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

The chemical composition and physical properties of aerosols significantly affect human health, cloud physics, and local climate. Hygroscopicity, a crucial physical property, represents the ability of aerosol to absorb moisture from the surrounding atmosphere and act as cloud condensation nuclei. In this study, we applied two home-built Air Quality Box (AQB) systems co-locating with Taiwan EPA Nanzi station (22°44’12” N, 120°19’42” E) in Kaohsiung, Taiwan, from 4 to 19 February 2021. AQB is composed of low-cost sensors to monitor the ambient gaseous pollutants (CO, CO₂, NO, NO₂, O₃, SO₂, and Non-Methane Hydrocarbons), aerosol particles (number size distribution between the diameter of 0.38-17 μm), and the meteorological parameters (T, RH, and P). As to PM (Particulate matter), EPA station monitors PM concentration at the dry state by controlling the measurement at less than 50% while the optical particle counter (OPC) in AQB reflects the ambient PM directly. The difference between the two values represents the amount of absorbed liquid water in the ambient condition. With the consideration of OPC sensitivity and aerosol hygroscopicity, OPC sensitivity and k-Köhler equation are applied to derive the hygroscopicity parameter (k) for PM_2.5 (fine particles) and PM_2.5-10 (coarse particles with a diameter in the range of 2.5 to 10 µm). In our preliminary results, OPC sensitivity is different between fine and coarse particles at RH<50%, suggesting the requirement of sensitivity adjustment for simple OPC at different particle ranges. The derived k ranges from 0.15 to 0.29, and 0.05 to 0.13 for fine and coarse particles, respectively. The results are consistent with those derived from collected samples analyzed using ion chromatography in our previous Kaohsiung winter campaign. The developed method provides a comprehensive way to determine the hygroscopicity of ambient aerosols, which can be helpful for atmospheric models to compare the results for further efficiency evaluation of aerosol acting as cloud condensation nuclei and radiation calculation. Furthermore, the application of this method for the low-cost sensors applied widely nearby EPA stations is under analysis to evaluate the performance of low-cost OPC sensors and to retrieve the enhanced temporal and spatial aerosol hygroscopicity.