Samples of PM1 were collected at two urban background sites, one in Zabrze and one in Warsaw (Poland), in summer (24 June - 22 August 2014) and winter (8 January - 8 March 2015) in parallel. At each site, in each season, 60 diurnal (24-h) samples were taken. The samples were analyzed for carbon (organic OC, and elemental, EC) and water soluble ions (Na+, NH4 +, Cl−, SO4 2−, NO3 −) by using, respectively, a Sunset Laboratory carbon analyzer and a Herisau Metrohm AG ion chromatograph. Each 24-h sample of PM1 was checked for mass closure using the categorization of the chemical components into: primary (POM) and secondary (SOM) organic matter, secondary inorganic matter (SIM), elemental carbon (EC), and sodium and chlorine (Na_Cl). The seasonal effects, typical of urban areas in Poland, are clearly seen in the whole series of the 24-hour PM1 concentrations. Most of the PM1components and PM1 had their mean concentrations higher in winter than in summer. Secondary aerosol (SOM+SIM) accounts for approx. 55% of the PM1 mass in Warsaw and 40% in Zabrze.