Formation and chemical aging of secondary organic aerosol during the β-caryophyllene oxidation

Abstract. The secondary organic aerosol (SOA) production during the oxidation of β-caryophyllene by ozone (O₃) and hydroxyl radicals (OH) and the subsequent chemical aging of the products during reactions with OH were investigated. Experiments were conducted with ozone and with hydroxyl radicals at low NO_x (zero added NO_x) and at high NO_x (hundreds of parts per billion). The SOA mass yield at 10 μg m⁻³ of organic aerosol was 27% for the ozonolysis, 20% for the reaction with OH at low NO_x, and 38% at high NO_x under dry conditions, 20 °C, and ozone excess. Parameterizations of the fresh SOA yields have been developed. The average fresh SOA atomic O : C ratio varied from 0.24 to 0.34 depending on the oxidant and the NO_x level, while the H : C ratio was close to 1.5 for all systems examined. An average density of 1.06 ± 0.1 μg m⁻³ of the β-caryophyllene SOA was estimated. The exposure to UV light had no effect on the β-caryophyllene SOA concentration and aerosol mass spectrometer (AMS) measurements. The chemical aging of the β-caryophyllene SOA produced was studied by exposing the fresh SOA to high concentrations (10⁷ molecules cm⁻³) of OH for several hours. These additional reactions increased the SOA concentration by 15–40% and O : C by approximately 25%. A limited number of experiments suggested that there was a significant impact of the relative humidity on the chemical aging of the SOA. The evaporation rates of β-caryophyllene SOA were quantified by using a thermodenuder allowing us to estimate the corresponding volatility distributions and effective vaporization enthalpies.