Historical and future changes in air pollutants from CMIP6 models

Abstract

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.

Surface Ozone‐Meteorology Relationships: Spatial Variations and the Role of the Jet Stream

“…We characterize the daily variability of O 3 by the standard deviation, and two levels (8 and 10 ppbv) are highlighted with the thin dashed and thick contours in Figure 1a. The hemispheric distribution of mean summertime surface O 3 and its variability in Figure 1a is consistent with simulations from other models in a recent model intercomparison (Turnock et al, 2020).…”

Recent (1980 to 2015) Trends and Variability in Daily‐to‐Interannual Soluble Iron Deposition from Dust, Fire, and Anthropogenic Sources

“…Like the natural sources, discussed above, we consider three NH anthropogenic iron source regions to examine qualitative emission change between western countries (North America and Europe) and Asian countries (Figure S7). Anthropogenic iron emission trends have broadly followed observed and modeled PM 2.5 (particulate matter <2.5 μm in diameter) changes (Leibensperger et al, 2012; Tørseth et al, 2012; Turnock et al, 2020). Air quality improvements, through technological advances aimed at reducing PM 2.5 , are counterweighed by population growth and energy demands; in China and India the latter has outweighed the former in recent decades (Daskalakis et al, 2016) and thus when combined with exponential steel production growth (Figure S1) Asian iron emissions rise, while North American and European emissions fall.…”

ChAP 1.0: A stationary tropospheric sulphur cycle for Earth system models of intermediate complexity

“…The CMIP5 data are recently superseded by the CMIP6 (Coupled Models Intercomparison Project, phase 6) data sets (Hoesly et al, 2018;Turnock et al, 2020). However, because the CMIP5 data are sufficient to validate our scheme, and because we expect that this scheme would need some (but not major) retuning when it is implemented into an Earth System Model, we limit our calculations in the present paper with the CMIP5 data.…”

“…Exclusion of such point-scale measurements is somewhat ameliorated by their assimilation into CAMS and EMEP. In addition, in future exercises, we plan to replace the CMIP5 forcing by the CMIP6 one (Hoesly et al, 2018;Turnock et al, 2020).…”