Representing ozone extremes in European megacities: the importance of resolution in a global chemistry climate model

Abstract

Abstract. The continuing growth of the world's urban population has led to an increasing number of cities with more than 10 million inhabitants. The higher emissions of pollutants, coupled to higher population density, makes predictions of air quality in these megacities of particular importance from both a science and a policy perspective. Global climate models are typically run at coarse resolution to enable both the efficient running of long time integrations, and the ability to run multiple future climate scenarios. However, when considering surface ozone concentrations at the local scale, coarse resolution can lead to inaccuracies arising from the highly nonlinear ozone chemistry and the sensitivity of ozone to the distribution of its precursors on smaller scales. In this study, we use UM-UKCA, a global atmospheric chemistry model, coupled to the UK Met Office Unified Model, to investigate the impact of model resolution on tropospheric ozone, ranging from global to local scales. We focus on the model's ability to represent the probability of high ozone concentrations in the summer and low ozone concentrations, associated with polluted megacity environments, in the winter, and how this varies with horizontal resolution. We perform time-slice integrations with two model configurations at typical climate resolution (CR, ~150 km) and at a higher resolution (HR, ~40 km). The CR configuration leads to overestimation of ozone concentrations on both regional and local scales, while it gives broadly similar results to the HR configuration on the global scale. The HR configuration is found to produce a more realistic diurnal cycle of ozone concentrations and to give a better representation of the probability density function of ozone values in urban areas such as the megacities of London and Paris. We find the observed differences in model behaviour between CR and HR configurations to be largely caused by chemical differences during the winter and meteorological differences during the summer.

The influence of model spatial resolution on simulated ozone and fine particulate matter for Europe: implications for health impact assessments

“…2; ∼ −3 %, Table 1) and the largest difference in spring, as noted above (∼ 16 %, Table 1). Similar differences in July mean O 3 concentrations between a 150 and a 40 km resolution were also found by Stock et al (2014). Over the majority of the stations, during winter and spring, O 3 concentrations simulated at the finer resolution are lower than concentrations simulated at the coarse resolution (downward arrows, Fig.…”

“…The impact of model horizontal resolution on simulated O 3 concentrations has been primarily linked to less dilution of emissions when using a finer resolution (Valari and Menut, 2008;Tie et al, 2010;Colette et al, 2013;Stock et al, 2014;Schaap et al, 2015). Investigating the impact of increasing model horizontal resolution from 48 to 6 km on O 3 concentrations in Paris, Valari and Menut (2008) found modelled surface O 3 to be more sensitive to the resolution of input emissions than to meteorology.…”

“…Investigating the impact of increasing model horizontal resolution from 48 to 6 km on O 3 concentrations in Paris, Valari and Menut (2008) found modelled surface O 3 to be more sensitive to the resolution of input emissions than to meteorology. A number of other studies note the sensitivity of simulated O 3 to simulated nitrogen oxide (NO x ) concentrations that determine the extent of titration of O 3 by nitrogen monoxide (NO) (Stock et al, 2014;Markakis et al, 2015;Schaap et al, 2015). Furthermore, Stock et al (2014) found the impact of spatial resolution (150 km vs. 40 km) on simulated O 3 concentrations to vary with season across Europe.…”

“…A number of other studies note the sensitivity of simulated O 3 to simulated nitrogen oxide (NO x ) concentrations that determine the extent of titration of O 3 by nitrogen monoxide (NO) (Stock et al, 2014;Markakis et al, 2015;Schaap et al, 2015). Furthermore, Stock et al (2014) found the impact of spatial resolution (150 km vs. 40 km) on simulated O 3 concentrations to vary with season across Europe. In winter, higher NO x concentrations produced more pronounced titration effects on O 3 at 40 km resolution with a mean bias error (MBE) of 3.2 %, leading to lower O 3 concentrations than at 150 km resolution (MBE = 14.4 %).…”

Tropospheric Ozone Assessment Report: Assessment of global-scale model performance for global and regional ozone distributions, variability, and trends

“…Computational advances allow current-generation CCMs to perform global simulations at around 1° × 1° to 2° × 2° horizontal resolution (e.g., Lamarque et al, 2013 and references therein), with higher resolutions possible for shorter (~1 year) simulations (e.g., Lin et al, 2012aLin et al, , 2012bPfister et al, 2014;Stock et al, 2014;Zhang et al, 2014). Where high and low resolution versions of a given global model have been compared, the general picture is of reduced biases and better agreement with the probability distribution for extreme episodes, both due to changes in the effective timescales of mixing for the chemistry (see also Section 3.4), and representation of the meteorology (Wild and Prather 2006;Pfister et al, 2014;Stock et al, 2014).…”