Tropospheric ozone changes and ozone sensitivity from present-day to future under shared socio-economic pathways

Abstract

Abstract. Tropospheric ozone is important to future air quality and climate. We investigate ozone changes and ozone sensitivity to changing emissions in the context of climate change from the present day (2004–2014) to the future (2045–2055) under a range of shared socio-economic pathways (SSPs). We apply the United Kingdom Earth System Model, UKESM1, with an extended chemistry scheme including more reactive volatile organic compounds (VOCs) to quantify ozone burdens as well as ozone sensitivities globally and regionally based on nitrogen oxide (NOx) and VOC concentrations. We show that the tropospheric ozone burden increases by 4\u2009% under a development pathway with higher NOx and VOC emissions (SSP3-7.0), but decreases by 7\u2009% under the same pathway if NOx and VOC emissions are reduced (SSP3-7.0-lowNTCF) and by 5\u2009% if atmospheric methane (CH4) concentrations are reduced (SSP3-7.0-lowCH4). Global mean surface ozone concentrations are reduced by 3–5\u2009ppb under SSP3-7.0-lowNTCF and by 2–3\u2009ppb under SSP3-7.0-lowCH4. However, surface ozone changes vary substantially by season in high-emission regions under future pathways, with decreased ozone concentrations in summer and increased ozone concentrations in winter when NOx emissions are reduced. VOC-limited areas are more extensive in winter (7\u2009%) than in summer (3\u2009%) across the globe. North America, Europe and East Asia are the dominant VOC-limited regions in the present day but North America and Europe become more NOx-limited in the future mainly due to reductions in NOx emissions. The impacts of VOC emissions on O3 sensitivity are limited in North America and Europe because reduced anthropogenic VOC emissions are offset by higher biogenic VOC emissions. O3 sensitivity is not greatly influenced by changing CH4 concentrations. South Asia becomes the dominant VOC-limited region under future pathways. We highlight that reductions in NOx emissions are required to transform O3 production from VOC- to NOx-limitation, but that these lead to increased O3 concentrations in high-emission regions, and hence emission controls on VOC and CH4 are also necessary.\n