Abstract
Abstract. US ambient ozone concentrations have two components: US background ozone and enhancements produced from the country's anthropogenic precursor emissions. Only the enhancements effectively respond to national emission controls. We investigate the temporal evolution and spatial variability in the largest ozone concentrations, i.e., those that define the ozone design value (ODV) upon which the National Ambient Air Quality Standard (NAAQS) is based, within the northern tier of US states. We focus on two regions: rural western states, with only small anthropogenic precursor emissions, and the urbanized northeastern states, which include the New York City urban area, the nation's most populated. The US background ODV (i.e., the ODV remaining if US anthropogenic precursor emissions were reduced to zero) is estimated to vary from 54 to 63 ppb in the rural western states and to be smaller and nearly constant (45.8±3.0 ppb) throughout the northeastern states. These US background ODVs correspond to 65 % to 90 % of the 2015 NAAQS of 70 ppb. Over the past 2 to 3 decades US emission control efforts have decreased the US anthropogenic ODV enhancements at an approximately exponential rate, with an e-folding time constant of ∼22 years. These ODV enhancements are relatively large in the northeastern US, with state maximum ODV enhancements of ∼35–64 ppb in 2000, but are not discernible in the rural western states. The US background ODV contribution is significantly larger than the present-day ODV enhancements due to photochemical production from US anthropogenic precursor emissions in the urban as well as the rural regions investigated. Forward projections of past trends suggest that average maximum ODVs in northeastern US will drop below the NAAQS of 70 ppb by about 2021, assuming that the exponential decrease in the ODV enhancements can be maintained and the US background ODV remains constant. This estimate is much more optimistic than in the Los Angeles urban area, where a similar approach estimates the maximum ODV to reach 70 ppb in ∼2050 (Parrish et al., 2017a). The primary reason for this large difference is the significantly higher US ODV background (62.0±2.0 ppb) estimated for the Los Angeles urban area. The approach used in this work has some unquantified uncertainties that are discussed. Models can also estimate US background ODVs; some of those results are shown to correlate with the observationally based estimates derived here (r2 values for different models are ∼0.31 to 0.90), but they are on average systematically lower by 4 to 13 ppb. Further model improvement is required until their output can accurately reproduce the time series and spatial variability in observed ODVs. Ideally, the uncertainties in the model and observationally based approaches can then be reduced through additional comparisons.
Highlights
The US has a long-standing air quality problem associated with elevated ozone concentrations (e.g., NRC, 1991)
We examine the time series of ozone design value (ODV) from the western rural states (Sect. 3.1), fit the time series of ODVs from the northeastern states to Eq (1) (Sect. 3.2), and discuss the results in the context of the conceptual model introduced above. This model considers the recorded ODVs to comprise two contributions: (i) an approximately constant US background ODV identified with y0 in Eq (1) and (ii) US anthropogenic ODV enhancements, which are approximated by the second term in Eq (1)
The analysis presented in this paper is applied to ODVs from eight northeastern US states and contrasted with ODVs from three sparsely populated rural western states in the northern US; it has two complementary parts
Summary
The US has a long-standing air quality problem associated with elevated ozone concentrations (e.g., NRC, 1991). Parrish et al (2017a) estimate that this remaining ODV (denoted as US background ODV) would be 62.0 ± 1.9 ppb in the Los Angeles urban area This contribution is the limit to which the ODVs can be reduced by US emission controls alone; it is so large that there is little margin for enhancement of ambient ozone concentrations by photochemical production from US anthropogenic precursor emissions before the NAAQS of 70 ppb is exceeded. We aim to quantify the temporal evolution and spatial variability in the US anthropogenic ODV enhancements and, based on past trends, project the expected time required for the maximum ozone concentrations to decrease to the 70 ppb NAAQS in the northeastern US.
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