Developing an Oxygen-17 Isotope-Coupled WRF-Chem Model for Elucidating Sulfate Formation Mechanisms in China Haze and Beyond: Part I. Model Description and Initial Assessments.

Atmospheric sulfate is a major aerosol component that affects atmospheric chemistry, air quality, and climate, yet its formation pathways remain poorly constrained. The oxygen-17 isotope anomaly of sulfate (Δ17O(SO42-)) has been sought to distinguish sulfate formation pathways, as different oxidants impart distinct Δ17O signatures. Here we develop a Δ17O(SO42-) tracing framework within the WRF-Chem model that tracks sulfate from each formation pathway throughout its atmospheric lifecycle, enabling more accurate quantification of pathway contributions than conventional methods based solely on local production rates. The results reveal a large difference on pathway attributions compared with the conventional method. What is more, while the model captures the spatial pattern of Δ17O(SO42-) across China on monthly scales, evaluation against daily observations from Hefei reveals a critical discrepancy, i.e., the model reproduces sulfate concentrations well but fails to capture daily variations in Δ17O(SO42-). This inconsistency demonstrates that models can yield the correct sulfate mass through incorrect combinations of formation pathways. Our results highlight that concentration-based model validation is insufficient for pathway attribution and underscore the need to integrate the Δ17O constraint as a routine benchmark for diagnosing and correcting chemical mechanisms in air quality models.