Stable oxygen isotope constraints on nitrate formation in Beijing in springtime

Area of Science:

• Atmospheric Chemistry
• Environmental Science
• Isotope Geochemistry

Background:

• Aerosol nitrate (NO₃⁻) is a major contributor to haze pollution.
• Quantifying NO₃⁻ formation mechanisms is crucial for pollution control but remains understudied.

Purpose of the Study:

• To investigate aerosol nitrate formation pathways in Beijing using stable oxygen isotopes.
• To differentiate contributions from various nitrate production routes during polluted and non-polluted conditions.

Main Methods:

• Collection of total suspended particulate (TSP) samples in Beijing during spring 2013.
• Analysis of NO₃⁻ concentration and its oxygen isotope composition (δ¹⁸O-NO₃⁻).
• Application of a Bayesian isotope mixing model to estimate fractional contributions of different formation pathways.

Main Results:

• Higher NO₃⁻ concentrations and δ¹⁸O-NO₃⁻ values were observed on polluted days (PD) compared to non-polluted days (NPD).
• Isotopic analysis suggests increased contributions from pathways involving N₂O₅ + H₂O/Cl⁻, NO₃ + VOCs, and ClNO₃ + H₂O during PD (53%-89%).
• The NO₂ + OH pathway dominated nitrate production during NPD (37%-69%).

Conclusions:

• Oxygen isotope analysis of nitrate is a powerful tool for elucidating atmospheric formation pathways.
• Specific pathways significantly contribute to nitrate pollution under different atmospheric conditions in Beijing.
• Understanding these pathways is key to developing effective haze mitigation strategies.