Shifting emissions to low latitudes had a greater influence on global tropospheric ozone than changing emission magnitude, 1980-2010

Global anthropogenic emissions of short-lived O3 precursors (NOx, NMVOCs and CO) have increased since 1980. These increases are greatest in developing countries (China and India), while emissions have generally decreased in North America and Europe. Consequently, emissions have shifted southwards. Modeling studies have shown that the tropospheric O3 burden and resulting radiative forcing are more sensitive to emission changes in the tropics and Southern Hemisphere than in other regions. Here we separate the influence of the change in the spatial distributions of short-lived global anthropogenic emissions from that of the change in the emission magnitude and the change in global CH4 between 1980 and 2010. We use the global CAM-chem model with 2008-2012 GEOS-5 meteorology for all simulations, neglecting possible effects of climate change. The global tropospheric O3 burden is estimated to have increased by 28.12 Tg from 1980 to 2010, with the largest increases over 30°S—30°N (17.93 Tg). Of the total O3 burden change, the influence of the change in the spatial distributions of emissions contributes 16.39 Tg, roughly double the effect of the change in emission magnitude (8.59 Tg) and of the global CH4 change (7.48 Tg). The three-month O3 season MDA8 surface O3 has decreased over the U.S. and Europe, and increased over East and South Asia from 1980 to 2010, mainly because of changes in the emission distributions. Significant increases in the zonal annual average O3 are modeled in the upper troposphere (500 to 150 hPa) between 15°N and 40°N. Upper tropospheric O3 also increases over the U.S. and Europe, despite peroxyacetyl nitrate (PAN) decreases in the middle to upper troposphere resulting from regional emission reductions. These upper tropospheric changes result from the change in the spatial distributions of emissions, with little from contributions from the changes in emissions magnitude or methane, and reflect the intercontinental transport of O3. We conclude that the changing spatial distribution of emissions has been the most important influence on global tropospheric ozone, more important even than the change in the global emission magnitude. This work highlights the dominant role of the spatial distribution of emissions, and suggests that future ozone burdens will be determined mainly by emissions from the tropics.