Atmospheric photooxidation and ozonolysis of sabinene: Reaction rate constants, product yields and chemical budget of radicals

Abstract. The oxidation of sabinene by the hydroxyl radical (OH) and ozone (O₃) was investigated under atmospherically relevant conditions in the atmospheric simulation chamber SAPHIR at Forschungszentrum Jülich, Germany. The rate constants of the reactions of sabinene with OH and with O₃ were determined. The temperature dependence between 284 K to 340 K of the rate constant of the reaction of sabinene with OH, k_SAB+OH, was measured for the first time using an OH reactivity instrument, resulting in an Arrhenius expression of (1.67 ± 0.16) × 10⁻¹¹ × exp((575 ± 30) T⁻¹) cm³ s⁻¹. The values agree with those determined in chamber experiments in this work and reported in the literature for ~298 K within the uncertainties of measurements. The ozonolysis reaction rate constant of sabinene (k_SAB+O3) determined in chamber experiments at a temperature of (278 ± 2) K is (3.7 ± 0.5) × 10⁻¹⁷ cm³ s⁻¹, which is 55 % lower than the value reported in the literature for room temperature. The measurement of products from the oxidation of sabinene by OH resulted in an acetone yield of (21 ± 15) %, a formaldehyde yield of (46 ± 25) %, and a sabinaketone yield of (19 ± 16) %. All yields determined in the chamber experiments agree well with values from previous laboratory studies within their uncertainties. In addition, the formaldehyde yield determined in this study is consistent with that predicted by the sabinene OH-oxidation mechanism which was devised from quantum chemical calculations by Wang and Wang (2018), whereas the acetone yield is about 15 % absolute higher than that predicted by the mechanism. In the ozonolysis experiments, the analysis if product measurements results in an acetone yield of (5 ± 2) %, a formaldehyde yield of (48 ± 15) %, a sabinaketone yield of (31 ± 15) %, and an OH radical yield of (18 ± 25) %. The OH radical yield is lower than expected from the theoretical mechanism in Wang and Wang (2017), but the value still agrees within the uncertainty. An analysis of the chemical budget of OH radicals was performed for the chamber experiments. The analysis reveals that the destruction rate of OH radcials matches the production rate of OH suggesting that there is no significant missing OH source for example from isomerization reactions of peroxy radicals for the experimental conditions in this work.