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
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
References (33)
- et al.
Variability of indicator values for ozone production sensitivity: a model study in Switzerland and San Joaquin Valley (California)
Atmospheric Environment
(2001) Ozone process insight from field experiments—Part III: Extent of reaction and ozone formation
Atmospheric Environment
(2000)- et al.
Spatial mapping of VOC and NOx limitations of ozone formation in central California
Atmospheric Environment
(2001) - et al.
Ozone response of precursor controls: comparison of data analysis methods with the predictions of photochemical air quality simulation models
Atmospheric Environment
(2001) - et al.
The use of ambient data to corroborate analyses of ozone strategies
Atmospheric Environment
(1999) - et al.
The application of data from photochemical assessment monitoring stations to the observation-based model
Atmospheric Environment
(2000) - et al.
A photochemical extent parameter to aid ozone air quality management
Atmospheric Environment
(1997) - et al.
The evaluation of some photochemical smog reaction mechanisms-I. Temperature and initial composition effects
Atmospheric Environment
(1992) - et al.
Comparison of ozone simulations using MM5 and CALMET/MM5 hybrid meteorological fields for the July/August, 2000 CCOS Episode
Atmospheric Environment
(2006) Ozone process insights from field experiments—Part II: Observation-based analysis for ozone production
Atmospheric Enviroment
(2000)