Calculation of aerosol particle hygroscopic properties from OPC derived PM2.5 data

Hygroscopic growth of aerosol particles due to increasing relative humidity in the atmosphere is characterized by their hygroscopicity parameter κ and their hygroscopic growth factor GF. A technique to calculate the two using PM2.5 data from two optical counting sensors is examined. Only one of the two instruments is equipped with a drying channel, and therefore differences between ambient and dry air concentrations can be observed when both are working simultaneously. Based on the definition of the hygroscopic growth factor as the ratio between an aerosol particle's wet and dry diameter, a relationship including PM values is reached through the assumption of particle spherical shape and basic geometry. Aerosol particles from marine and urban sources were sampled during a week of measurements in two locations, in Norderney, Germany during April 2021 and in Rødby, Denmark, during September 2022. Calculated GF and κ values were related to the origin of the air-mass using back trajectory modeling (NOAA HYSPLIT) and by comparing the results to their expected values and fits on theoretical growth curves. It was found that κ = 0 . 6 ± 0 . 1 $\kappa =\nobreak 0.6 \pm\nobreak 0.1$ when continental air was sampled (agreeing with ammonium sulphate's κ = 0 . 6 1 $\kappa =\nobreak 0.61$ ) and κ = 1 . 1 ± 0 . 1 $\kappa =\nobreak 1.1 \pm\nobreak 0.1$ when marine air was sampled (agreeing with sea salt's same value). The GF estimates also matched their respective expected values within a deviation of 1σ. For the measurements in Rødby, particle number size distributions for the cases of marine sourced concentrations showed a peak at 1 to 2 μm, which is similar to previous studies of Baltic sea aerosol particle size distribution and structure.