Abstract. A dual-chamber system was deployed in two different environments to study the potential of ambient air, which was directly injected into the chambers, to form secondary organic and inorganic aerosol. A total of 16 experiments took place during March 2022 in a polluted environment in the Po Valley, Italy, which is dominated by anthropogenic emissions. Another 15 experiments were conducted in the Pertouli forest, Greece, which is dominated by biogenic emissions. In both campaigns, ambient air containing highly oxidized (average O:C 0.7–0.8) aerosol was the starting point of the experiments, and its chemical evolution under the presence of OH radicals was followed. In the Po Valley secondary organic aerosol (SOA) formation was observed in all experiments but one, and the SOA formed ranged from 0.1 to 10 µg m−3. Experiments conducted under more polluted conditions (usually at night and early morning) had significantly higher SOA formation, with the concentration of the organic aerosol at the end being about 4 times higher than the initial level. Also, production of 4–230 µg m−3 of ammonium nitrate was observed in all experiments due to the high levels of ammonia in this area. The produced SOA appeared to increase as the ambient relative humidity increased, but other parameters could also be responsible for this. There was not a clear relationship between the SOA and temperature, while higher SOA production was observed when the PM1 levels in the Po Valley were high. Contrary to the Po Valley, only one experiment in the Pertouli forest resulted in the formation of detectable SOA (about 1 µg m−3). This experiment was characterized by higher ambient concentrations of both monoterpenes and isoprene. In two experiments, some SOA was formed, but its concentration dropped below detection levels after 30 min. This behavior is consistent with local formation in a chamber that was not well mixed. Although both environments have organic aerosol (OA) with O:C in the range of 0.7–0.8, the atmosphere of the two sites had very different potentials of forming SOA. In the Po Valley, the system reacts rapidly, forming large amounts of SOA, while in Pertouli the corresponding SOA formation chemistry appears to have been practically terminated before the beginning of most experiments, so there is little additional SOA formation potential left.
