Nitrogen Isotope Differences between Major Atmospheric NO<sub><i>y</i></sub> Species: Implications for Transformation and Deposition Processes
Xueyan Liu, Yimeng Yin, Wei Song
Abstract
Nitrogen isotopes (δ15N) can trace sources of major inorganic NOy species (here referring to nitrogen dioxide (NO2), nitric acid (HNO3), and nitrate (NO3–)) in the atmosphere. However, δ15N variations during transformation and deposition processes of NOy species remain poorly understood. Here, we dissected δ15N differences between NO2, HNO3, particulate NO3–, and precipitation NO3– by converging observations of their concentrations and δ15N from literature sources. δ15N values of HNO3 were higher by 6.9 ± 4.6‰ than those of NO2, indicating that the hydroxyl radical pathway dominated the HNO3 formation. δ15N values of particulate NO3– were higher by 4.9 ± 4.2‰ than those of HNO3, indicating HNO3 as a main precursor of particulate NO3–. δ15N values of precipitation NO3– were lower than those of HNO3 (by 2.8 ± 3.3‰) and particulate NO3– (by 7.5 ± 3.5‰) but higher than those of NO2 (by 5.1 ± 4.5‰). Relative contributions of NO2, HNO3, and particulate NO3– to precipitation NO3– were estimated as 45 ± 10%, 32 ± 5%, and 23 ± 7%, respectively, which differed from their fractions in ambient atmosphere (57 ± 14%, 14 ± 8%, and 29 ± 11%, respectively). This revealed NO2 as an important precursor of precipitation NO3– and a preferential scavenger of HNO3 by precipitation. This work is beneficial for more accurate evaluations on sources and processes of atmospheric NOy pollutants.