On the budget of hydroxyl radicals at Schauinsland : role of biogenic volatile organic compounds
- Publisher: Forschungszentrum, Zentralbibliothek
The concentration of hydroxyl radicals is estimated from the degradation of NOx and selected Volatile Organic Compounds (VOCs) during the transport of polluted air masses from the City of Freiburg to the Schauinsland mountain through a steep valley called "Großes Tal" . The approach is based upon chemical and meteorological measurements male during the SLOPE (Schauinsland Ozone Precursor Experiment) campaign at two ground based sites and aboard a small aircraft . Separation of chemistry and transport is achieved through the degradation of chemical compounds with signiffcantly different reactivity towards OH . The transport time of the air between the two measurements (100±20 min) and the infiuence of inixing with background air on the calculated OH concentration is quantified with the help of the results from a dispersion experiment with SF6 and from airborne chemical measurements made during SLOPE. Five in-situ gas chromatography (GC) systems were deployed for the VOC measurements. In addition, samples were collected at the surface sites on charcoal adsorption tubes and analysed by gas chromatography/mass spectrometry (GCIMS) . The uncertainties of the different measurements are discussed on the basis of intercomparisons with a standard mixture and simultaneous measurements of ambient air. Deviations of the in-situ GCs at the standard mixture are mostly within 5-30 % . The GC/MS measurements were systematically lower by about 25 % . Several compounds are identified that were measured by identical systems with an inter-system comparability of heiter than ±10 %. These compounds are utilized for the estimation of OH concentrations . When averaged over all identified compounds in the Tange C2-C15 and all instruments, the overall uncertainty of the total measured VOC concentrations during SLOPE is estimated to ±25 %. By combining all ground based VOC measurements (C 2-C15), it was found that the total VOC concentration is dominated by anthropogenic hydrocarbons (alkanes and aromatics) at the site . The contribution of biogenic VOCs to the total VOC reactivity (E[VOCi]•k0 ) in the air masses is about 30 % at the entrance of the valley and about 70 % at Schauinsland. OH concentrations of 1±0.2. 10' cm-3 are estimated from the degradation of VOCs and NOx between the two measuring sites. The results of a parameter analysis showed that iiixiiig of tLhiic plume -wü'ih-Ll. agcu background air can lead to an overestirnation of the OH concentrations . Emissions in the valley between the two measuring sites, would lead to an underestimation of the OH concentrations . The in.fluence of rnixing of the plume with background air, as characterized by airborne measurements, is 30 % . After background correction, the resulting OH concentration (7±2. 105 cm-3) is almost a factor of two larger than what is calculated with a chemical box model constraint by the measured trace gas concentrations and photolysis rates. The OH concentrations estimated at Schauinsland are in reasonable agreement with the results of the estimates reported elsewhere, taking into account the time of the year and the fast that the most estimates were diurnal averages. The radical budget can be closed within the experimental uncertainties, when an upper limit is adopted for the photolysis of nitrous acid, in addition to the radical production from photolysis of ozone, carbonyl compounds as well as ozonolysis of unsaturated VOCs. Based upon the available evidence, oxidants other than OH should have negligible influence on the estimated OH concentrations from the VOC decay.