Elsevier

Atmospheric Environment

Volume 40, Issue 27, September 2006, Pages 5156-5166
Atmospheric Environment

Evaluation of the ability of indicator species ratios to determine the sensitivity of ozone to reductions in emissions of volatile organic compounds and oxides of nitrogen in northern California

https://doi.org/10.1016/j.atmosenv.2006.03.060Get rights and content

Abstract

Six indicator ratios were evaluated for their regulatory application in California using three-dimensional (3-D) photochemical transport modeling output for an ozone episode during 31 July–2 August 2000. The evaluation was based on four criteria with increasing usefulness for 8-h ozone controls. The four criteria can be briefly described as: (1) smooth functional behavior between ozone benefits/disbenefits and the magnitude of the indicator ratio, (2) narrow separation between beneficial and detrimental regimes for oxides of nitrogen (NOx) emissions controls, (3) narrow transition regime between where volatile organic compound controls are more beneficial than NOx controls and vice versa, and (4) invariance of these transition ranges in time and space. None of the indicator ratios met all criteria. We present results for five sub-regions in northern California. The transition regime for criteria (3) was found to be much wider than that for criteria (2) for all sub-regions. Hence, the indicator ratios may be of only limited usefulness in determining precursor limitations and might be more valuable in diagnosing if NOx controls are beneficial or detrimental.

Introduction

One of the key tasks in the preparation of State Implementation Plans to comply with the ozone National Ambient Air Quality Standards of the US Environmental Protection Agency (US EPA) is the determination of which precursor(s) to control. While the reduction of volatile organic compounds (VOC) emissions cannot be counterproductive in polluted air, reduction of nitrogen oxides (NOx) emissions could increase surface ozone under certain conditions.

A widely accepted method to assess the sensitivity of ozone to emission reductions of VOC and NOx is to employ a three-dimensional (3-D) grid-based photochemical transport model (CTM). First the acquisition or generation of appropriate initial and boundary conditions, meteorological fields, and emission estimates are required to employ a state-of-the-science CTM. Once an acceptable base case is developed and the performance of the modeling system, including the emission, meteorology, and photochemical models, is validated with intensive field measurements, the CTM can then be used to assess the impact of VOC and NOx controls on ozone mixing ratios. Significant resources and expertise are required to conduct intensive field measurements, employ models, and interpret the results. This burden of resources and expertise has driven the development of precursor limitation indicators, collectively known as “indicator-based analyses”, which are based on routine field measurements without advanced modeling.

In general, the indicator-based analysis can be divided into two categories (Kleinman, 2000). The first category includes methods that attempt to predict the sensitivity of instantaneous net ozone production to precursor emission reductions. That is, if P([O3])=d[O3]/dt is the instantaneous net production rate of ozone at a given time, methods in the first category include potential indicators of dP([O3])/dEVOC and dP([O3])/dENOx, where EVOC represents the emissions of VOC, ENOx represents that of NOx, and [O3] is the observed ozone mixing ratio. These methods are also known as local methods because they attempt to provide a measure of how instantaneous net ozone production would respond to controls of precursors at a given location and time. Examples of these methods are the “constrained steady-state method”, the “photo-stationary state method”, and the “radical budget method” (Cardelino and Chameides, 1995, Cardelino and Chameides, 2000; Kleinman, 2000 and references therein; Tonnesen and Dennis, 2000a, Tonnesen and Dennis, 2000b).

The second category includes methods that attempt to predict the sensitivity of ozone concentration to precursor emission reductions. That is, they are expected to represent d[O3]/dEVOC and d[O3]/dENOx. These methods are also known as non-local methods because they attempt to provide a measure of how the ozone concentration at a given location would respond to reductions of all emissions that contributed to the formation of that ozone. This would include local emissions as well as emissions from different locations in the past. In a Lagrangian framework, this would include all emissions along the back-trajectory of an air parcel. Examples of these methods include the use of [VOC]/[NOx] (NRC, 1991); [NOy] (Milford et al., 1994); [O3]/[NO2], [HCHO]/[NOy], and [H2O2]/[HNO3] (Sillman, 1995); [O3]/[NOy], [O3]/[HNO3], and [H2O2]/[NOz] (Sillman et al., 1997) as indicators. Here, NOy represents NOx and all of its oxidation products including nitric acid (HNO3), peroxyacetyl nitrate (PAN), other organic nitrates, and particulate nitrate. It is customary to use NOz to represent all oxidation products of nitrogen except NOx, so that [NOy]–[NOx]=[NOz]. We also include in this non-local category a method that evolved into what is now known as the Smog Production (SP) algorithm (Johnson, 1984; Johnson and Quigley, 1989; Johnson et al., 1990; Johnson and Azzi, 1992; Hess et al., 1992a, Hess et al., 1992b, Hess et al., 1992c; Chang and Suzio, 1995; Chang et al., 1997; Blanchard, 2000, Blanchard, 2001; Blanchard et al., 1999; Blanchard and Fairley, 2001; Blanchard and Stoeckenius, 2001; Blanchard and Tanenbaum, 2003).

In this paper, we evaluate a subset of the second category of indicators against photochemical model predictions for possible regulatory application in northern California, including the Central Valley. For this evaluation we have selected [O3]/[NOy], [O3]/[NOz], [Total Peroxides]/[HNO3], [HCHO]/[NOy], [O3]/[NOx], and the extent of reaction (ER) of the SP algorithm. The first four ratios were selected because they can be derived from basic equations of atmospheric chemistry with reasonable assumptions (Sillman, 1995; Kleinman, 2000). [Total Peroxides] is used in place of [H2O2] because the former represents radical termination more completely than the latter (Sillman, 2002). Note that we selected [O3]/[NOx] despite the fact that NOx does not have a “memory” of the past as NOy and NOz could have. There is a desire to use NOx as a surrogate for NOy because NOx measurements are common while NOy measurements are not. The ER of the SP algorithm was selected, despite its lack of a sound theoretical basis, because of its independent development based on smog chamber measurements. The [O3]/[NOy], [O3]/[NOz], and ER are mentioned as possible corroborative methods in the US EPA final modeling guidance for 8-h ozone (EPA, 2005). The [VOC]/[NOx] ratio and [NOy] were not considered here, because they have already been proven poor indicators (Milford et al., 1989; Wolff and Korsog, 1992).

Section snippets

Methods

We selected a pre-existing base-case photochemical air quality simulation as the test bed to compare the ability of indicator ratios to predict the VOC-NOx sensitivity of ozone concentrations in the northern California modeling domain (Fig. 1). The modeling domain includes a part of the Pacific Ocean in the west, the Mojave Desert in the east, the northern Sacramento Valley in the North, and the Tehachapi Mountains in the south. This base case air quality simulation represents the July

Results and discussion

We first present the modeled surface ozone distribution, together with the impact of emission reductions. Fig. 2 shows the baseline, 1-h surface O3 distribution over the CCOS model domain at 4 p.m., 31 July 2000. Peak O3 appeared near the lower right corner, where wildfires were present. Fig. 3, Fig. 4 show the impact of 25% emissions reductions of NOx and VOC, respectively, on 1-h O3 mixing ratios. Please note that throughout this document, we represent ozone benefits due to emission

Conclusions

We evaluated six indicator ratios for their regulatory application in California using simulated data from a 3-D, fine-grid photochemical transport model for an ozone episode during 31 July–2 August 2000. The evaluation was based on four criteria with increasing usefulness for 8-h ozone controls. Most of the six indicator ratios are shown to meet a few but in no case all of the criteria. We presented the 8-h ozone cutoff ranges for NOx disbenefit and benefit regimes, and that for VOC and NOx

Acknowledgments

We thank Paul Allen, Eugene Yang, Daniel Chau, Kemal Gürer, Kathleen Fahey, and John DaMassa of California Air Resources Board for providing us with model inputs and critically reviewing this manuscript.

Disclaimer. This paper has been reviewed by the staff of the California Air Resources Board and has been approved for publication. Approval does not signify that the contents necessarily reflect the views and policies of the California Air Resources Board, nor does mention of trade names or

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