The School of Earth and Atmospheric Sciences Presents Dr. Wendell Walters, Brown University

Investigating Atmospheric Reactive Nitrogen Utilizing Novel Isotopic Constraints

Anthropogenic activities have significantly perturbed the global nitrogen (N) budget through emissions of reactive nitrogen (Nr) primarily in the form of nitrogen oxides (NOx = NO + NO2) and ammonia (NH3) from accelerated fossil fuel combustion and agricultural activities since the second half of the 20th century. 

These compounds have important human health, climate, and environmental consequences including the formation of fine particulate matter, influence on the atmospheric oxidation budget, and deposition of excess Nr in sensitive ecosystems. Therefore, constraining Nr emission sources and their chemical, human health, and climate feedbacks is critical for future policy regulations to mitigate these affects. 

Here, I will present recent results of tracking the contribution of traffic-derived NH3 emissions and its role in fine particulate matter formation in a coastal urban environment utilizing novel N stable isotope analysis (δ15N) of NH3 and its secondary product, ammonium (NH4 + ). New methodologies and theoretical frameworks have been developed to accurately and precisely characterize and interpret δ 15N of NH3 and pNH4 + in ambient environments. The δ15N composition (or “fingerprint”) of NH3 has been characterized in vehicle-derived plumes in both stationary and mobile measurement campaigns in Northeastern United States as well as in Shenyang, China. 

Utilizing this newly established δ15N “fingerprint” the sources of NH3 in an urban environment have been determined in a year-long field study. Other on-going and future applications will (1) characterize the chemical coupling between NOx and biological volatile organic compounds utilizing oxygen stable isotopes (Δ17O & δ18O) in controlled laboratory chamber studies, (2) determine seasonal sea-atmosphere NH3 bi-directional flux in a large estuary, and (3) track modern Martian atmospheric nitrate formation in controlled experimental studies.