Nitrate Isotopic Composition in Rainfall and Fine Particulate Matter: Back Trajectory, Meteorology, and Source - Receptor Relationship Analysis

Abstract

The southeastern portion of North Carolina is a dense crop and animal (swine) agricultural region which previous research suggests emits a significant portion of the state's nitrogen emissions. These findings indicated that transporting air over this region can effect nitrogen concentrations in precipitation at sites at least 50 miles away. The present study was able to combine isotope compositional information with the concept of back trajectory analysis to examine the relationship between this regional nitrogen emission source and receptors independent of pollutant concentration information. The Hybrid Single Particle Lagrangian Integrated Trajectory (HYSPLIT) model was used to determine potential sources of nitrogen in rainwater collected at an urban receptor site in Raleigh, North Carolina, during the first three quarters of 2004. The delta 15N isotope ratio signatures of each sample were used to further differentiate between sources of the rainwater nitrogen. This study examined the importance of pollutant sources such as animal agricultural activity and meteorology on rainfall chemistry as well as their implications on fine particulate matter formation. Additionally, meteorological conditions associated with anomalous springtime fine particulate matter concentrations found in coastal New Hanover County, North Carolina were investigated.

Samples that transited the dense crop and animal (swine) agricultural region of east-southeastern North Carolina (i.e. the source region) had lower delta 15N isotope ratios in the nitrate ion (average = -2.1 ± 1.7 per mil) than those from a counterpart nonagricultural region (average = 0.1 ± 3.0 per mil.) However, the limited data set does not offer any conclusive evidence of similar patterns in ammonium ions. An increase in PM fine mass concentrations in the urban receptor site (yearly average = 15.1 ± 5.8 μg/m3) was also found to correspond to air transport over the dense agricultural region relative to air which was not (yearly average = 11.7 ± 5.8 μg/m3).

Fine particulate matter concentrations over the course of four years in four counties around the state were examined, and peak levels of PM2.5 were found in the summer at 3 of the 4 sites. The fourth site, located in costal New Hanover County had a fine particulate matter peak in the spring, which appeared to be unusual as most literature indicates that conditions for high PM2.5 levels are generally found in the summer. Investigation of the phenomenon revealed that historically, meteorological conditions including relative humidity, temperature, and wind direction allow for a peak to exist much earlier in the year at such a location. Transport of pollutants from the dense swine region to the Northwest of New Hanover County is more common in the springtime, along with relative humidity that is in a range witch allows an increase in particulate matter to occur.