Nitrogen isotope differences between atmospheric nitrate and corresponding nitrogen oxides: A new constraint using...

Area of Science:

• Environmental Chemistry
• Isotope Geochemistry
• Atmospheric Science

Background:

• Effective mitigation of nitrogen (N) emissions relies on accurately tracking N sources.
• Stable isotope mixing models, using N and O isotopes in atmospheric nitrate (NO₃^-), are used to assess NO_x source contributions.
• A key challenge is accurately determining the isotopic differences between NO₃^- and its precursor NO_x (ε_(NO2→NO3-) values).

Purpose of the Study:

• To evaluate the ε_(NO2→NO3-) values by incorporating the hydroxyl radical (•OH) oxidation pathway (NO₂ → NO₃^-) using triple oxygen isotopes (Δ¹⁷O).
• To compare Δ¹⁷O-based ε values with traditional δ¹⁸O-based ε values.
• To re-evaluate the source contributions of atmospheric NO₃^- using the newly derived ε_{17O-based(NO2→NO3-)} values.

Main Methods:

• Analysis of NO₃^- in PM_2.5 samples collected in Beijing using stable isotope analysis.
• Application of triple oxygen isotope (Δ¹⁷O) values to determine ε_(NO2→NO3-) during the NO₂ oxidation to NO₃^-.
• Utilizing the Stable Isotope Analysis in R (SIAR) model with updated ε_(NO2→NO3-) values to quantify source contributions.

Main Results:

• The Δ¹⁷O-based ε values (ε_{17O-based(NO2→NO3-)} = 15.6 ± 7.4‰) differed significantly from δ¹⁸O-based ε values (ε_{18O-based(NO2→NO3-)} = 33.0 ± 9.5‰).
• Using ε_{18O-based(NO2→NO3-)} values led to underestimation of coal combustion (CC-NO_x) by 64% and overestimation of microbial N cycle (MC-NO_x) by 216%.
• The new ε_{17O-based(NO2→NO3-)} values significantly reduced uncertainties in the source apportionment of atmospheric NO₃^-.

Conclusions:

• Triple oxygen isotopes (Δ¹⁷O) provide a more accurate constraint for ε_(NO2→NO3-) values compared to dual oxygen isotopes (δ¹⁸O).
• The improved ε_{17O-based(NO2→NO3-)} values enhance the reliability of stable isotope mixing models for attributing atmospheric NO₃^- sources.
• This advancement is crucial for effective N emission mitigation strategies and understanding atmospheric chemistry.