The 38th annual Air Pollution Workshop was held April 9-13, 2006 in Charlottesville, VA. In recent years, a one day symposiums has been held on the Monday before the Air Pollution Workshop. The theme for the 2006 Symposium was “Air Pollution and Vegetation Effects Research in National Parks and Natural Areas: Implications for Science, Policy, and Management”.

Johnny Boggs presented research findings on the effects of nitrogen deposition on conifer and deciduous forests in the northeastern US, and Steve McNulty presented estimates of critical nitrogen and sulfur loads for forests in the lower 48 states using a simple mass balance equation. Submitted papers for the symposium will be published in a special issue of Environmental Pollution.

McNulty also made a workshop presentation on sensitivity and uncertainty analyses for the national critical loads estimates.

Effects of nitrogen deposition on high elevation spruce-fir and deciduous forests across the northeastern US

Johnny Boggs, Steven McNulty, and Linda Pardo

Abstract. The effects of nitrogen deposition on the health and productivity of high elevation spruce-fir forests has been a concern for several decades in Europe and the US. Recent studies have suggested that nitrogen deposition may also have negative impacts on the health deciduous forests. In 1999 we established a series of plots in deciduous forests to test this hypothesis. The plots were sampled adjacent to spruce-fir plots originally established in 1987/88 across a nitrogen deposition gradient in the northeastern US. A subset of the spruce-fir plots was also re-sampled. Foliar and forest floor samples were collected from 70 spruce-fir and 60 deciduous plots at different elevations and aspects to evaluate how both forest types were responding to equal doses of nitrogen deposition, and to determine if the ecological state of the original spruce-fir plots has changed during the past 12 years. Results indicated that forest floor chemistry (percent N, C:N ratios, pH, and net mineralization and nitrification potential) between deciduous and spruce-fir plots were positively correlated. Nitrogen deposition was negatively correlated with forest floor C:N ratios in both forest types. Additionally, spruce-fir forest floor C:N ratios decreased by 14% from 1987 to 1999 on Gore Mountain, the highest nitrogen deposition site. Nitrogen deposition is causing changes in forest floor and foliar chemistry in deciduous forests, suggesting that concerns about the effects of nitrogen deposition should not be limited to high elevation spruce-fir forests. Furthermore, nitrogen deposition has continued to influence forest floor and foliar chemistry of red spruce forests over the past 12 years.

Estimates of critical nitrogen and sulfur loads across the lower 48 US using a simple mass balance equation

Steven McNulty, Erika Cohen, Harbin Li, and Jennifer Moore Myers

Abstract. The 1990 Clean Air Act mandated reductions in sulfur and nitrogen emissions collectively termed “acid rain”. Although significant advances have been made in reducing sulfur emissions, little progress has been made in reducing nitrogen. Continued concern regarding the impacts of acidic deposition on forest ecosystems has prompted the development of critical loads assessments. A critical load can be defined as a quantitative estimate of an exposure to one or more pollutants below which significant harmful effects on specified sensitive elements of the environment do not occur according to present knowledge. When pollutant loads exceed the critical load it is considered that there is an increase risk of harmful ecosystem impacts. The excess over the critical load has been termed the exceedance. A larger exceedance is often considered to pose a greater risk of ecosystem harm. Historically, critical load assessments in the US are conducted on relatively small geographic areas, using various methods and assessment tools. These methodological differences make national level critical loads assessments impossible. Therefore, we elected to use a simple mass balance equation to calculate the critical load and exceedance at 4 km2 spatial resolution across all forests in the continental US. The resultant model generated maps are designed to initiate the discussion for more sophisticated process level modeling of critical loads assessments which are to follow. Critical load model input data reliability, and model parameter sensitivity, methods for model validation, and geographical areas of concern will be presented.