Organic peroxides' gas-particle partitioning and rapid heterogeneous decomposition on secondary organic aerosol

Abstract. Organic peroxides, important species in the atmosphere, promote secondary organic aerosol (SOA) aging, affect HO_x radicals cycling, and cause adverse health effects. However, the formation, gas-particle partitioning, and evolution of organic peroxides are complicated and still unclear. In this study, we investigated in the laboratory the production and gas-particle partitioning of peroxides from the ozonolysis of α-pinene, which is one of the major biogenic volatile organic compounds in the atmosphere and an important precursor for SOA at a global scale. We have determined the molar yields of hydrogen peroxide (H₂O₂), hydromethyl hydroperoxide (HMHP), peroxyformic acid (PFA), peroxyacetic acid (PAA), and total peroxides (TPOs, including unknown peroxides) and the fraction of peroxides in α-pinene/O₃ SOA. Comparing the gas-phase peroxides with the particle-phase peroxides, we find that gas-particle partitioning coefficients of PFA and PAA are 10⁴ times higher than the values from the theoretical prediction, indicating that organic peroxides play a more important role in SOA formation than previously expected. Here, the partitioning coefficients of TPO were determined to be as high as (2–3)  ×  10⁻⁴ m³ µg⁻¹. Even so, more than 80 % of the peroxides formed in the reaction remain in the gas phase. Water changes the distribution of gaseous peroxides, while it does not affect the total amount of peroxides in either the gas or the particle phase. Approx. 18 % of gaseous peroxides undergo rapid heterogeneous decomposition on SOA particles in the presence of water vapor, resulting in the additional production of H₂O₂. This process can partially explain the unexpectedly high H₂O₂ yields under wet conditions. Transformation of organic peroxides to H₂O₂ also preserves OH in the atmosphere, helping to improve the understanding of OH cycling.