Assessing chemistry schemes and constraints in air quality models used to predict ozone in London against the detailed Master Chemical Mechanism

Air pollution is the environmental factor with the greatest impact on human health in Europe. Understanding the key processes driving air quality across the relevant spatial scales, especially during pollution exceedances and episodes, is essential to provide effective predictions for both policymakers and the public. It is particularly important for policy regulators to understand the drivers of local air quality that can be regulated by national policies versus the contribution from regional pollution transported from mainland Europe or elsewhere. One of the main objectives of the Coupled Urban and Regional processes: Effects on AIR quality (CUREAIR) project is to determine local and regional contributions to ozone events. A detailed zero-dimensional (0-D) box model run with the Master Chemical Mechanism (MCMv3.2) is used as the benchmark model against which the less explicit chemistry mechanisms of the Generic Reaction Set (GRS) and the Common Representative Intermediates (CRI v2-R5) schemes are evaluated. GRS and CRI are used by the Atmospheric Dispersion Modelling System (ADMS) and the regional chemistry transport model EMEP, respectively. The MCM model uses a near explicit chemical scheme for the oxidation of volatile organic compounds (VOCs) and is constrained to observations of VOCs, NOx, CO, HONO, photolysis frequencies and meteorological parameters from ClearfLo. The influence of VOCs, NO, HONO and j(NO2) on local O3 production has been investigated. The total VOC concentration measured during the summer and winter ClearfLo observation periods was found to be on average six times greater than that of the generic VOC mix employed in the GRS. The GRS model output is sensitive to the total VOC concentration, with a ~5.5-fold increase in the modelled O3 concentration when the VOC concentration is increased to represent the concentrations observed. Up to 3 ppb of nitrous acid (HONO) was observed during ClearfLo during the daytime in the winter and here we demonstrate that this radical precursor plays a dominant role in local O3 production particularly in winter when other sources of radicals are limited. Constraining the GRS- and CRIv2-R5- box models to measured HONO concentrations leads to a 200% increase in O3 predicted by the GRS box model and a 136% increase in O3 predicted by the CRIv2-R5 model in the winter with more modest increases predicted during the summer.