Interannual variability of secondary organic carbon driven by seasonal shifts in formation regimes in southern Taiwan
Journal
Atmospheric Environment
Journal Volume
382
Start Page
122143
ISSN
13522310
Date Issued
2026-10-01
Author(s)
Ye, Jun-Fa
Chan, Chih-Yu
Wu, Jian-Xian
Tang, Shi-Ya
Chen, Tse-Lun
Young, Li-Hao
Peng, Yen-Ping
Chen, Wei-Hsiang
Lin, Ming-Yeng
Tsay, Si-Chee
Lin, Neng-Huei
Abstract
Secondary organic aerosol (SOA) formation in urban environments is often interpreted in terms of precursor abundance and atmospheric oxidation capacity; however, these factors alone do not always explain observed variability. This study investigates SOC controls during 2024-2025 at Nanzi, southern Taiwan, as part of the Kao-Ping Experiment (KPEx) within the ASIA-AQ campaign framework. During early spring, organic carbon (OC) concentrations exhibited a pronounced interannual contrast, with March-April 2024 levels nearly twice those of 2025. EC-tracer analysis attributed this enhancement primarily to secondary organic carbon (SOC). Observational analyses indicated that spring 2024 experienced weaker ventilation, elevated aromatic precursors, and higher BTEX-derived SOA formation potential; however, oxidation capacity alone could not explain the SOC enhancement. To disentangle interacting controls, an interpretable machine learning framework combining XGBoost and SHapley Additive exPlanations (SHAP) was applied to 9877 hourly observations (R2 = 0.69; RMSE = 1.03 μg/m3). SHAP analysis revealed that while seasonal structure constrained SOC variability in both years, distinct formation regimes emerged during early spring. In March-April 2024, enhanced SOC was preferentially associated with lower aerosol pH under elevated aerosol liquid water content (ALWC) and moderate-to-high oxidation conditions, indicating multiphase or acid-catalyzed processing. In contrast, March-April 2025 showed stronger dependence on Ox and boundary-layer dynamics, consistent with an oxidation-driven regime. These findings demonstrate that interannual SOC differences can arise from seasonally specific shifts in oxidation-thermodynamic coupling, and suggest that the ASIA-AQ campaign period coincided with a distinct acidity-sensitive SOC regime, underscoring the importance of multi-year context when interpreting campaign observations.
Subjects
Aerosol pH
ALWC
ASIA-AQ/KPEx
Interpretable machine learning
SOC
Publisher
Elsevier Ltd
Type
journal article