Evaluation of the daytime tropospheric loss of 2-methylbutanal

Abstract

Saturated aldehydes, e.g. 2-methylbutanal (2 MB, CH3CH2CH(CH3)C(O)H), are emitted into the atmosphere by several biogenic sources. The first step in the daytime atmospheric degradation of 2 MB involves gas-phase reactions initiated by hydroxyl (OH) radicals, chlorine (Cl) atoms, and/or sunlight. In this work, we report the rate coefficients for the gas-phase reaction of 2 MB with OH (kOH) and Cl (kCl), together with the photolysis rate coefficient (J), in the ultraviolet solar actinic region in Valencia (Spain) at different times of the day. The temperature dependence of kOH was described in the 263–353 K range by the following Arrhenius expression: kOH(T)=(8.88±0.41)×10-12 exp[(331±14)/T] cm3 molec.−1 s−1. At 298 K, the reported kOH and kCl are (2.68±0.07)×10-11 and (2.16±0.32)×10-10 cm3 molec.−1 s−1, respectively. Identification and quantification of the gaseous products of the Cl reaction and those from the photodissociation of 2 MB were carried out in a smog chamber by different techniques (Fourier transform infrared spectroscopy, proton transfer time-of-flight mass spectrometry, and gas chromatography coupled to mass spectrometry). The formation and size distribution of secondary organic aerosols formed in the Cl reaction were monitored by a fast mobility particle sizer spectrometer. A discussion on the relative importance of the first step in the daytime atmospheric degradation of 2 MB is presented together with the impact of the degradation products in marine atmospheres.