposition were 10%, 15%, and 12% for sulfate, nitrate, and acidity, respectively. Source–receptor relationships associated with the 30 events were examined for three sites located in Michigan, North Carolina, and upstate New York. It was found that the amount of time that transport was to these areas from the U.S. Midwest (an area of high SO₂ emissions) was represented to within 20%.

Abstract

Running 3-day periods from 1979 to 1985 were categorised into one of 20 meteorological categories. These categories were developed through the cluster analysis of 3-day progressions of 85-kPa wind flow over eastern North America. The purpose for developing the categories was to identify recurring atmospheric transport patterns that were associated with differing amounts of wet sulfate (SO²⁻ ₄) and nitrate (NO⁻ ₃) deposition at a variety of locations in eastern North America. Identification of these patterns was necessary to facilitate the selection of time periods for simulation by the Regional Acid Deposition Model and in the development of a method for estimating long-term acidic deposition over eastern North America from a limited number of model runs. The effectiveness of this method (referred to as the aggregation method) was expected to be dependent on the ability of the categories to separate structure in wet deposition patterns. This paper describes the determination of the 20 meteorological categories and demonstrates that there were differences in their meteorological and chemical behavior and in their frequency of occurrence. Observations of precipitation and wet SO²⁻ ₄ and NO⁻ ₃ deposition from 22 sites in eastern North America and multiple regression models were used to demonstrate that there were statistically significant differences in deposition among categories and that knowledge of meteorological category explained some of the variation in wet deposition. The best statistical correlation, which was based upon precipitation amount, time of year, and meteorological category, explained 35%–83% (28%– 76%) of the observed variation in wet SO²⁻ ₄ (NO⁻ ₃) deposition depending on location. On average, across all sites and for both SO²⁻ ₄ and NO⁻ ₃, knowledge of category accounted for about 4% of the variation. The minimum amount explained by category was 1% and the maximum was 13%.

Abstract

In this paper, the authors present an analysis of correlations between SO₂ emissions and wet SO²⁻ ₄ concentrations over eastern North America that includes adjustments for the impact of meteorological variability. The approach uses multiple-regression models and readily available meteorological information to analyze precipitation chemistry data collected from 1979 to 1986 at six Utility Acid Precipitation Study Program site. On an event-to-event basis, from 25% to 50% of the variation in concentrations, depending on site, was found to be related to meteorology. Precipitation amount, temperature, upwind emissions, and upwind mean lower-tropospheric relative humidity (indicator for upwind precipitation) were related to the natural log of SO²⁻ ₄ concentrations. Inclusion of this information resulted in a decrease in the uncertainty associated with the emission change to concentration relationship at all sites, but the results were inconsistent Year-to-year and season-to-season changes in SO₂ emissions were found to be significantly related (p < 0.033) to variations in event and seasonal concentrations at three of the sites, but not at the other three sites. Possible explanations for this discrepancy are discussed in this paper. When all sites were examined simultaneously, strong statistical correlations (p < 0.007) were found between emissions and concentrations, indicating that SO²⁻ ₄ concentrations decreased in response to seasonal and annual emission changes.