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Despite efforts to control atmospheric pollutant emissions, ambient air quality remains a major concern in many parts of the world. Air pollution has significant negative impacts on human health (Pope et al., 2002; Dockery et al., 1993; Jerrett et al., 2009). More than 80% of the world’s population is exposed to pollutant concentrations exceeding the World Health Organization (WHO) recommended levels (Brauer et al., 2012) and around 3.6 million deaths can be attributed to ambient air pollution with another 4 million from household related sources (Lim et al., 2012). Moreover, air pollution can alter ecosystems, damage buildings and monuments, as well as influence earth’s energy balance and therefore climate change.
Long-term global scenarios for air pollutant emissions have been used for atmospheric chemistry and Earth system model simulations intended to examine future changes in climate, air, and water systems. These scenarios reflect plausible future emissions based on socioeconomic, environmental, and technological trends. These scenarios are generally produced by integrated assessment models (IAMs) (Moss et al., 2010), which project economic growth, population, energy consumption, land-use and agriculture along with associated GHG and pollutant emissions. Recent examples include in particular, the Representative Concentration Pathway (RCP) scenarios (van Vuuren et al., 2011a), which were the multi-model global scenarios of greenhouse gases and air pollutants used in the Coupled Model Intercomparison Project phase 5 (CMIP5) (Taylor et al., 2011). The RCPs were developed to span a range of climate forcing levels and were not associated with specific socio-economic narratives. These scenarios reflected the prevailing view that air quality policies will be successfully implemented globally and that emissions control technology will continue to evolve and as a result show significant declines in particulate matter (PM) and ozone precursor emissions over the 21st century at a global level (Amann et al., 2013; van Vuuren et al., 2011b). More recent scenarios have included alternative assumptions on pollution control, in an effort to better understand the role of air pollution control in terms of reference scenario development and the co-benefits from climate policies (see for example Rogelj et al., 2014; Rao et al., 2013; West et al., 2013; Chuwah et...) . While providing a wider range of pollution futures, the assumptions on air pollution control in these scenarios are, however, still largely independent of underlying scenario narratives.
It is generally assumed in long-term scenarios, implicitly, that pollutant concentration goals will continue to be more ambitious over time, once incomes become sufficiently large. However, the time, stringency, and enforcement success of future targets for a particular region cannot generally be known and must ideally be treated as scenario variable. In a long-term scenario context, it is further necessary that assumptions on air pollution control are consistent with the underlying challenges to climate change mitigation and adaptation. Pollution outcomes in such scenarios can then be expected to be a cumulative result of a range of variables including socio-economic development, technological change, efficiency improvements and policies directed at pollution control as well as alternative concerns including climate change, energy access, and agricultural production.
The Shared Socio Economic Pathways (SSPs) (Kriegler et al., 2012) are a new generation of scenarios and storylines primarily framed within the context of climate change mitigation and adaptation. The SSP narratives (van Vuuren et al., 2014; O’Neill et al., 2014) comprise a textual description of how the future might unfold, including a description of major socio-economic, demographic, technological, lifestyle, policy, institutional and other trends. In this paper, our overarching goal is to develop plausible ranges of future air pollutant emission development pathways in the SSP scenarios, which are based on internally consistent and coherent assumptions on the degree and implementation of future air pollution control. Other papers in this Special Issue summarize parallel efforts in terms of elaboration of developments in the energy system, land use and greenhouse gas emissions in the SSP scenarios (Bauer et al., 2017; Popp et al., 2017).
The structure of the paper is as follows. We first describe the development of a set of alternative assumptions on the degree and implementation of ‘pollution control’ in the SSP scenarios. These assumptions then reflect historical evidence and prevailing attitudes and progress on pollution control and potential attitudes to the health and environmental impacts of air pollution in the future. We further postulate a link between these alternative development pathways for pollution control and a specific SSP narrative. We also describe quantitative guidance with regards to implementation of these assumptions in IAMs. Finally, the paper summarizes key results from different IAM interpretations of the SSP scenarios in terms of air pollutant emissions and regional ambient air quality.