OH Oxidation of α-Pinene in the Atmosphere Simulation Chamber SAPHIR: Investigation of the Role of Pinonaldehyde Photolysis as an HO2 Source

Abstract:

About one third of the land surface is covered by forests, emitting approximately 75% of the total biogenic volatile organic compounds (BVOCs). The main atmospheric sink of these BVOCs during daytime is the oxidation by the hydroxyl radical (OH). Over the last decades field campaigns investigating the radical chemistry in forested regions showed that atmospheric chemistry models are often not able to describe the measured OH concentration well. At low NO concentrations and an OH reactivity dominated by BVOCs the OH was underestimated. This discrepancy could only partly be explained by the discovery of new OH regeneration pathways in the isoprene oxidation mechanism. Field campaigns in the U.S.A and Finland (Kim 2013 ACP, Hens 2014 ACP) demonstrated that in monoterpene (e.g. α-pinene) dominated environments model calculations also underpredict the observed HO₂ and OH concentrations significantly even if the OH budget was closed by the measured OH production and destruction terms. These observations suggest the existence of an unaccounted source of HO₂. One potential HO₂ source in forests is the photolysis of monoterpene degradation products such as aldehydes. In the present study the photochemical degradation mechanism of α-pinene was investigated in the Jülich atmosphere simulation chamber SAPHIR. The focus of this study was in particular on the investigation of the role of pinonaldehyde, a main first generation product of α-pinene, as a possible HO₂ source. For that purpose the pinonaldehyde yields of the reaction α-pinene + OH were determined at ambient monoterpene concentrations (<5 ppb) under low NO_x as well as high NO_x conditions. The pinonaldehyde yield under high NO_x conditions (30.5 %) is in agreement with literature values of Wisthaler (2001 AE) and Aschmann (2002 JGR), under low NO_x conditions the yield (10.8 %) is approximately a factor of three lower than the value published by Eddingsaas (2012 ACP). In a second set of experiments the photolysis rate of pinonaldehyde was determined from the decay of the measured concentration time series of pinonaldehyde in the presence of an OH scavenger. The photolysis experiments showed that the pinonaldehyde photolysis is about a factor of 5 faster than assumed by MCM 3.2, leading to a significant rise in the modelled HO₂ concentration.