Attribution of ground-level ozone to anthropogenic and natural sources of NO<sub>x</sub> and reactive carbon in a global chemical transport model

Abstract

<jats:p>Abstract. We perform a source attribution for tropospheric and ground-level ozone using a novel technique which accounts separately for the contributions of the two chemically distinct emitted precursors (reactive carbon and oxides of nitrogen) to the chemical production of ozone in the troposphere. By tagging anthropogenic emissions of these precursors according to the geographical region from which they are emitted, we determine source/receptor relationships for ground-level ozone. Our methodology reproduces earlier results obtained through other techniques for ozone source attribution, and also delivers additional information about the modelled processes responsible for intercontinental transport of ozone, which is especially strong during the spring months. The current generation of chemical transport models used to support international negotiations aimed at reducing the intercontinental transport of ozone show especially strong inter-model differences in simulated springtime ozone. Current models also simulate a large range of different responses of surface ozone to methane, one of the major precursors of ground-level ozone. Using our novel source attribution technique, we show that emissions of NOx from international shipping over the high seas play a disproportionately strong role in our model system to the hemispheric-scale response of surface ozone to changes in methane, as well as to the springtime maximum in intercontinental transport of ozone and its precursors. We recommend a renewed focus on improvement of the representation of the chemistry of ship NOx emissions in current-generation models. We demonstrate the utility of ozone source attribution as a powerful model diagnostic tool, and recommend that similar source attribution techniques become a standard part of future model inter-comparison studies. </jats:p>

Sources of Surface O&lt;sub&gt;3&lt;/sub&gt; in the UK: Tagging O&lt;sub&gt;3&lt;/sub&gt; within WRF-Chem

“…Consistent with previous work (e.g, Karamchandani et al, 2017;Lupaşcu and Butler, 2019;Butler et al, 2020), the hemispheric O3 level, represented here by the LB source region, dominated the monthly O3 concentrations in the UK during the entire study period with a mean relative contribution of ~71%, exhibiting a maximum in May (mean 76%), a minimum in June (mean 66%), and an increase in August (mean 72%); see Fig. Ireland (July and August).…”

“…The method used here is based on Lupaşcu and Butler (2019), who performed an attribution of O3 exclusively to NOx precursors, as in Emmons et al (2012). Butler et al (2020) has shown that anthropogenic emissions of NMVOC play only a small role in regional scale O3 production, with the VOC precursors most important for O3 production being methane and biogenic NMVOC. As this study is primarily focused on the anthropogenic influence on O3, the use of NOx tagging for O3 source attribution is considered appropriate.…”

“…Several studies have investigated the contribution of intercontinental transport to O3 in Europe, in particular from North America and Asia, using different modelling techniques (e.g., Li, 2002;Derwent et al, 2004;Auvray and Bey, 2005;Sudo and Akimoto, 2007;Derwent et al, 2008;Emmons et al, 2012;Derwent et al, 2015;Mertens, 2017;Butler et al, 2018;Lupaşcu and Butler, 2019;Butler et al, 2020). However, these studies do not provide a quantitative estimate of the contribution of the different source regions to the total amount of O3 over the UK at a regional scale, but rather an estimate at a national scale or at individual locations across Europe or for the European region as a whole.…”