Historical and future changes in air pollutants from CMIP6 models
Authors: Turnock Steven T. , Allen Robert J. , Andrews Martin , Bauer Susanne E. , Deushi Makoto , Emmons Louisa , Good Peter , Horowitz Larry , John Jasmin G. , Michou Martine , Nabat Pierre , Naik Vaishali , Neubauer David , O'Connor Fiona M. , Olivié Dirk , Oshima Naga , Schulz Michael , Sellar Alistair , Shim Sungbo , Takemura Toshihiko , Tilmes Simone , Tsigaridis Kostas , Wu Tongwen , Zhang Jie 
Published: November 2020

Abstract. Poor air quality is currently responsible for large impacts on human health across the world. In
addition, the air pollutants ozone (O3) and particulate matter less than 2.5 µm in
diameter (PM2.5) are also radiatively active in the atmosphere and can influence
Earth's climate. It is important to understand the effect of air quality and climate mitigation
measures over the historical period and in different future scenarios to ascertain any impacts
from air pollutants on both climate and human health. The Coupled Model Intercomparison
Project Phase 6 (CMIP6) presents an opportunity to analyse the change in air pollutants simulated by the
current generation of climate and Earth system models that include a representation of chemistry
and aerosols (particulate matter). The shared socio-economic pathways (SSPs) used within CMIP6
encompass a wide range of trajectories in precursor emissions and climate change, allowing for an
improved analysis of future changes to air pollutants. Firstly, we conduct an evaluation of the
available CMIP6 models against surface observations of O3 and PM2.5. CMIP6
models consistently overestimate observed surface O3 concentrations across most regions
and in most seasons by up to 16 ppb, with a large diversity in simulated values over
Northern Hemisphere continental regions. Conversely, observed surface PM2.5
concentrations are consistently underestimated in CMIP6 models by up to 10 µg m−3,
particularly for the Northern Hemisphere winter months, with the largest model diversity near
natural emission source regions. The biases in CMIP6 models when compared to observations of
O3 and PM2.5 are similar to those found in previous studies. Over the
historical period (1850–2014) large increases in both surface O3 and PM2.5
are simulated by the CMIP6 models across all regions, particularly over the mid to late 20th
century, when anthropogenic emissions increase markedly. Large regional historical changes are
simulated for both pollutants across East and South Asia with an annual mean increase of up to
40 ppb for O3 and 12 µg m−3 for PM2.5. In future
scenarios containing strong air quality and climate mitigation measures (ssp126), annual mean
concentrations of air pollutants are substantially reduced across all regions by up to
15 ppb for O3 and 12 µg m−3 for PM2.5. However, for
scenarios that encompass weak action on mitigating climate and reducing air pollutant emissions
(ssp370), annual mean increases in both surface O3 (up 10 ppb) and
PM2.5 (up to 8 µg m−3) are simulated across most regions, although, for
regions like North America and Europe small reductions in PM2.5 are simulated due to the
regional reduction in precursor emissions in this scenario. A comparison of simulated regional
changes in both surface O3 and PM2.5 from individual CMIP6 models highlights
important regional differences due to the simulated interaction of aerosols, chemistry, climate
and natural emission sources within models. The projection of regional air pollutant
concentrations from the latest climate and Earth system models used within CMIP6 shows that the
particular future trajectory of climate and air quality mitigation measures could have important
consequences for regional air quality, human health and near-term climate. Differences between
individual models emphasise the importance of understanding how future Earth system feedbacks
influence natural emission sources, e.g. response of biogenic emissions under climate change.