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 take up 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, showing high correlations with data from the Taiwan Environmental Protection Administration (TW-EPA). With the application of κ-Köhler equation and certain assumptions, AQB-monitored PM concentrations are converted to dry particle mass concentration, providing optical particle counter sensitivity correction and resulting in improved correlation with TW-EPA data. The derived single hygroscopicity parameters (κ) range from 0.15 to 0.29 for integrated fine particles (PM2.5) and 0.05 to 0.13 for coarse particles (PM2.5−10), consistent with results of ionic chromatography analysis from a previous winter campaign nearby. Moreover, the analysis of PM10 division into PM2.5 and PM2.5−10, considering composition heterogeneity, provided improved dry PM10 concentration as the sensitivity coefficients for PM2.5−10 were notably higher than for PM2.5. Our methodology provides a comprehensive approach to assess ambient aerosol hygroscopicity, with 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.