Deriving the hygroscopicity of ambient particles using low-cost optical particle counters

Abstract. This study investigates the chemical composition and physical properties of aerosols, which play a crucial role in influencing human health, cloud physics, and local climate. Our focus centers on the hygroscopicity of ambient aerosols, a key property reflecting the ability to absorb moisture from the atmosphere and serve as cloud condensation nuclei. Employing home-built Air Quality Box (AQB) systems equipped with low-cost sensors, we assess the ambient variability of particulate matter (PM) concentrations to determine PM hygroscopicity. The AQB systems effectively captured meteorological parameters and most pollutant concentrations, with high correlations observed compared to Taiwan Environmental Protection Administration (EPA) data. With the application of κ-Köhler equation and certain assumptions, AQB-monitored PM concentrations are converted to dry particle mass concentration, showing improved correlation with EPA data and optical particles counter sensitivity correction. The derived κ values range from 0.15 to 0.29 for integrated fine particles (PM_2.5) and 0.05 to 0.13 for coarse particles (PM_2.5-10), consistent with results of ionic chromatography analysis for samples from a previous winter campaign nearby. Moreover, the analysis of PM₁₀ division into PM_2.5 and PM_2.5-10, considering composition heterogeneity, provided improved dry PM₁₀ concentration as the sensitivity coefficients for PM_2.5-10 were notedly higher than for PM_2.5. Our methodology provides a comprehensive approach to assess ambient aerosol hygroscopicity, offering significant implications for atmospheric modeling, particularly in evaluating aerosol efficiency as cloud condensation nuclei and in radiative transfer calculations. Overall, the AQB systems proved to be effective in monitoring air quality and deriving key aerosol properties, contributing valuable insights into atmospheric science.