5.  Final Action on Ammonia

	Ammonia (NH3) is a gaseous pollutant that is emitted by natural and
anthropogenic sources.  Emissions inventories for ammonia are considered
to be among the most uncertain of any species related to PM.  Ammonia
serves an important role in neutralizing acids in clouds, precipitation,
and particles.  In particular, ammonia neutralizes sulfuric acid and
nitric acid, the two key contributors to acid deposition (acid rain). 
Deposited ammonia also can contribute to problems of eutrophication in
water bodies, and deposition of ammonium particles may effectively
result in acidification of soil as ammonia is taken up by plants.  The
NARSTO Fine Particle Assessment indicates that reducing ammonia
emissions where sulfate concentrations are high may reduce PM2.5 mass
concentrations, but may also increase the acidity of particles and
precipitation.  An increase in particle acidity is suspected to be
linked with human health effects and with an increase in the formation
of secondary organic compounds.  Based on this information and further
insights gained from the NARSTO Fine Particle Assessment, it is apparent
that the formation of particles related to ammonia emissions is a
complex, nonlinear process.

	Though recent studies have improved our understanding of the role of
ammonia in aerosol formation, ongoing research is required to better
describe the relationships between ammonia emissions, particulate matter
concentrations, and related impacts.  The control techniques for ammonia
and the analytical tools to quantify the impacts of reducing ammonia
emissions on atmospheric aerosol formation are both evolving.  Also,
area-specific data are needed to evaluate the effectiveness of reducing
ammonia emissions on reducing PM2.5 concentrations in different areas,
and to determine where ammonia decreases may increase the acidity of
particles and precipitation.  

Due to the considerable uncertainty related to ammonia, our final rules
do not require ammonia to be regulated as a PM2.5 precursor but do give
States the option to regulate ammonia as a precursor to PM2.5 in
nonattainment areas for purposes of NSR on a case-by-case basis. 
Consistent with our proposal, if a State demonstrates to the
Administrator’s satisfaction or EPA demonstrates that ammonia
emissions in a specific nonattainment area are a significant contributor
to that area’s ambient PM2.5 concentrations, the State would regulate
ammonia as a PM2.5 precursor under the NSR program in that nonattainment
area.  Once this demonstration is made, ammonia would be a “regulated
NSR pollutant”  under  NA NSR  for that particular nonattainment area,
and the State would need to regulate construction and modification of
stationary sources that increase emissions of ammonia in that area to
assure that these emissions do not interfere with reasonable further
progress or the ability of that area to attain or maintain the PM2.5
NAAQS.   In all other nonattainment areas in that State and nationally, 
ammonia  would not be subject to the NSR program.  In addition, the
action of any State identifying ammonia emissions as a significant
contributor to a nonattainment area’s PM2.5 concentrations, or our
approval of a non-attainment SIP doing so, does not make ammonia a
regulated NSR pollutant for the purposes of PSD in any attainment or
unclassifiable areas nationally.

 NARSTO (2004) “Particulate Matter Assessment for Policy Makers: A
NARSTO Assessment.”  P. McMurry, M. Shepherd, and J. Vickery, eds.
Cambridge University Press, Cambridge, England. ISBN 0 52 184287 5.

