Attributing Recent Tropospheric Ozone Trends (1995-2014) in the Presence of Climate Variability: Fingerprinting Anthropogenic and Biomass Burning Sources

Tropospheric ozone is an air pollutant and greenhouse gas. Its concentrations have been observed to increase during recent decades throughout much of the troposphere although sparse observations limit confidence in the magnitude, and in some cases the sign, of the trend. Our prior work detected a human “fingerprint” in upper tropospheric ozone trends in a 17-year OMI/MLS satellite dataset starting in 2005, motivating us to further attribute tropospheric ozone trends to specific sources of ozone precursors. Here, we quantify the time-evolving impacts of surface anthropogenic emissions (SfcA), aircraft emissions (AC) and biomass burning (BB) on tropospheric ozone trends over 1995-2014 with a new set of ‘‘all-but-one-forcing’’ initial-condition ensembles in WACCM6, in which emissions from the sector of interest are fixed at 1995 level. Each of the 12 initial-condition ensemble members uses initial conditions, sea surface temperatures and sea ice from a parallel ensemble member generated previously with the fully coupled CESM2-WACCM6 climate model. We evaluate the WACCM6 ozone simulations with measurements from IAGOS commercial aircraft, ozonesondes and the OMI/MLS satellite product. The size of the ensembles enables us to isolate the impacts of each individual forcing from the noise of internal climate variability. We identify spatiotemporal patterns of the ozone response to precursor emissions from each of our selected sources (SfcA, AC and BB), referred to as “fingerprints”. We find that SfcA are the primary driver of large-scale patterns of externally forced historical tropospheric ozone trends, which we identified in prior work, with some influence from AC in Northern Hemisphere summer. The impacts of BB are mainly reflected in a lower tropospheric ozone anomaly over South America in 1997-98 associated with the unusually strong El Niño event that year. We emphasize that BB emissions are treated as a forcing in our simulations, and thus the same across all ensemble members. We also draw on an available 4-member initial-condition ensemble with ozone depleting substances (ODS) fixed to 1950 levels and find that positive tropospheric ozone trends would be larger if ODS had not declined over 1995-2014. We examine fingerprints from the natural lightning NO_X (LNO_X) source that varies with climate and find it increases over time over most land areas. Our findings can help guide future mitigation efforts by applying the source-specific fingerprints to identify the major drivers of observed tropospheric ozone trends.