Atmospheric lifetimes and fates of a series of sesquiterpenes

Abstract

Using relative rate techniques, rate constants for the gas phase reactions of OH radicals and NO 3 radicals with the sesquiterpenes α‐cedrene, α‐copaene, β‐caryophyllene, α‐humulene, and longifolene have been measured at 296±2 K and atmospheric pressure of air. The rate constants obtained (in units of 10 −11 cm 3 molecule −1 s −1 ) for the OH radical and NO 3 radical reactions, respectively, are α‐cedrene, 6.7±1.4 and 0.82±0.30; α‐copaene, 9.0±1.9 and 1.6±0.6; β‐caryophyllene, 20 +5 −9 and 1.9±0.8; α‐humulene, 29 +7 −10 and 3.5±1.3; and longifolene, 4.7±1.0 and 0.068±0.021, where the indicated errors include the estimated overall uncertainties in the rate constants for the reference compounds. These rate constants and those for the corresponding O 3 reactions are combined with estimated ambient lower tropospheric concentrations of OH radicals, NO 3 radicals, and O 3 to allow the lifetimes of these sesquiterpenes with respect to these chemical loss processes to be calculated. The sesquiterpenes studied are all reactive, with calculated overall lifetimes of a few hours or less. In particular, β‐caryophyllene and α‐humulene are highly reactive toward O 3 and NO 3 radicals, with calculated lifetimes due to these reactions of 1–2 min, and β‐caryophyllene and α‐humulene are therefore not expected to be present in ambient air at measurable concentrations.