Influence of aromatics on tropospheric gas-phase composition

Abstract

Abstract. Aromatics contribute a significant fraction to organic compounds in the troposphere and are mainly emitted by anthropogenic activities and biomass burning. Their oxidation in lab experiments is known to lead to the formation of ozone and aerosol precursors. However, their overall impact on tropospheric composition is uncertain as it depends on transport, multiphase chemistry, and removal processes of the oxidation intermediates. Representation of aromatics in global atmospheric models has been either neglected or highly simplified. Here, we present an assessment of their impact on gas-phase chemistry, using the general circulation model EMAC (ECHAM5/MESSy Atmospheric Chemistry). We employ a comprehensive kinetic model to represent the oxidation of the following monocyclic aromatics: benzene, toluene, xylenes, phenol, styrene, ethylbenzene, trimethylbenzenes, benzaldehyde, and lumped higher aromatics that contain more than nine C atoms. Significant regional changes are identified for several species. For instance, glyoxal increases by 130 % in Europe and 260 % in East Asia, respectively. Large increases in HCHO are also predicted in these regions. In general, the influence of aromatics is particularly evident in areas with high concentrations of NOx, with increases up to 12 % in O3 and 17 % in OH. On a global scale, the estimated net changes of trace gas levels are minor when aromatic compounds are included in our model. For instance, the tropospheric burden of CO increases by about 6 %, while the burdens of OH, O3, and NOx (NO+NO2) decrease between 3 % and 9 %. The global mean changes are small, partially because of compensating effects between high- and low-NOx regions. The largest change is predicted for the important aerosol precursor glyoxal, which increases globally by 36 %. In contrast to other studies, the net change in tropospheric ozone is predicted to be negative, −3 % globally. This change is larger in the Northern Hemisphere where global models usually show positive biases. We find that the reaction with phenoxy radicals is a significant loss for ozone, on the order of 200–300 Tg yr−1, which is similar to the estimated ozone loss due to bromine chemistry. Although the net global impact of aromatics is limited, our results indicate that aromatics can strongly influence tropospheric chemistry on a regional scale, most significantly in East Asia. An analysis of the main model uncertainties related to oxidation and emissions suggests that the impact of aromatics may even be significantly larger.

Development and evaluation of a new compact mechanism for aromatic oxidation in atmospheric models

“…The Module Efficiently Calculating the Chemistry of the Atmosphere (MECCA; Sander et al (2011)) includes detailed aromatic chemistry described by Cabrera-Perez et al (2016). The chemistry of toluene and benzene is taken from MCM without simplification, but with some important updates (notably to phenoxy-phenylperoxy radical cycling) described in detail by Cabrera-Perez et al (2016) and Taraborrelli et al (2021). MECCA lumps the xylene isomers and sets their chemistry identical to that of toluene, aside from the branching ratios in their initial reaction with OH.…”

“…MECCA lumps the xylene isomers and sets their chemistry identical to that of toluene, aside from the branching ratios in their initial reaction with OH. Despite its large number of species and reactions, MECCA has been used in a global 3-D model study by Taraborrelli et al (2021).…”

“…NO 2 , which provides a terminal sink and the only route to fragmentation products from this radical system. Here, we treat the phenoxy-phenylperoxy system similarly to their representation in recent work by Taraborrelli et al (2021), including the phenylperoxy + NO 2 reaction (Jagiella and Zabel, 2007), the phenoxy + O 3 reaction (Tao and Li, 1999), explicit treatment of phenyl hydroperoxide and nitrophenols (lumped with nitrocatechols), and HONO formation from nitrophenol photolysis (Bejan et al, 2006;Chen et al, 2011).…”

Oxidation of low-molecular weight organic compounds in cloud droplets: global impact on tropospheric oxidants

“…c Stratospheric-tropospheric exchange. d O 3 loss due to reaction with phenoxy radicals from oxidation of aromatics (Taraborrelli et al, 2021). e Calculated based on mean burden and the mean total production.…”

“…Further, it is often limited to basic sulfur dioxide (SO 2(aq) ) oxidation as the only incloud O 3(aq) destruction pathway in the aqueous phase (Ervens, 2015). The reduced Chemical Aqueous Phase Radical Mechanism (CAPRAM-RED) is based on CAPRAM 3.0i (Tilgner and Herrmann, 2010) and represents about 200 reactions (Deguillaume et al, 2009). So far, it has been applied in 2-D applications using the regional chemistry transport model COSMO-MUSCAT (Deguillaume et al, 2009;Schrödner et al, 2014).…”