Sulphur Oxidation States in Atmospheric Particulate Matter Studied by XANES: A Comparative Study from Europe, Asia, and North America

Abstract

Understanding the oxidation state of sulphur in fine atmospheric particulate matter (PM_2.5) is essential for constraining secondary aerosol formation, climate forcing, and health impacts. This study utilises comparison of sulphur K-edge X-ray absorption near-edge structure spectra, combined with linear combination fitting of reference spectra, to characterise sulphur speciation in PM_2.5 from Kraków (Poland), Qingdao (China), and Atlanta (USA). Across all regions, sulphate species S⁽⁺⁶⁾ dominate, confirming their role as the principal end product of atmospheric sulphur oxidation, but the distribution of intermediate and reduced sulphur forms varies systematically with emission sources and meteorological conditions. In Kraków, wintertime PM_2.5 is richer in ammonium bisulphates relative to summer, consistent with enhanced solid-fuel combustion and more acidic aerosol conditions. In Qingdao, coarse particles contain detectable S⁽⁺⁴⁾ species associated with gypsum and calcium sulphite, linked to interactions between anthropogenic sulphur and mineral dust, whereas fine particles are dominated by ammonium sulphate. In the Atlanta region, ammonium sulphate is prevalent at all sites, with additional contributions from gypsum and metal-containing sulphates that vary between urban, rural, and background locations. Based on literature data employing a consistent spectroscopic methodology for a specific region across three contrasting environments, this study identifies region-specific sulphur speciation  fingerprints  and  demonstrates  the capability of X-ray absorption near-edge structure to link emission inventories, atmospheric transformation processes, and policy-relevant indicators of aerosol composition.