Formation of highly oxygenated organic aerosol in the atmosphere: Insights from the Finokalia Aerosol Measurement Experiments

[1] Aged organic aerosol (OA) was measured at a remote coastal site on the island of Crete, Greece during the Finokalia Aerosol Measurement Experiments (FAME‐08 and FAME‐ 09), which were part of the EUCAARI intensive campaigns. Quadrupole aerosol mass spectrometers (Q‐AMSs) were employed to measure the size‐resolved chemical composition of non‐refractory submicron aerosol (NR‐PM 1), and to estimate the extent of oxidation of the OA. The experiments provide unique insights into ambient oxidation of aerosol by measuring at the same site but under different photochemical conditions. NR‐PM 1 concentrations were about a factor of three lower during FAME‐09 (winter) than during FAME‐08 (summer). The OA sampled was significantly less oxidized and more variable in composition during the winter than during the early summer. Lower OH concentrations in the winter were the main difference between the two campaigns, suggesting that atmospheric formation of highly oxygenated OA is associated with homogeneous pho-tochemical aging.