Distinctive timing of U.S. historical ozone change determined by climate and anthropogenic emissions

Abstract:

The ground-level ozone over the United States is known to have been influenced by historical climate change and anthropogenic precursor emissions, whereas their individual contributions and the associated mechanisms of influence remain less understood. Here we analyze 25-year (1990-2014) annual mean ground-level ozone across the U.S. Empirical Orthogonal Function (EOF) and linear fitting show that ozone has experienced a notable rate of growth at 0.13 ppb/yr averaged over U.S. About 74% of this growth is attributed to enhancement in nighttime ozone (at 0.19 ppb/yr), with the remaining 26% due to daytime ozone increase (at 0.07 ppb/yr). To relate ozone interannual variations to climate change, we employ the Atlantic Multi-decadal Oscillation (AMO) and Southern Oscillation (SO) indices to infer the states of climate over the U.S. We find large correlations between ozone interannual variability and these climate indices: 0.65 for SO index, 0.73 for AMO index, and 0.82 for their combined index. The correlations are much higher (0.69, 0.78 and 0.86) for daytime ozone. Sensitivity simulations conducted with the GEOS-Chem chemical transport model reveal that climate variability has determined ozone interannual variability, particularly for daytime ozone, while anthropogenic emissions reductions have particularly driven the growth in nighttime ozone. These results better connect ozone air pollution with human activity and climate change.

Figure. Slopes for the linear fitting analyse of the ground-level ozone variations on the inter-annual timescale for AQS measurements during 1990-2014 (a) for daytime ozone and (b) for nighttime ozone. Also shown are the slopes for interannual variation during 2004-2011 (c) for daytime ozone and (d) for nighttime ozone.