Soil and litter emission sources as important contributors to ozone production from volatile organic compounds in island tropical forests

Abstract

While studies have confirmed that volatile organic compounds (VOCs) emitted directly by tropical island forest vegetation significantly influence ozone (O₃) production and climate change through atmospheric oxidation processes, the environmental effects of long-neglected soil and litter emission sources as key potential contributors to VOCs, particularly their driving mechanisms in near-surface O₃ pollution formation, remain understudied. This investigation combines field observations with machine learning models to investigate the emission characteristics, sources, and contributions of VOCs from tropical island forests to O₃ and secondary organic aerosol (SOA) formation. The results reveal discrepancies between traditional ozone formation potential (OFP) estimates and machine learning-based assessments. OFP calculations identified acetaldehyde and methanol as the dominant contributors to O₃ formation, while toluene and monoterpenes were primary drivers of SOA formation. However, the XGBoost model integrated with the SHapley Additive exPlanations (SHAP) framework, which quantifies the dynamic impacts of VOCs under real-world atmospheric conditions, demonstrated that isoprene made the most significant contribution to O₃ formation (|SHAP|=5.9), surpassing other VOCs. For SOA formation, benzene and toluene showed the highest contributions, with |SHAP| values of 1.2 and 0.8, respectively. By calculating initial VOC concentrations and applying the Positive Matrix Factorization (PMF) model, we identified four VOC sources: soil and litter emissions (41 %), oxidative formation (28.5 %), anthropogenic transport (16.6 %), and direct plant emissions (13.9 %). Photochemical reactions caused significant losses of plant-derived VOCs during transport; after accounting for photochemical losses, the contribution of direct plant emissions increased from 4.6 % to 13.9 %. SHAP analysis highlighted that soil and litter emissions contributed most significantly to O₃ formation (|SHAP|=14.7), offering theoretical advantages over traditional OFP estimates that prioritized plant emissions. The SHAP framework, derived from observational data mining, effectively mitigated biases caused by temporal or regional variations and provided a more accurate quantification of rapidly consumed VOCs during active photochemical processes, thereby addressing limitations of conventional OFP methods. These findings indicate that VOCs from soil and litter emissions in tropical forest regions exert a substantial influence on local O₃ formation.

Keywords: Hainan; Ozone; Tropical forests; Volatile organic compounds; XGboost.