Long-term measurement of biogenic volatile organic compounds in a rural background area: Contribution to ozone formation

摘要

The measurement of biogenic volatile organic compounds (BVOCs) in ambient air is of special interest in rural background areas such as the Valderejo Natural Park (VNP). VNP is a mixed forest area away from important urban centers and where ozone episodes often occur. The main BVOCs at VNP are monoterpenes and isoprene emitted by conifers and deciduous trees, respectively. Volatile organic compounds (VOCs) are routinely measured in ambient air by using a gas chromatography system fitted with flame ionization detection (GC-FID). In order to obtain comparable results with other sites around the world our system was configured following closely the US EPA guidelines for C2-C11 ozone precursors' measurement in Photochemical Assessment Monitoring Stations (PAMS). On this system isoprene is correctly quantified but monoterpenes suffer some losses and degradation during sampling and analysis, producing two significant chromatographic peaks. In this work, we propose a method to quantify the monoterpenes from response factors, in order to recover their mixing ratios from historical chromatographic databases.Monoterpenes' mixing ratios have been recovered to create a 10-year database (2009-2018) containing the results of average hourly mixing ratios of more than 60 VOCs, including isoprene and monoterpenes. The highest BVOC mixing ratios are recorded in the summer, both the average seasonal mixing ratio (monoterpenes: 0.32 ppbv, isoprene: 0.06 ppbv) and the maximum average hourly values (monoterpenes: 8.07 ppbv, isoprene: 1.78 ppbv). These biogenic compounds have a distinct daily cycle. Isoprene shows its highest mixing ratios in the middle of the day. The mixing ratio of monoterpenes remains at low values during the day, because of their high photochemical activity, and higher values are observed during the night. Regarding the Ozone Formation Potential of non-oxygenated VOCs at VNP, the greatest contribution corresponds to BVOCs (an average of 51% obtained from 10 years of measurements).