Impact of the chemical regime on highly oxygenated molecule and secondary organic aerosol formation: Effects of varying the importance of NO, HO₂·, RO₂· reactions on α-pinene photooxidation products

Abstract. An important source of secondary organic aerosol (SOA) are highly oxygenated molecules (HOMs) formed by atmospheric oxidation of volatile organic compounds. HOM formation is governed by the fate of HOM peroxy radicals (RO₂·) which depends on the availability of reaction partners (RO₂·, HO₂·, NO) and on their overall lifetime – factors often insufficiently explored in laboratory studies.

We performed α-pinene photooxidation experiments systematically exploring these parameters and present the impacts on HOM and SOA formation within a generic framework explaining the changes in HOM production. Steady-state experiments were performed in the SAPHIR-STAR atmospheric simulation chamber. The reaction regime was shifted by increasing HO₂· and NO, separately and simultaneously, while keeping the α-pinene primary oxidation conditions constant. (NH₄)₂SO₄ particles were added to observe gas-phase HOM condensation and investigate product volatilities.

We find decreasing SOA formation potential when moving away from RO₂·-dominated regimes. One reason is the suppression of HOM accretion product formation from HOM-RO₂·+RO₂·. Alkoxy radicals (RO·) from RO₂·+RO₂· or RO₂·+NO play another important role. RO· are crucial intermediates in certain HOM formation pathways but also produce lower-mass, more fragmented HOM with higher volatility, decreasing SOA formation potential. Additionally, RO₂·+NO forms organic nitrates, which we show have higher volatility than other termination products. Our mechanistic considerations illustrate which factors impact the HOM product distribution and explain the reduced SOA formation through changes in HOM composition and volatility.