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Jeffrey R. Brook, Perry J. Samson, and Sanford Sillman

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%.

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Jeffrey R. Brook, Perry J. Samson, and Sanford Sillman

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%.

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Jeffrey R. Brook, Perry J. Samson, and Sanford Sillman

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.

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Frank J. Marsik, Kenneth W. Fischer, Tracey D. McDonald, and Perry J. Samson

Abstract

During the summer of 1992, measurements of the boundary layer mixing height were conducted at five locations around the city of Atlanta, Georgia, as part of the 1992 Atlanta Field Intensive of the Southern Oxidants Research Program on Ozone Non-Attainment. These measurements were made during a series of “high-ozone-event days” for the purpose of acquiring information about the temporal evolution of the convective mixed layer. The information acquired from these systems was included in a database of meteorological variables for use in the photochemical modeling efforts associated with the study. The following measurement systems were selected for use in this study by organizers of the 1992 Atlanta Field Intensive: one rawinsonde system, four radar wind profiler–RASS (radar acoustic sounding system) systems, and two lidar systems.

A comparison of the mixing-height estimates from each of the measurement systems used during the 1992 Atlanta Field Intensive was performed in an effort to evaluate the consistency of the estimates between the different systems and, further, to evaluate the relative performance of each system during the study period. Statistical analyses were performed on the dataset, with in-depth statistical analyses presented for two specific days: 30 July and 4 August 1992. Results indicate that there is often disagreement in the mixing-height estimates between the various systems, particularly during the early morning and late afternoon. It is believed that the differences between estimates are the result of 1) the physical limitations of the different instrument system 2) the assumptions used with each system as to which tracer most accurately defines the structure of the convective boundary layer, and 3) the spatial inhomogeneity of convective boundary layer structure across the region studied.

In general, the rawinsonde system appeared to give the most accurate mixing-height estimates under the meteorological conditions studied. The lidar estimates were comparable to the rawinsonde estimates, except during the early morning hours, when some estimates were erroneously high. This overestimation occurred when the return signal from aerosols suspended aloft during the previous day's mixing exceeded that from new aerosols being mixed into the developing convective boundary layer. Radar wind profiler estimates were also erroneously high during the morning hours, due mainly to the use of a setup configuration that precluded detection of the newly developing mixed layer below 400–600 m.

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Michael J. Markus, Bruce H. Bailey, Ronald Stewart, and Perry J. Samson

Abstract

Low-level (<2 km) cloud frequencies have been derived for the Appalachian Mountain region for the period 1985–88 based on in situ measurements by optical cloud and relative humidity sensors, and regional analyses incorporating the U.S. Air Force Real-Time Nephanalysis (RTNEPH) database. Statistics include cloud frequency as a function of elevation, season and time of day. The in situ results reveal that the higher Appalachian peaks (>1400 m) were in cloud an average of 29%–37% during the period, while peaks near 1000 m experienced cloud 11%–19% of all hours. RTNEPH regional results indicate that low-level cloud was most frequent between 900 m and 1300 m with a maximum at 1100 m. Orographic effects are probably responsible for the difference in these findings. Drought conditions during the period reduced overall cloudiness in the southern portions of the Appalachians, while more normal amounts were observed in northern areas. Cloud was found to be more abundant at night over the mountains in contrast to trends observed at regional airport sites.

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