资源环境科技发展态势分析平台

US background ozone (O-3) includes O-3 produced from anthropogenic O-3 precursors emitted outside of the USA, from global methane, and from any natural sources. Using a suite of sensitivity simulations in the GEOS-Chem global chemistry transport model, we estimate the influence from individual background sources versus US anthropogenic sources on total surface O-3 over 10 continental US regions from 2004 to 2012. Evaluation with observations reveals model biases of +0-19 ppb in seasonal mean maximum daily 8 h average (MDA8) O-3, highest in summer over the eastern USA. Simulated high-O-3 events cluster too late in the season. We link these model biases to excessive regional O-3 production (e.g., US anthropogenic, biogenic volatile organic compounds (BVOCs), and soil NOx, emissions), or coincident missing sinks. On the 10 highest observed O-3 days during summer (O-3_top10obs_JJA), US anthropogenic emissions enhance O-3 by 5-11 ppb and by less than 2 ppb in the eastern versus western USA. The O-3 enhancement from BVOC emissions during summer is 1-7 ppb higher on O-3_top10obs_JJA days than on average days, while intercontinental pollution is up to 2 ppb higher on average versus on O-3_top10obs_JJA days. During the summers of 2004-2012, monthly regional mean US background O-3 MDA8 levels vary by up to 15 ppb from year to year. Observed and simulated summertime total surface O-3 levels on O-3_top10obs_JJA days decline by 3 ppb (averaged over all regions) from 2004-2006 to 2010-2012, reflecting rising US background (+2 ppb) and declining US anthropogenic O-3 emissions (-6 ppb) in the model. The model attributes interannual variability in US background O-3 on O-3_top10obs days to natural sources, not international pollution transport. We find that a 3-year averaging period is not long enough to eliminate interannual variability in background O-3 on the highest observed O-3 days.