Biomass-Burning Contributions to Rainwater and Aerosol Constituents May Be Systematically Overestimated in Urban China

Potassium (both water-soluble K⁺ and elemental K) and levoglucosan (LG) are commonly used as chemical markers for emissions from biomass burning (BB). However, their application in complex urban environments, particularly in coal-dominated regions, may be compromised by overlapping sources and atmospheric processes. Using a decade-long data set (2005–2015) of rainwater chemistry and high-resolution aerosol measurements (2018–2019) in Shanghai, China's largest megacity, this study quantitatively assesses the impacts of coal combustion on potassium and LG levels. Our analysis reveals that over 80% of K⁺ in rainwater and ∼53% of aerosol potassium originate from coal combustion as indicated by strong correlations with industrial SO₂ emissions (r² = 0.80–0.95). Similarly, LG concentrations are significantly impacted by coal combustion with molecular diagnostic ratios aligning with coal-derived endmembers particularly during autumn. Considering the prevalence of coal-based energy systems in most Chinese cities, we argue that pollution attributed to BB in rainwater and aerosol across urban China may be systematically overestimated when relying on currently broadly used BB tracers. By integrating long-term observational data with receptor modeling, we further underscores the necessity of refining source attribution methodologies to better distinguish between BB and coal combustion impacts on air quality and climate in urban areas.