“Atmospheric Ammonia: Importance, Trends and Challenges”

Rick D. Saylor

Atmospheric Research & Analysis Incorporated

Wednesday, February 11, 2009 at 3:30 p.m.

(Refreshments at 3:00 pm)

327 McVey Hall

ABSTRACT: Ammonia (NH3) is an important atmospheric constituent for a variety of reasons. As the dominant alkaline compound in the atmosphere, its neutralization of gaseous sulfuric (H2SO4) and nitric (HNO3) acids forms ammonium (NH4+) salts that contribute significantly to the total atmospheric burden of fine particles, thereby impacting air quality and atmospheric visibility. The NH3-NH4+ system is an important component of the global nitrogen cycle, linking organic nitrogen in decaying biomass to the return of nitrogen to the atmosphere. Dry deposition of NH3 or wet deposition of NH4+ may contribute to the acidification or eutrophication of terrestrial or aquatic ecosystems, may contribute to the alteration of the nitrogen status of forests, or may directly impact the health or disease- and frost-susceptibility of forests and other vegetation. Furthermore, NH3 is a major component of total reactive nitrogen, the continued growth of which has been identified as a major global concern. Recent investigations suggest that NH3 is a critical factor in the nucleation of new particles in sulfur-rich environments. For these reasons, it is important to have a clear understanding of the sources, deposition, and atmospheric behavior of NH3.

Major sources of NH3 include livestock wastes (e.g., from cattle feedlots, poultry houses, and hog farms), volatilization of fertilizers applied to cropland, the exhaust of vehicles equipped with catalytic converters, sewage treatment facilities, industries, and human respiration. Although NH3 emissions are not expected to be increasing substantially in North America, recent data from the southeastern U.S. suggests that gaseous NH3 levels have been increasing in recent years. We hypothesize that decreasing nitric and sulfuric acid levels in the atmosphere, resulting from promulgated regulations on the emission of nitrogen oxides and sulfur dioxide, are leading to the partitioning of more NH3 from the particle phase into the gaseous phase. Higher gas-phase NH3 levels in the future may have an adverse impact on terrestrial and aquatic ecosystems.

The interaction of NH3 with the surface is particularly interesting since some plants can be emission sources of this species if the external nitrogen supply to the plant is large. As a consequence, modeling the atmosphere-surface exchange of NH3 and NH4+ requires a sophisticated treatment of bi-directional transport between the atmosphere and plant and soil surfaces, something which existing 3-D atmospheric chemistry models are not equipped to handle. Furthermore, species that are co-deposited with NH3 onto vegetation, including gaseous SO2, fine particle H2SO4, and gaseous HNO3, may react under certain conditions to decrease the pH of aqueous films existing on leaf surfaces, thereby increasing the uptake of gaseous NH3. Or, conversely, soil-derived coarse particles may co-deposit on these leaf surfaces and serve to increase the pH of the aqueous environment, thereby decreasing the uptake of gaseous NH3. Taking into account these chemical processes on vegetation surfaces is a challenge for models of NH3-NH4+ atmosphere-surface exchange, but will be necessary for future modeling studies examining the impact of nitrogen deposition to sensitive ecosystems.