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 Obs ervatory, Austria (3106 m) in Fall/Winter 1999/2000, showing high mixing ra tios of anthropogenic and biogenic VOCs at this high attitude site due to u pward mixing of air masses (Geophys. Res. Lett. 2F (2001) 507). Here we giv e an interpretation of proton-transfer-reaction (PTR-MS) mass scans obtaine d in November 1999 based on fragmentation data, GC-PTR-MS analysis and the variability-lifetime relationship, described by the power law, delta (ln(x) ) = A tau (-b). The variability-life time 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) x 10(5) molecules cm(-3). Additionally we were able to indirectly determine a diurnal HO-profile with peak values of (1.3 +/-0.5) x 10(6) molecules cm( -3) around midday. HO-reaction rate coefficients for higher aldehydes (hept anal to nonanal) were estimated due to photochemical losses during a stagna nt 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 var iability in VOC concentrations. This information is essential in understand ing the sources and photochemical processing of VOCs detected in ambient ai r at field measurement sites. (C) 2001 Elsevier Science Ltd. All rights res erved.