This study shows the possibility to use OPC-N3 for calculation of hygroscopic properties of the aerosol. This can be done as particulate matter (PM) values measured by OPC-N3 depend on air humidity. Therefore, in order to obtain non biased PM values, one needs to make corrections.
The work shows the first results of aerosol hygroscopicity conducted in Poland. The study was performed during Spring 2020 (lock-down period) and Winter 2020. The research was conducted in the Geophysics Institute at the University of Warsaw, close to the city center. Two OPC-N3 sensors were connected to the outlet from two legs of the Aerosol Conditioning System ACS1000. In one of them, low relative humidity was kept below 37%, and in the other, the relative humidity was changed in the range of 50-90% in cycles.
The calculation of growth factor was done by dividing the PM1 measured from wet pipe by PM1 measured in the dry channel. The hygroscopicity parameter κ was calculated from κ-Köhler theory, showing a fluctuation of the κ parameter which depends on aerosol type.
The mean value of κ during Spring measurements was 0.22.  The observed κ for Winter mean was 0.044. The values of hygroscopicity of aerosol in winter are smaller than the ones corresponding to Spring, in line with respect to previous literature reports.
As measurements of PM done by OPC-N3 can be biased by high relative humidity, we propose a method to make the correction depending on the surrounding humidity.