Influence of Biogenic VOCs on Photooxidant Formation: Simulation Experiments in EUPHORE and Comparison with Model Calculations

US/German Workshop Riverside 1999 L. Ruppert 1 Influence of Biogenic VOCs on Photooxidant Formation: Simulation Experiments in EUPHORE and Comparison with Model Calculations L. Ruppert Fraunhofer Institut Atmosphärische Umweltforschung, IFU Kreuzeckbahnstr. 19, D-82467 Garmisch-Partenkirchen, e-mail: [email protected] Introduction The possible contribution of biogenic VOCs to tropospheric ozone formation even in highly developed regions like central Europe or the eastern United States is strongly discussed. Isoprene and monoterpenes account for a main fraction of the biogenic VOC emissions. Several recent field studies have shown situations where isoprene in fact dominates the daytime photochemistry and thereby contributes strongly to the regional ozone formation (Staffelbach et al., 1997; Biesenthal et al., 1998; Roberts et al., 1998). Despite recent progress, the degradation mechanisms of monoterpenes are far from being sufficiently understood and even in the case of isoprene the quantified products account for only 60 70% of the carbon balance. Due to the complexity of the compounds and their reactions, causing exceptional analytical difficulties, this knowledge is likely to improve only gradually. This work represents a complementary, not alternative, integrated approach to the investigation of mechanistic details in laboratory studies. Experimental Several smog chamber runs have been carried out in the outdoor simulation chamber EUPHORE in Valencia, Spain (Becker, 1996). A three-component VOC mixture (base mix) was chosen as a reference case. The composition of the base mix (n-butane, 50%-C, ethene and toluene 25%-C, each) was chosen (1) to represent the proportions of the main VOC classes, alkanes, alkenes and aromatic hydrocarbons, typically measured in relatively polluted ambient air and (2) to deal with compounds whose oxidation pathways are relatively well known, in order to reduce the uncertainties for the modeling of the reference system. By adding relatively small amounts of a fourth VOC of interest, e.g. a biogenic one, changes in the NOx transformation and ozone formation relative to the base mix runs should be observed. This experimental approach is mainly based on similar experiments by Carter et al., 1995, who used indoor smog chambers with artificial light sources. In this contribution we report on experiments with isoprene, α-pinene, limonene and the potential fuel additive di-ethoxy methane (DEM) as additives to the base mix. The initial carbon and NOx concentrations of 2 ppm-C and 200 ppb, respectively, as well as the proportions of the base mix VOCs were kept constant in all experiments. In every experiment the mixture was exposed to sunlight for at least 7 h, centered around midday. Besides the reacting VOCs, ozone, NO and NO2, also the concentrations of a number of reaction products could be measured time-resolved, e.g. formaldehyde, 2-butanone, methyl vinyl ketone, methacrolein and PAN. Abstract US/German Workshop Riverside 1999 L. RuppertUS/German Workshop Riverside 1999 L. Ruppert 2 Experimental Results In Fig. 1 the concentration-time profiles of several species from two experiments, a base case and one with added α-pinene, are plotted for comparison. The experiments were run on two consecutive days with clear sky conditions so that light, temperature and chamber conditions are virtually equal during both runs. The most obvious difference lies in the enhanced ozone formation in the α-pinene experiment, which continues even after the α-pinene has completely reacted. The accurate data analysis revealed that in the α-pinene experiment, compared to the base run, [OH] is slightly suppressed in the beginning of the run, when the overall OH-reactivity is substantially increased due to the highly reactive α-pinene, but significantly increased in the second half of the experiment, after the α-pinene had reacted. 0 50 100 150 200 250 300 350 400 7:30 8:30 9:30 10:30 11:30 12:30 13:30 14:30 15:30 16:30 GMT ozone