Past Trends: How have the contribution of and sensitivity to anthropogenic emissions sources outside Europe and North America changed over time?
The current levels of intercontinental transport and hemispheric baseline concentrations of O3 and PM are a result of emissions that, on a global basis, increased rapidly between 1950 and 1990. Since 1990, global emissions leading to O3 and PM concentrations have experienced little change or have begun to decrease. In Europe and North America, which have been the dominant sources of anthropogenic emissions until recent decades, emissions of most precursors are constant or declining, due to the implementation of air pollution control policies. In East Asia and South Asia, emissions of precursors have risen dramatically in recent years, due to economic growth and development in these regions.
Between 1990 and 2010, anthropogenic emissions of PM increased little globally, but shifted geographically, with a 30% decline in Europe and North America and a 50% increase in Asia [Klimont 2017]. Similarly, between 2000 and 2010, global anthropogenic emissions of ozone precursors (NOX, CO, and NMVOCs) grew modestly. However, emissions in Europe and North America decreased by 10% to 50% while emissions in South Asia and East Asia and other regions of the world increased by 10% to 50% [Turnock 2018]. Globally, anthropogenic CH4 emissions increased by 17% between 1990 and 2012, with decreases in Europe, little change in North America, and strong increases in East Asia, South Asia, and other regions of the world. The changing spatial patterns of emissions have shifted ozone precursors into the tropics where ozone production is more efficient. Zhang et al. suggest that this equatorward shift of emissions has increased the total global ozone burden more than the combined effect of the increase in global methane emissions and the increase in the total mass of non-methane precursor emissions.
Between the HTAP1 simulations based on 2001 and the HTAP2 simulations based on 2008-2010, there were no significant changes in the sensitivities of O3 and PM2.5 concentrations in Europe and North America to 20% perturbations in anthropogenic emissions in other regions. However, changes in emissions patterns have changed the absolute and relative contributions of regional and extra-regional sources. For example, Jonson et al.  found that for European annual average surface O3 levels, the contribution of European emissions had decreased significantly between 2001 and 2010, while the contribution of North American emissions had declined far less and the contribution of East Asian emissions had increased. Thus, the relative contribution of extra-regional sources increased. Decreases in emissions in North America over the last decade are likely to have contributed to flattening or declining trends observed recently at remote sites in Europe. Likewise, controls on the electricity and industrial sectors in China have led to decreasing emissions of some pollutants in the last 3-5 years. The implications of these recent emissions decreases and their influence on observed trends at remote sites in North America and Europe should be explored.