Evaluation of the daytime tropospheric loss of 2-methylbutanal

Abstract

Abstract. Saturated aldehydes, e.g. 2-methylbutanal (2MB, CH3CH2CH(CH3)C(O)H), are emitted into the atmosphere by several biogenic sources. The first step in the daytime atmospheric degradation of 2MB 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 2MB 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 molecule-1 s-1. At 298 K, the reported kOH and kCl are (2.68±0.07)×10-11 cm3 molecule-1 s-1 and (2.16±0.16)×10-11 cm3 molecule-1 s-1. Identification and quantification of the gaseous products of the Cl-reaction and those from the photodissociation of 2MB 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 was monitored by a fast mobility particle sizer spectrometer. A discussion on the relative importance of the first step in the daytime atmospheric degradation of 2MB is presented together with the impact of the degradation products in marine atmospheres.