Interannual Variability of BVOC Emissions in an Alpine City

Abstract. Terpenoid emissions above urban areas are a complex mix of biogenic and anthropogenic emission sources. In line with previous studies we found that summertime terpenoid emissions in an alpine city were dominated by biogenic sources, but especially at lower temperatures the anthropogenic influences were non-negligible. Inter-seasonal emission measurements revealed consistency for monoterpenes and sesquiterpenes, but a large difference in isoprene between the summers 2015 and 2018. Standardized emission potentials for monoterpenes and sesquiterpenes were 0.12 nmol m-2 s-1 and 3.0·10-3 nmol m-2 s-1 in 2015 and 0.11 nmol m-2 s-1 and 3.4·10-3 nmol m-2 s-1 in 2018, respectively. Observed isoprene emissions were about four times higher in 2018 than in 2015. This factor decreased to 2.3 after standardizing isoprene emissions to 30 °C air temperature and photosynthetic active radiation (PAR) of 1000 μmol m-2 s-1. Based on emission model parameterizations, increased leaf temperatures can explained ~50 % of these differences, but standard emission potentials remained higher in 2018, when a heat wave persisted. Potential other reasons for the differences such as emission parameterization, footprint changes, water stress conditions and tree trimming are investigated.