Chemical composition of the BBOA particles. The AMS results (Figure and Table ) indicate that both the BBOA samples were mainly organic (> 90 wt %), which is consistent with previous measurements of BBOA from laboratory burns of pine wood. The O:C was 0.73-0.74, which is higher than for freshly emitted laboratory BBOA and unaged field BBOA but consistent with some BBOA aged for intermediate timescales. Phase behavior. Both samples showed two phases across the full range of RH values (Figure ). Phase behaviour results were the same when increasing and decreasing the RH. Others have shown that phase separation can occur in mixtures of primary organic aerosol (POA) and secondary organic aerosol (SOA), and in mixtures of different types of SOA, if the organic molecules making up the mixtures have a difference of > 0.2 in O:C values. In addition, previous studies have shown that unaged BBOA can contain organic molecules with a wide range of O:C values. Therefore, phase separation in our BBOA can be explained by a large difference in O:C values of the organic molecules making up the BBOA. Our samples also likely contain secondary BBOA in addition to primary BBOA. Secondary BBOA should have organic molecules with higher O:C values than primary BBOA, further enhancing phase separation when mixed with primary BBOA. Phase separation in aerosol particles can also be driven by the coexistence of organic and inorganic salts which are soluble in water, but the AMS results (Table ) indicate that BBOA samples from both sites were mainly organic. Hence, the presence of inorganic salts was most likely not the main driver for the two phases. It is likely that very low polarity compounds in the BBOA samples, such as long-chain hydrocarbons, were not extracted by methanol and water. If these compounds are present in the whole BBOA prior to sampling, they should have an even lower O:C than any compounds we extracted, thereby increasing the tendency for there to be phase separation.