Measurement report: Size-resolved seasonal study of inorganic ions and isotopic carbon signatures of aerosol particles at the Wadden Sea

Abstract. Measurements of aerosol size-resolved chemical composition and source contribution of carbonaceous aerosols provide a unique opportunity to characterize continental outflow over the sea. Studies at coastal sites usually show an interesting mix of continental and marine aerosol sources. However, few data exist on the size dependence of the aerosol chemical composition and isotopic source apportionment of organic carbon (OC). This study aims to quantify seasonal, air-mass, and size-dependent variations in inorganic aerosol composition and OC sources. It also highlights isotope measurements as a tool to study sources and atmospheric processing of OC. Size-resolved measurements of inorganic ions, levoglucosan, and total carbon (TC) were combined with ¹³C and ¹⁴C analysis of OC at different desorption temperatures, which allows to investigate subgroups like secondary organic aerosol (SOA) and aged organic aerosol. During continental outflow conditions, nitrate, ammonium, and TC dominated, while sulfate and TC dominated under marine influence. Sulfate reached up to 0.5 μg/m³ both in marine and regional continental air, indicating the importance of ship emissions. Fossil OC fractions were highest in particles <250 nm, characteristic for traffic emissions. ¹³C/¹²C-ratios of ambient OC deviate strongly from expected signatures of primary sources and indicate SOA contribution to more volatile carbon and smaller particles. The ¹³C/¹²C-ratios of less volatile OC at larger sizes are consistent with a significant influence of photochemical processing. The results highlight shipping emissions as the main source of sulfate aerosols at this coastal site and demonstrate the highly processed nature of OC in the European continental outflow.