Using total OH reactivity to assess isoprene photooxidation via measurement and model

• Published in: Atmospheric environment (1994) 2014-06, Vol.89, p.453-463
• Main Authors: Nölscher, A.C., Butler, T., Auld, J., Veres, P., Muñoz, A., Taraborrelli, D., Vereecken, L., Lelieveld, J., Williams, J.
• Format: Article
• Language: English
• Subjects: Applied sciences; Atmospheric pollution; Exact sciences and technology; Hydroxyl radical; Isoprene photooxidation; Mainz Isoprene Mechanism Extended MIME; Master Chemical Mechanism MCM 3.2; Pollutants physicochemistry study: properties, effects, reactions, transport and distribution; Pollution; Total OH reactivity
• ISSN: 1352-2310; 1873-2844
• DOI: 10.1016/j.atmosenv.2014.02.024

The Tropics provide a reactive atmospheric environment with high levels of biogenic emissions, rapidly growing anthropogenic influence, high solar radiation and temperature levels. The major reactive biogenic emission is isoprene which reacts rapidly with the primary daytime oxidant OH, the hydroxyl radical. This key photooxidation process has recently been the focus of several experimental and computational studies. A novel isoprene degradation mechanism was recently proposed (MIME) supplementing the commonly used MCM 3.2 scheme. This study examined the photooxidation of isoprene in the controlled conditions of the Valencia atmospheric reaction chamber, EUPHORE (EUropean PHOtoREactor). Besides the detection of isoprene and its major oxidation products formaldehyde, methyl vinyl ketone (MVK) and methacrolein (MACR), the total loss rate of OH (total OH reactivity) was measured. The total OH reactivity was compared to the individual measurements of isoprene and its oxidation products to assess the significant contributors to the overall OH loss rate. Measured total OH reactivity showed excellent agreement to the calculation based on individual compounds detected by a Proton-Transfer-Reaction-Time-Of-Flight-Mass-Spectrometer (PTR-TOF-MS). On average 97% of the measured total OH reactivity could be explained by isoprene and its major oxidation products. Total OH reactivity was also compared to various isoprene degradation schemes to evaluate known mechanisms. The MCM 3.2 isoprene mechanism reproduced the temporal degradation of total OH reactivity (and isoprene) reasonably well with a 57% (and 95%) agreement within the model uncertainties and a linear curve fit slope of 0.69 (and 1.02) for a model to measurement correlation. Large discrepancies between modeled values and all observed compounds were found for the recent isoprene oxidation scheme in MIME. Possible mechanistic reasons are discussed and improvements proposed. The subsequently modified version of MIME differed from the measured total OH reactivity only about 12% at the end of the experiment and represented best the overall temporal profile (linear curve fit slope of correlation: 0.95). •Isoprene photooxidation was examined via total OH reactivity measurements.•Isoprene and major products accounted for the observed total OH reactivity.•The commonly used MCM 3.2 reproduced the experimental findings reasonably well.•The recent isoprene chemistry in MIME showed large discrepancies to the measurements.•