Ambient sesquiterpene concentration and its link to air ion measurements

摘要

Ambient air ion size distributions have been measured continuously at theFinnish boreal forest site in Hyyti?l? since spring 2003. In general,these measurements show a maximum of air ions below 1.0 nm in diameter. Butthis physical characterization does not provide any information about theion's chemical composition, which is one key question regarding theexplanation of nucleation events observed. In this study we propose a link ofthe observed maximum of negative air ions between 0.56 and 0.75 nm to theso-called stabilised Criegee biradical, formed in the reaction of biogenicsesquiterpenes with ozone and predominantly destroyed by its reaction withambient water vapour. Calculations of the electron and proton affinities of120 kJ mol−1 (1.24 eV) and of 960 kJ mol−1 supportthis link. Other possible candidates such as sulphuric acid derived clustersare unable to explain the observations made. By using this approach, we areable to calculate the ambient concentration of sesquiterpenes at the air ioninstrument inlet with a high time resolution on the daily and seasonal scale.The estimated concentration is found to reveal the same seasonal pattern asemission measurements conducted at shoot level. As expected for biogenicVOCs, the concentration is obtained highest during summer (maximum values ofabout 100 pptv) and smallest during winter (minimum less than 1 pptv).Because of the sesquiterpenes high reactivity and its low ambientconcentrations, this approach can be a first step in understanding theiremission and their impact on atmospheric chemistry in more detail. Thefindings presented are highly relevant for emission budgets too, since borealforests are extended over large areas of the globe.