We assigned half the instrument detection limit to all instrument readings below this limit. Based on the results from Ambrose (2017) and Slemr et al. (2016) , we considered 1 pg of mass on the gold trap as the instrumental detection limit under automated peak calibration. At a 15-minute sampling interval and the used flow rate of 1.3 L/min (STP), this corresponds to a detection limit of 0.051 ng m -3 . For uncertainty calculations, we assumed a rectangular (i.e. uniform) distribution between 0.0 ng m -3 and 0.051 ng m -3 .