α-Pinene nitrates: synthesis, yields and atmospheric chemistry

Abstract. The biogenic volatile organic compound α-pinene is one of the dominant monoterpenes emitted to the Earth's atmosphere at an estimated rate of ~50 Tg C yr⁻¹. Its atmospheric oxidation products in the presence of NO can lead to ozone production, as well as production of secondary organic aerosol (SOA). The major oxidation pathway of α-pinene is reaction with OH, which in the presence of NO can form either α-pinene nitrates or convert NO to NO₂, which can photolyze to form ozone. In this work, we successfully synthesized four α-pinene hydroxy nitrates through three different routes, and have identified these 4 individual isomers in α-pinene/OH/NO reaction chamber experiments. From the experiments, we determined their individual production yields, estimated the total RONO₂ yield, and calculated the relative branching ratios of the nitrate precursor peroxy radicals (RO₂). The combined yield of the four α-pinene nitrates was found to be 0.130 (±0.035) at atmospheric pressure and 296 K, and the total organic nitrate yield was estimated to be 0.19 (+0.10/−0.06). We also determined the OH rate constants for two of the isomers, and have calculated their overall atmospheric lifetimes, which range between 22 and 38 h.