Heterogeneous formation and light absorption of secondary organic aerosols from acetone photooxidation: Remarkably enhancing effects of seeds and ammonia

Abstract. Secondary organic aerosols (SOA) from highly volatile organic compounds (VOCs) are currently not well represented in numerical models as their heterogeneous formation mechanisms in the atmosphere remain unclear. Based on the smog chamber experiments, here we investigated the yield and formation pathway of SOA from acetone photooxidation in the presence of preexisting haze particles ((NH₄)₂SO₄, and NH₄HSO₄) and mineral dusts (Na₂SO₄) under ammonia-rich conditions. Our results showed that the yield of acetone-derived SOA can be remarkably enhanced via multiphase reactions in the presence of these preexisting seeds especially for the mineral dusts, suggesting that heterogeneous reactions of highly volatile VOCs is an important source of atmospheric SOA. We found that aerosol acidity is a key factor controlling the formation pathways of SOA, in which carbonyls produced from acetone photooxidation dissolve into the aqueous phase of the preexisting seeds and oligomerize into SOA that consist of larger molecules on the acidic aerosols but smaller molecules on the neutral mineral aerosols. Moreover, light absorption ability of SOA formed on (NH₄)₂SO₄ aerosols is stronger than that formed on Na₂SO₄ mineral particles especially in the presence of ammonia. Based on the yields obtained, we estimated the importance of acetone-derived SOA in the global atmosphere, which is 9.5–18.4 Tg yr^-1, equivalent to 8.5–16.4 % of the global SOA budget, suggesting that heterogeneous formation of highly volatile VOCs such as acetone is an importance source of SOA in the atmosphere and should be accounted for in the future model studies.