Source Apportionment of Sulfur and Nitrogen Species at Rocky Mountain National Park using Modeled Conservative Tracer Releases and Tracers of Opportunity

Visibility degradation and changes in ecosystem function in Rocky Mountain National Park (RMNP) are occurring because of emissions of nitrogen and sulfate species along the Front Range of the Colorado (CO) Rocky Mountains, as well as sources farther east and west. The nitrogen compounds include oxidized, reduced, inorganic, and organic nitrogen. The Rocky Mountain Atmospheric Nitrogen and Sulfur study (RoMANS) was initiated to better understand the origins of sulfur and nitrogen species as well as the complex chemistry occurring during transport from source to receptor. This included a source apportionment assessment to identify the relative contributions to atmospheric sulfur and nitrogen species in RMNP from within and outside of the state of CO; from emission sources along the Colorado Front Range; as well as the relative contributions from mobile sources, agricultural activities, and large and small point sources within CO. As part of the study, a monitoring program was conducted for two 5-week periods, one during the spring, the other during late summer. Monitoring data of ammonium/ammonia, nitrogen oxide/nitrates, and sulfur dioxide/sulfates were combined with tracers of opportunity and modeled releases of conservative tracers from source regions around the United States to apportion these species to their respective sources, using a variety of receptor modeling tools. The preliminary results show that during the spring a larger fraction of nitrogen and sulfur species came from sources within CO than during the summer. Specifically, during the spring, sources within CO contributed more than 80% of the ambient NH3, NH4+, HNO3, and particulate NO3-; 50% of SO2 and 30% of particulate SO4= came from sources within CO. During the summer period CO sources contributed to about 75% of the ambient NH3 and 55-60% of the NH4+ and HNO3; 30% of SO2, and 20% of particulate SO4=.