|Review of “Estimating background contributions and US anthropogenic enhancements to maximum ozone concentrations in the northern US” by D. Parrish|
This manuscript attempts to provide observationally-based estimates of background ozone and anthropogenic ozone production in the northern US using time series of ozone design values (ODVs) for the rural Northwest and the northeastern US. The analysis is based on multiple unsubstantiated and likely incorrect assumptions. I agree with the editor’s decision to not publish precisely because of the author’s argument that this is likely to be a high-impact paper given that it estimates background ozone to be significantly higher than previous analyses in most areas. While the author argues that the potential importance of the paper means it should be published so that it can become part of the open debate regarding background ozone, the assumptions that underlie the analysis are, in my opinion, so problematic that it should not be published until they can be justified via modeling or some other formal analysis. The discussion of the potential weaknesses of the analysis does not provide sufficient caution regarding the interpretation of the results and the uncertainties associated with the estimates of background ozone and anthropogenic enhancements to ODVs are misleadingly small given the number of assumptions and the disregard for temporal and spatial covariations among the ODVs. I believe that the manuscript has a likelihood of skewing the conversation about background ozone because it seems to provide highly significant, observation-based estimates of the background but in fact is based on an overly-simplistic view of ozone chemistry and makes so many “leaps of faith” regarding ozone’s response to changing emissions that its results are meaningless without significant work to lay the grounds for those assumptions.
The first assumption is that ODVs everywhere in the northern U.S. have decreased following an exponential curve in response to emissions controls. While this exponential decrease appears to be fairly well-defined for Los Angeles, it is so poorly defined for the regions in this analysis that a second and more objectionable assumption must be made that the timescale (tau) is the same for the rural northwest, the highly variable northeast, and Los Angeles. The need for this assumption arises from the fact that the ODVs have only been decreasing since 2000 in many of the time series analyzed and the decrease has been small relative to that in Los Angeles, so that the exponential decay is shallow and not well-defined. In fact, it is clear that in many cases a linear decrease would fit the ODV data as well if not better than an exponential. Assuming the exponential form and the same tau everywhere implies that 1) anthropogenic precursor emissions have decreased exponentially in response to emissions controls in all regions analyzed and that 2) “control strategies have produced approximately equal relative reductions in anthropogenic ozone enhancements through the country.” This is such an overly-simplified view of the highly heterogeneous system of emission control and of ozone’s highly nonlinear response to emissions changes that it is difficult to overstate how breathtaking it is. There are two throwaway statements in the paper regarding the possible influence of “volatile compounds”, but the role of VOCs in ozone chemistry, the fact that ozone can be either VOC- or NOx-limited and that the response to emissions controls can be opposite for two nearby locations depending on the NOx / VOC mix are completely ignored. So too are the differences in emissions controls that have been applied in Los Angeles and those that have been applied in the Northeast (there is a brief discussion of this issue on page 18, lines 20-26, but the author appears to believe that the question of whether precursor emission reductions have been similar between Los Angeles and the Northeast is unknowable, when in fact a simple analysis of emissions inventories would provide a great deal of relevant information) and the transition of substantial regions of the Northeast from VOC-limited to NOx-limited during the period from 2000 to the present. Furthermore, by the author’s own description, the equation used to describe the decrease in ODVs “assumes that all sectors of anthropogenic US ozone precursor emissions have been reduced by emission controls at approximately the same rate. However, in some respects this is a poor approximation in that some emission sectors have received lesser efforts than others. Any emissions that have not been reduced would tend to lead to an overestimate of US background ODV.” In fact, we know with absolute certainty that all precursor emissions have not been reduced at the same rate. Given this and the fact that any increase in tau also leads to a lower estimate of background ozone, the confidence limits provided for the background ozone estimates are ridiculously small and the estimates are not meaningful.
Another objectionable assumption is that background ozone has remained constant over the entire observational record and that all of the processes that might change background ozone act merely to produce variability in ODVs rather than trends. In fact, we have good reason to believe that there should be trends in background ozone associated with wildfires and international emissions, and that these should be highly regionally variable.
The author argues that two additional analyses demonstrate the robustness of the results despite the assumptions inherent in the equation describing ODV changes. The first such analysis is that shown in Figure 10, where ODVs are reconstructed from Equation 1 using the tau value from Los Angeles and a single, regionally-averaged value of background ozone (yo). The only thing proven by this analysis, in which there is only one free parameter remaining (A), is that a very shallow exponential curve reasonably fits ~15 years of data for the Northeastern US. Given that there has been a relatively steady decline in ozone in the Northeastern US since 2000, this is a given once you fix a value for tau and yo. The fit improves if you consider only the maximum ODVs in each state each year, but no justification is provided as to why this should be the case. The second analysis that is meant to demonstrate the robustness of the results is, to my mind, a direct argument against their robustness. This analysis also relies on unsubstantiated assumptions that 1) all ODV time series follow the same functional form (it is not clear why this should be the case given the heterogeneity of ozone precursor emissions in the region and the nonlinearity of ozone chemistry) and 2) all of the time series will approach a common US background ODV (we have no a priori expectation of this given expected gradients in influence from international transport from Canada, variability in natural emissions, etc). Furthermore, one of four time series that differs substantially from all of the other time series in the Northeast (i.e. the New York City urban area, which has a higher background ozone value that most of the rest of the region in Table 2) is used as the reference. The regression analysis produces background ozone estimates ranging from 25 to 62 ppb, but a single, averaged value is calculated and compared to the result from the exponential fit analysis. Yet if one were to assume that the variability in the derived background ODV is real (and no justification is provided as to why it should not be) then the derivation of A values (or the taus – it is impossible to say which) in the exponential fits would be completely different. The uncertainties of 3.9 ppb and 1.7 ppb on the background ozone estimates do not consider regional and temporal coherence of the ODVs nor the uncertainties associated with the assumptions underlying each analysis and are likely grossly underestimated. While the author is correct that there is not a good way of accounting for these additional uncertainties, the low values provide a misleadingly precise estimate of background ozone.
In Section 4.3, the author treats his “observationally-derived estimates” of background ozone as the gold standard against which model-derived values should be evaluated. This discussion illustrates the danger of this paper. There is a natural inclination to treat observationally-based estimates as “truth”, when in fact the unsubstantiated assumptions underlying this analysis and the gross underestimate of uncertainties are not likely to be clear to most casual readers. Until additional analysis has been done to provide a foundation for the assumptions on which the analysis is based, this work should not be published.