Abstract
Abstract. Ground-level ozone is a secondary pollutant produced photochemically from reactions of NOx with peroxy radicals produced during volatile organic compound (VOC) degradation. Chemical transport models use simplified representations of this complex gas-phase chemistry to predict O3 levels and inform emission control strategies. Accurate representation of O3 production chemistry is vital for effective prediction. In this study, VOC degradation chemistry in simplified mechanisms is compared to that in the near-explicit Master Chemical Mechanism (MCM) using a box model and by "tagging" all organic degradation products over multi-day runs, thus calculating the tagged ozone production potential (TOPP) for a selection of VOCs representative of urban air masses. Simplified mechanisms that aggregate VOC degradation products instead of aggregating emitted VOCs produce comparable amounts of O3 from VOC degradation to the MCM. First-day TOPP values are similar across mechanisms for most VOCs, with larger discrepancies arising over the course of the model run. Aromatic and unsaturated aliphatic VOCs have the largest inter-mechanism differences on the first day, while alkanes show largest differences on the second day. Simplified mechanisms break VOCs down into smaller-sized degradation products on the first day faster than the MCM, impacting the total amount of O3 produced on subsequent days due to secondary chemistry.
Highlights
Ground-level ozone (O3) is both an air pollutant and a climate forcer that is detrimental to human health and crop growth (Stevenson et al, 2013)
This study compares the impacts of different simplification approaches of chemical mechanisms on O3 production chemistry focusing on the role of volatile organic compound (VOC) degradation products
PAN photolysis is only important in the free troposphere (Harwood et al, 2003) and was removed from MOZART-4, RACM2 and CB05 for the purpose of the study, as this study considers processes occurring within the planetary boundary layer
Summary
Ground-level ozone (O3) is both an air pollutant and a climate forcer that is detrimental to human health and crop growth (Stevenson et al, 2013). Despite decreases in emissions of O3 precursors over Europe since 1990, EEA (2014) reports that 98 % of Europe’s urban population are exposed to levels exceeding the WHO air quality guideline of 100 μg m−3 over an 8 h mean. These exceedances result from local and regional O3 precursor gas emissions, their intercontinental transport and the non-linear relationship of O3 concentrations to NOx and VOC levels (EEA, 2014). Effective strategies for emission reductions rely on accurate predictions of O3 concentrations using chemical transport models (CTMs) These predictions require adequate representation of gas-phase chemistry in the chemical mechanism used by the CTM. This study compares the impacts of different simplification approaches of chemical mechanisms on O3 production chemistry focusing on the role of VOC degradation products
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