Dual nitrate isotopes in dry deposition: Utility for partitioning NOx source contributions to landscape nitrogen deposition

Dry deposition is a major component of total atmospheric nitrogen deposition and thus an important source of bioavailable nitrogen to ecosystems. However, relative to wet deposition, less is known regarding the sources and spatial variability of dry deposition. This is in part due to difficulty in measuring dry deposition and associated deposition velocities. Passive sampling techniques offer potential for improving our understanding of the spatial distribution and sources of gaseous and aerosol N species, referred to here as dry deposition. We report dual nitrate isotopic composition (delta N-15 and delta O-18) in actively collected dry and wet deposition across the high-deposition region of Ohio, New York, and Pennsylvania. We also present results from initial tests to examine the efficacy of using passive nitric acid collectors as a collection medium for isotopic analysis at a site in New York. Isotopic values in actively collected dry deposition, including particulate nitrate and gaseous nitric acid, are compared with those in wet nitrate deposition and surrounding NOx emission sources. delta N-15 values in dry and wet fractions are highest at the westernmost sites and lowest at the easternmost sites, and stationary source NOx emissions (e. g., power plants and incinerators) appear to be the primary control on delta N-15 spatial variability. In contrast, delta O-18 values show a less consistent spatial pattern in dry deposition. Both delta N-15 and delta O-18 show strong seasonality, with higher values in winter than summer. Seasonal variations in stationary source NOx emissions appear to be the most likely explanation for seasonal variations in delta N-15, whereas seasonal variations in air temperature and solar radiation indicate variable chemical oxidation pathways control delta O-18 patterns. Additionally, we demonstrate the utility of passive samplers for collecting the nitric acid (HNO3) component of dry deposition suitable for isotopic analysis. We observe slight differences in delta N-15-HNO3 values between simultaneous samples collected actively and passively (0.6%). However, we observe a larger offset in delta O-18 values between actively and passively collected samples; the causes for this offset warrant further investigation. Nonetheless, passive sample collection represents a significant cost savings over active sampling techniques and could allow a more extensive understanding of patterns of dry deposition and associated insights to nitrogen sources across landscapes.