Abstract
Abstract. The degradation mechanism of 1,3,5-trimethyl- benzene (TMB) as implemented in the Master Chemical Mechanism version 3.1 (MCM) was evaluated using data from the environmental chamber at the Paul Scherrer Institute. The results show that the MCM provides a consistent description of the photo-oxidation of TMB/NOx mixtures for a range of conditions. In all cases the agreement between the measurement and the simulation decreases with decreasing VOC-NOx ratio and in addition with increasing precursor concentration. A significant underestimation of the decay rate of TMB and thus underestimation of reactivity in the system, consistent with results from previous appraisals of the MCM, was observed. Much higher nitrous acid (HONO) concentrations compared to simulations and expected from chamber characterization experiments were measured during these smog chamber experiments. A light induced NO2 to HONO conversion at the chamber walls is suggested to occur. This photo-enhanced NO2 to HONO conversion with subsequent HONO photolysis enhances the reactivity of the system. After the implementation of this reaction in the model it describes the decay of TMB properly. Nevertheless, the model still over-predicts ozone at a later stage of the experiment. This can be attributed to a too slow removal of NO2. It is also shown that this photo-enhanced HONO formation is not restricted to TMB photo-oxidation but also occurs in other chemical systems (e.g. α-pinene). However, the influence of HONO as a source of OH radicals is less important in these more reactive systems and therefore the importance of the HONO chemistry is less obvious.
Highlights
Volatile organic compounds (VOC) are emitted into the atmosphere from anthropogenic and biogenic sources
In this paper we describe the evaluation of the Master Chemical Mechanism of 1,3,5-trimethlybenzene photooxidation using the data from the Paul Scherrer Institute (PSI) chamber
We propose a photo-enhanced conversion of NO2 to HONO on the chamber walls in the presence of VOCs to be responsible for part of the discrepancies between model and measurements
Summary
Volatile organic compounds (VOC) are emitted into the atmosphere from anthropogenic and biogenic sources. Aromatics are mostly emitted by fuel combustion and evaporation and will influence mainly urban areas where they contribute 16–44% to the total hydrocarbon mass emitted into the atmosphere (Calvert et al, 2002; Derwent et al, 2000; Molina et al, 2007) They contribute significantly to the production of ozone and other secondary pollutants in these areas. There have been a number of studies evaluating the Master Chemical Mechanism against data of different compounds from various environmental chambers e.g. aromatics, including TMB (Bloss et al, 2005a; Wagner et al, 2003), ethene (Zador et al, 2005), isoprene and its oxidation products (Pinho et al, 2005) as well as various alkenes (Hynes et al, 2005; Pinho et al, 2006) and monoterpenes (Pinho et al, 2007; Saunders et al, 2003b). We propose a photo-enhanced conversion of NO2 to HONO on the chamber walls in the presence of VOCs to be responsible for part of the discrepancies between model and measurements
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