Abstract
Abstract. US background ozone (O3) includes O3 produced from anthropogenic O3 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 O3 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) O3, highest in summer over the eastern USA. Simulated high-O3 events cluster too late in the season. We link these model biases to excessive regional O3 production (e.g., US anthropogenic, biogenic volatile organic compounds (BVOCs), and soil NOx, emissions), or coincident missing sinks. On the 10 highest observed O3 days during summer (O3_top10obs_JJA), US anthropogenic emissions enhance O3 by 5–11 ppb and by less than 2 ppb in the eastern versus western USA. The O3 enhancement from BVOC emissions during summer is 1–7 ppb higher on O3_top10obs_JJA days than on average days, while intercontinental pollution is up to 2 ppb higher on average versus on O3_top10obs_JJA days. During the summers of 2004–2012, monthly regional mean US background O3 MDA8 levels vary by up to 15 ppb from year to year. Observed and simulated summertime total surface O3 levels on O3_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 O3 emissions (−6 ppb) in the model. The model attributes interannual variability in US background O3 on O3_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 O3 on the highest observed O3 days.
Highlights
In the USA, ozone (O3) is regulated as a criteria pollutant under the National Ambient Air Quality Standard (NAAQS)
Due to nonlinearities in atmospheric photochemistry, these “zero out” estimates of source contributions depend on the presence of all other precursor emissions at present-day levels
We conclude that a 3-year smoothing period is not long enough to eliminate the interannual variability in maximum daily 8 h average (MDA8) O3 levels entirely, and in the western USA (WUS), this interannual variability tends to reflect variations in O3_USB
Summary
In the USA, ozone (O3) is regulated as a criteria pollutant under the National Ambient Air Quality Standard (NAAQS). We apply a global chemistry transport model alongside O3 observations to examine which sources are influencing average versus high-O3 events, and the extent to which they vary from year to year. Identifying the extent to which these biases reflect poor representation of US anthropogenic versus background O3 sources is relevant for assessing uncertainties in estimates of background O3 on days when the O3 NAAQS is exceeded. We estimate the influence from various individual background sources on O3 concentrations and the interannual variability in background O3 levels with a focus on the highest 10 events in each of the 10 U.S EPA regions during each summer (JJA) or year. We aim to answer the following questions: (1) which sources exert the strongest influence on O3 on the 10 days with the highest model biases against observations? (2) Which background sources influence total O3 the most on average versus the 10 highest O3 days? (3) Which sources influence the interannual variability of O3 in each region on average versus the 10 highest O3 days?
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