The chemical composition of submicron aerosol particles was characterized using an aerosol chemical speciation monitor (ACSM) at a mixed-mature forest site at the Aukstaitija Integrated Monitoring Station in the eastern part of Lithuania. Four organic aerosol (OA) factors were determined by positive matrix factorization (PMF) analysis applied for the ACSM data: semi-volatile oxygenated OA (SVOOA, the contribution to the total OA mass concentration was 33%), low-volatile oxygenated OA (LVOOA, 39%), cooking related OA (COA, 15%) and biomass burning OA (BBOA, 13%). In our study secondary organic aerosol (SOA) has been identified as one of the most important contributors to the submicron particle (PM1) mass concentration. A comparison between SVOOA and submicron forest organic aerosol mass (SFOM) was performed, and a good correlation of 0.75 between them was found. In our study the SOA concentration dependence on temperature was explored in 3 means: firstly, by investigating aerosol mass spectra measured during hot and cold days. It has been found that during hot days events signal intensities of m/z 42, 43, 45, 53 and 59 were about 2.1-2.7 times higher compared to those during cold days. Secondly, by investigating the temperature dependence of SVOOA and estimated SFOM. It has been found that SFOM and SVOOA concentrations increase with temperature. Thirdly, by assessing the influence of temperature related stomatal conduction on SVOOA concentration: no direct influence on SVOOA concentration was observed.