Hygroscopic enhancement of suburban aerosol light scattering measured using a single-nephelometer system in Central Europe

Abstract. Most atmospheric aerosol particles are hygroscopic, meaning they absorb water from the surrounding air, altering their size, shape, overall chemistry, refractive index, and thus light-scattering properties – an effect with important implications for Earth's radiative balance. The scattering enhancement factor, f(RH), and backscattering enhancement factor, f(RH)bsp, quantify the increase in light scattering under elevated relative humidity (RH). These parameters are typically measured using two nephelometers operating under dry (RH<40 %) and humidified (RH>80 %) conditions, a method prone to inter-instrument uncertainties. This study presents a single-nephelometer system that reduces measurement uncertainty associated with inter-instrument comparison and enables the study of aerosol hygroscopic behavior in the inadequately represented European urban environment. The system was deployed at a suburban site in Prague, Suchdol, Czech Republic, from November 2022 to August 2023. Results revealed low aerosol hygroscopicity, likely due to a well-mixed aerosol population dominated by black and brown carbon. Both enhancement factors peaked in spring, coinciding with the enhanced formation of secondary aerosols and particle growth, which modified aerosol size distributions and hygroscopicity. Low hygroscopic enhancement values in summer reflected a composition shift toward black carbon-dominated aerosols from traffic emissions, with particle growth being disrupted, potentially due to the structural compaction of black carbon aggregates under high RH. While f(RH) and f(RH)bsp generally increased with decreasing concentrations of light-absorbing particles, organic carbon, particularly its most volatile fractions, significantly enhanced aerosol hygroscopicity in the urban environment. Despite overall low aerosol hygroscopicity, increased RH significantly influenced aerosol climate-relevant optical properties.