Variability-lifetime relationship of VOCs observed at the Sonnblick Observatory 1999—estimation of HO-densities

An extensive dataset of VOC measurements was collected at the Sonnblick Observatory, Austria (3106 m) in Fall/Winter 1999/2000, showing high mixing ratios of anthropogenic and biogenic VOCs at this high altitude site due to upward mixing of air masses (Geophys. Res. Lett. 2F (2001) 507). Here we give an interpretation of proton-transfer-reaction (PTR-MS) mass scans obtained in November 1999 based on fragmentation data, GC-PTR-MS analysis and the variability-lifetime relationship, described by the power law, σ(ln(x))=Aτ−b. The variability-lifetime plot of anthropogenic VOCs gave a proportionality factor A of 1.40 and a,b exponent (sink term) of 0.44 and allowed an estimate of average HO-densities on the order of (1.5±0.4)×105 molecules cm−3. Additionally we were able to indirectly determine a diurnal HO-profile with peak values of (1.3±0.5)×106 molecules cm−3 around midday. HO-reaction rate coefficients for higher aldehydes (heptanal to nonanal) were estimated due to photochemical losses during a stagnant air episode (27 November) and from the variability-lifetime relationship. Combining long term PTR-MS analysis of VOCs and the variability-lifetime method provides a valuable tool for assessing the dominant cause of the variability in VOC concentrations. This information is essential in understanding the sources and photochemical processing of VOCs detected in ambient air at field measurement sites.