United States Environmental Protection Agency	Office of Research and Development

National Exposure Research Laboratory
Research Abstract

Government Performance Results Act (GPRA) Goal # 4
Annual Performance Measure # 274

Significant Research Findings:

Estimates of the Distribution and Patterns of Atmospheric Nitrogen
Stressors' Airborne Concentration and Deposition in the Eastern US

Scientific	To determine the impact(s) of legislatively driven pollutant control strategies,

Problem and	baseline conditions must be defined. This research documents the seasonal

Policy Issues	behavior and spatial patterns of both dry and wet nitrogen species in the eastern

US just prior to the January, 1995 implementation of Phase I of the Clean Air Act
Amendments of 1990.

Research	The goal was to define the seasonal and spatial patterns of nitrogen species"

Approach	concentration and deposition in the eastern US. Monitoring data from two national

networks were used to perform this research. Dry data came from EPA's
CASTNET (Clean Air Status and Trends Network) and wet data came from
NADP (National Acid Deposition Program). Weekly dry and wet concentration,
and dry and wet deposition of selected nitrogen species monitored at or near 34
rural sites in the east were adjusted for known biases and composed into seasonal
values according to peer-reviewed protocols. To ensure representative results,
five-year (1990-1994) averages were examined.

Results and	In the East, dry concentration, deposition velocity, precipitation, and dry, wet, and

Impact	total deposition of each of the monitored chemical constituents display regular

seasonal cycles of behavior. With few exceptions, seasonal extremes tend to occur
in the summer and winter. Much of the seasonal behavior may be explained by
summertime enhancement of atmospheric chemistry and convective processes.
Nitric acid (HN03) is the dominant contributor to dry deposition of measured
oxidized nitrogen (NOx) and measured total nitrogen (N). Wet deposition is a
major contributor to total deposition, generally peaking in the summer or spring.
Total NOx, and N deposition peak in the summer, with summertime measured total
deposition exceeding the winter value by a factor of ~2. Dry deposition of HN03,
NOx, and N is the highest at the high elevation subset of sites. The proportion of
dry to total deposition also appears to be highest at high elevation sites in the East,
underscoring the importance of dry deposition as a stressor to high elevation
ecosystems in the eastern US.

This documentation of the seasonal behavior and spatial patterns of both dry and
wet nitrogen species in the eastern US just prior to the implementation of Phase I
of the Clean Air Act Amendments of 1990 (i.e., January 1, 1995) will help define
the baseline against which future changes resulting from legislatively mandated
control strategies can be detected, measured and demonstrated.

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Research
Collaboration and
Research
Products

Examples of recent publications from this study include:

Shadwick, D. S., Sickles, J. E.. II, 2004. Sample size for seasonal mean concentration
deposition velocity, and deposition: a resampling study. Atmospheric Enviromnent 38,
477-489.

Sickles, J. E., II, Shadwick, D. S., 2002a. Biases in the Clean Air Status and Trends
Network filter pack results associated with sampling protocol. Atmospheric Enviromnent
29, 4687-4698.

Sickles, J. E., II, Shadwick, D. S., 2002b. Precision of atmospheric dry deposition data
from the Clean Air Status and Trends Network. Atmospheric Environment 36, 5671-5686.

Future Research A study contrasting the behavior of nitrogen species in the first half of the 1990's
(1990-1994) with that in the second half (1995-1999) is underway. Upon its
completion, a second study will begin of the changes occurring after 2000 when
several oxides of nitrogen (NOx) control measures commenced.

Contacts for

Additional

Information

Questions and inquiries can be directed to:

Joseph E. Sickles, II, Ph.D.

U.S. EPA, Office of Research and Development
National Exposure Research Laboratory, MD-E243-05
Research Triangle Park, North Carolina 27711
Phone: 919/541-2446
E-mail: sickles.ioseph@epa.gov

Funding for this project was through the U.S. EPA's Office of Research and
Development, National Exposure Research Laboratory, and the work was
conducted by the Environmental Sciences Division, Landscape Characterization
Branch. Additional funding for this research was administered under EPA contract
number 68-W01032.

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