A coupled modeling and observational approach to understanding oxygen-18 in atmospheric nitrate
δ15N and δ18O values of atmospheric nitrate are fairly well documented in the literature. Some research has suggested δ 15N in atmospheric nitrate is an indicator of NOx source in the atmosphere, where δ18O of atmospheric nitrate tends to be an indicator of atmospheric oxidation. δ18O in particular however is not as well understood as it can be. Many papers suggest that δ 18O values in atmospheric NO3-, and variations therein, indicate specific HNO3 production pathways or other such atmospheric chemistry, though most of these assessments are qualitative. Presented here is the first coupled modeling and observational approach to fully understanding δ 15N and δ18O values in atmospheric nitrate. A model for the prediction of δ18O in atmospheric HNO 3 has been developed which utilizes mass-balance relationships to estimate δ 18O values in atmospheric HNO3. Precipitation samples collected by the National Atmospheric Deposition Program at the Indiana Dunes National Lakeshore between 2001 and 2003 are analyzed for δ15N and δ18O. These measured values are compared with the first predictions of δ18O in atmospheric NO3 - to help to quantitatively determine the specific atmospheric chemistry behind the trends. This first attempt at a model which predicts δ 18O of atmospheric HNO3 does not produce completely accurate results, however it shows potential. The shortfalls of the model exist primarily in its underestimation of δ18O values of atmospheric HNO3 when compared to the measured samples from the Indiana Dunes National Lakeshore. A correction to some components of the estimation help to rectify some of this underestimation, although predicted values still deviate from those measured. The RACM model (Stockwell et al., 1997), which is coupled with this isotopic mass-balance model, predicts the contribution of each major HNO3 production pathway however the calculations of RACM are based on air quality data that was incomplete for this study location. Ultimately the accuracy of the model depends on accurate and complete known datasets for calculation. Despite not showing absolute accuracy in the prediction of δ 18O values of HNO3, this model shows tremendous potential in helping to better understand the chemical reactions which control the δ 18O of atmospheric HNO3.^
Greg Michalski, Purdue University.
Atmospheric Chemistry|Biogeochemistry|Environmental Sciences