Characterization of biogenic volatile organic compounds and their oxidation products at a stressed pine forest close to a biogas power plant

Abstract. In this study, we present real-time measurements of organic aerosol (OA) and biogenic volatile organic compounds (BVOCs) at a pine forest stressed by bark beetles and previous droughts close to a biogas power plant (BPP) in western Germany during June 2020. A proton-transfer-reaction time-of-flight mass spectrometer coupled with a particle inlet (CHARON-PTR-ToF-MS) and a Vocus-PTR-ToF-MS were deployed to measure OA and BVOCs. During the entire measurement period, the average concentration of monoterpenes (2.5 ± 5.3 ppb) was higher than isoprene (0.58 ± 0.54 ppb) and sesquiterpenes (0.01 ± 0.01 ppb). The OA composition mainly consisted of semi-volatile organic compounds formed from monoterpene oxidation. Based on a wind direction analysis, BVOC data were categorized into two groups with main influence from the BPP (WD-BPP) and the forest (WD-forest), respectively. In the WD-BPP group, high concentrations of monoterpenes and sesquiterpenes were attributed to BPP emissions. In the WD-forest group, higher temperatures enhanced the biogenic emissions of isoprene, monoterpenes, and sesquiterpenes especially during daytime, exceeding their photochemical consumption. Positive matrix factorization analysis of VOCs revealed substantial contributions of gaseous organic acids from BVOC oxidation during daytime, while weakly oxidized monoterpene products dominated during nighttime. Moreover, increasing relative humidity promoted the gas-to-particle partitioning of gaseous weakly oxidized monoterpene products, leading to an increase of nighttime OA mass. This study highlights that the variations of BVOCs and their oxidation products are influenced by meteorology, local BPP emissions, and chemical transformation processes at this stressed forest.