Atmospheric oxidation of 1-butene initiated by OH radical: Implications for ozone and nitrous acid formations

Abstract

The present study combines (i) field measurements of 1-butene over Xi'an and surrounding areas during a heavy ozone pollution episode with (ii) quantum chemical calculations on the photochemical transformation of 1-butene initiated by OH radical. The measurements show that the diurnal variation of both 1-butene and ozone exhibits a negative relationship during the sampling campaign. The concentrations of 1-butene vary from 0.04 to 0.67 ppbv, and the lowest and highest values emerge in the noon time (1 p.m.–3 p.m.) and in the night time (1 a.m.–3 a.m.). The concentrations of O3 range from 32.36 to 88.82 ppbv, and the high values appear in the noon time. Quantum chemical calculations show that the OH-addition pathways are more energetically preferable than H-abstraction channels, and the rate coefficient exhibits a negative T-dependence in the temperature range of 273–400 K. The autoxidation of peroxy radicals (RO2) formed from the addition of molecular oxygen to OH-addition products S2 and S3 leads to the formation of highly oxygenated molecules (HOMs), C4H9O5, in which the 1,5-H shift reaction is favoured. In the low-NOx regions, RO2 radicals mainly react with HO2 radical leading to hydroperoxide ROOH on both the singlet and triplet PESs, in which the triplet PES is preferable. In the high-NOx regions, RO2 radicals mainly react with NO leading to the formation of RO· + NO2, aldehyde + HONO and organic nitrate, in which the HONO formation appears to be previously unconsidered in the photochemical transformation of alkenes. The branching ratio of RO· + NO2 increases slowly with increasing temperature (from 51.3 to 52.7%), whereas aldehyde + HONO gradually decreases with the temperature rising (from 48.7 to 47.3%). The formation of organic nitrate is negligible over the temperature range of 273–400 K. These findings are expected to deepen our understanding the photochemistry oxidation of alkene under realistic atmospheric conditions.