Preprints
https://doi.org/10.5194/egusphere-2025-923
https://doi.org/10.5194/egusphere-2025-923
19 May 2025
 | 19 May 2025
Status: this preprint is open for discussion and under review for Atmospheric Chemistry and Physics (ACP).

Modeling atmospheric sulfate oxidation chemistry via the oxygen isotope mass-independent fractionation using the Community Multiscale Air Quality Model (CMAQ)

Huan Fang and Wendell Walters

Abstract. Atmospheric sulfate formation influences climate and air quality, yet its chemical pathways remain difficult to constrain. This study utilizes the oxygen isotope anomaly (Δ17O) of sulfate aerosol (ASO4) as a tracer to distinguish formation processes. We modeled Δ17O(ASO4) using the Community Multiscale Air Quality Model (CMAQ), focusing on 2006 and 2019 to quantify key ASO4 formation pathways and their response to U.S. emission changes. In 2006, Δ17O(ASO4) values were predicted to be below 1 ‰ in the Gulf Coast indicated acidic, ASO4-rich conditions dominated by S(IV) + H2O2 oxidation, while values above 1 ‰ in the West suggested less acidic conditions, leading to enhanced ASO4 production via S(IV) + O3 oxidation. Peak Δ17O(ASO4) values of ~5 ‰ in April across the Western U.S. reflected O3-driven ASO4 formation during high ammonia (NH3) emissions from fertilization. Between 2006 and 2019, mean Δ17O(ASO4) increased by up to 2 ‰, driven by declining sulfur dioxide (SO2) emissions from regulatory measures. Model comparisons with historical measurements show reasonable agreement in the acidic southeastern U.S. (RMSE = 0.3 ‰, Baton Rouge, LA). However, the model overpredicts Δ17O(ASO4) in the West (RMSE = 0.5 ‰, La Jolla, CA; RMSE = 2.1 ‰, White Mountain Research Center, CA), particularly during periods of high NH3 emissions. This overestimation suggests an excessive model response to aqueous S(IV) + O3 reactions. These results emphasize the need for expanded Δ17O(ASO4) measurements and improved model constraints to better capture evolving emission trends and regulatory impacts on sulfate formation.

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Huan Fang and Wendell Walters

Status: open (until 30 Jun 2025)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on egusphere-2025-923', Anonymous Referee #2, 04 Jun 2025 reply
  • CC1: 'Comments on novelty and literature context of Δ17O-sulfate modeling study using CMAQ by Fang and Walters', Shohei Hattori, 11 Jun 2025 reply
    • CC2: 'Correction to reference list', Shohei Hattori, 11 Jun 2025 reply
Huan Fang and Wendell Walters

Data sets

Simulating Δ17O of sulfate aerosol within the contiguous United States to trace the formation processes Huan Fang https://doi.org/10.5281/zenodo.14954960

Huan Fang and Wendell Walters

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Short summary
The Sulfur Tracking Mechanism in CMAQ was used to model the oxygen isotope anomaly (Δ17O) of aerosol sulfate (ASO4) for the first time. This approach allows for a qualitative analysis of sulfate (SO42-) formation processes and comparison with corresponding measurements.
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