Analysis of the dependency of atmospheric formaldehyde - as a proxy for bVOC emissions - on vegetation status over a Central European city and potential implications for surface ozone exceedances

In the city centre of Vienna, Austria ozone (maximum 8 hour mean) mda8 exceedances of the threshold value of 120 μg/m³ can occur from as early as March until September, which coincides with the main local vegetation season. Biogenic volatile organic compounds (bVOCs), which are mainly emitted by forests, but also other vegetation as agricultural field crops and are precursor substances to atmospheric formaldehyde (HCHO). Thereby they contribute to the production of ozone in and around the city. On the other hand, vegetated areas reduce the ozone concentration by uptake via stomatal and cuticular pathways and soil uptake.In this study the dependency of HCHO mixing ratios, obtained from path averaged MAX-DOAS UV retrievals over the Vienna city centre, on meteorological parameters (air temperature, global radiation, boundary layer height) and vegetation drought stress indicators are analysed, focusing on the difference between drought and non-drought conditions. Following indicators are used: standardized precipitation index (SPI), relative soil saturation from the Agricultural Risk Information System (ARIS), vapour pressure deficit and satellite-based photosynthetically active radiation anomaly (fAPAR) as well as solar-induced chlorophyll fluorescence (SIF).A clear dependency of the HCHO on vegetation-related parameters and the area of origin of HCHO and its precursor substances is found. However, the strength of the relationship between the parameters changes depending on the vegetation status. The results of the observational HCHO analyses spanning 2017-2021 are compared with bVOCs estimates of the Model of Emissions of Gases and Aerosols from Nature (MEGAN). The observed ozone concentrations are compared with the ozone mixing ratios and dry deposition rates calculated by the chemical transport model developed at Meteorological Synthesizing Centre-West within the European Monitoring and Evaluation Program (EMEP MSC-W model), which includes the Deposition of Ozone for Stomatal Exchange (DO3SE) model, to better understand timing and magnitudes of sources and sinks. Possible consequences for exceedances of the mda8 ozone target value in the study region are discussed.