Urban and Remote cheMistry modELLing with the new chemical mechanism URMELL: part I gas-phase mechanism development†

Air quality is a globally pressing issue as it poses a major threat for human health and ecosystems. Non-methane volatile organic compounds (NMVOCs) are highly reactive substances and known for their impact on O₃, HO_x (OH + HO₂) and NO_x (NO + NO₂) concentrations. NMVOCs comprise a variety of anthropogenic and biogenic compounds with highly complex and entangled relations. Therefore, it is key to capture these interdependencies for any air quality assessment through modeling. Unfortunately, chemical mechanisms used for air quality modeling are often too simplified and partly outdated. Here, we present the development of the chemical mechanism URMELL (Urban and Remote cheMistry modELLing) comprising an extended chemical treatment of major anthropogenic and biogenic NMVOCs based on current knowledge. Box model simulations of standardized urban and remote conditions were performed with URMELL and other mechanisms, and the obtained concentration time profiles of key compounds were compared. High correlations (>0.9) with the benchmark mechanism MCMv3.3.1 are found for all urban conditions. For remote conditions, the simulations using URMELL have much higher oxidant concentrations, especially for OH reaching concentrations ∼10⁶ molecules per cm³ which is in the same range of measured ambient OH concentrations at remote isoprene-dominated sites. For further evaluation, URMELL was applied in the chemical transport model COSMO-MUSCAT and simulations for Germany in May 2014 were performed. Modeled O₃, NO and NO₂ concentrations were compared with 57 measurement sites indicating improved ozone correlations for urban as well as remote isoprene-influenced sites than the currently applied mechanism.