Quantifying the nitrogen isotopic fractionation of NOx in atmospheric simulation chamber
Abstract
Nitrogen isotopic compositions (d 15N) of atmospheric NO2 and nitrate are widely used to trace the emission sources of NOx. However, this approach sometimes can be biased due to the significant but not well-constrained isotopic fractionations between NO and NO2 in the atmosphere. The NO-NO2 isotopic fractionations are mainly controlled by two processes: equilibrium isotopic effect (EIE) between NO and NO2 molecules and kinetic isotopic effect (KIE) associated with NOx photochemistry (the Leighton cycle). EIE has been previously investigated by several studies which displayed some discrepancies and KIE has not been experimentally studied yet. We measured the EIE and KIE by analyzing the d 15N values of NO (d 15NNO), NO2 (d 15NNO2) and NOx (d 15NNOx) in a 10 m3 atmospheric simulation chamber under various NO/NO2 concentrations and lighting conditions. Our results showed the enrichment factors of EIE and KIE at room temperature are 1.0268± 0.0012 and 0.9895± 0.0005, respectively. Using these values, we further calculated the isotopic fractionations between NOx, NO and NO2 (d 15NNO2-d 15NNO, d 15NNO2-d 15NNOx) as a function of NOx concentration, NO2/NOx ratio, and NO2 photolysis frequency (jNO2). We find that the competition between EIE and KIE strongly depends on the NOx concentrations. EIE is the dominate factor at high NOx concentrations (>20 ppb), in which d 15NNO2 can be >20‰ higher than d 15NNOx. KIE mainly controls the isotopic fractionation between NO and NO2 at low NOx concentrations (<1 ppb) and high jNO2, in which the d 15NNO2 is 0-10‰ lower than the d 15NNOx. Our study also provided mathematical solutions to calculate the NO-NO2 isotopic fractionations at any given condition, enabling a more accurate NOx source appointment using stable nitrogen isotopes.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.A43Q3127L
- Keywords:
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- 0317 Chemical kinetic and photochemical properties;
- ATMOSPHERIC COMPOSITION AND STRUCTURE;
- 0340 Middle atmosphere: composition and chemistry;
- ATMOSPHERIC COMPOSITION AND STRUCTURE;
- 0365 Troposphere: composition and chemistry;
- ATMOSPHERIC COMPOSITION AND STRUCTURE;
- 0394 Instruments and techniques;
- ATMOSPHERIC COMPOSITION AND STRUCTURE