An innovative approach to measuring hygroscopic light scattering enhancement using a humidified single-nephelometer system

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 interinstrument uncertainties. This study presents a novel single-nephelometer system that reduces measurement uncertainty and studies 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, possibly influenced by favorable conditions for new particle formation and changes in aerosol composition, size distribution, and meteorological conditions. In contrast, low 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 low aerosol hygroscopicity, increased RH significantly influenced aerosol climate-relevant variables.