Particulate ammonium contribution to total deposition

Rich Scheffe1, Jesse Bash2, James Kelly3, Bret Schictel4, Donna Schwede5 and Jason Lynch6

      There has been a wealth of evidence over the last decade illustrating the rising importance of reduced inorganic nitrogen (NHx = ammonia gas, NH3,  plus particulate ammonium, p-NH4) in the overall atmospheric mass balance and deposition of nitrogen as emissions of oxidized nitrogen have decreased throughout a period of stable or increasing NH3 emissions.  In addition, the fraction of ambient ammonia relative to p-NH4 generally has risen as a result of decreases in both oxides of nitrogen and sulfur emissions.   EPA plans to consider ecological effects related to deposition of nitrogen, of which NHx is a contributing component, in the review of secondary National Ambient Air Quality Standards (NAAQS) for oxides of nitrogen and sulfur (NOx/SOx standard).   Although these ecological effects are associated with total nitrogen deposition, it will be important to understand the emissions sources contributing to the total nitrogen deposition and to understand how much of the total nitrogen deposition is from deposition of NHx versus other nitrogen species.  Because p-NH4 contributes to nitrogen deposition and can also be a significant component of particulate matter, there is a potential overlap in addressing nitrogen based deposition effects in the secondary PM and NOx/SOx NAAQS.  Consequently, there is a policy interest in quantifying the contribution of p-NH4 to total nitrogen deposition.  While dry deposition of p-NH4 is calculated through a variety of modeling approaches, the amount of precipitation based NH4 results from a variety of transfer processes of ambient p-NH4 and NH3 to the aqueous phase without delineating the relative contributions of ambient p-NH4 and NH3.   Preliminary results of a simple method utilizing available outputs from a chemical transport model (CTM) are used to demonstrate the relative contribution of p-NH4 to total nitrogen deposition and spur discussion of techniques to refine future assessments.


1U.S. EPA,
2U.S. EPA,
3U.S. EPA,
4U.S. NPS,
5U.S. EPA,
6U.S. EPA,