A SCREENING METHOD FOR OZONE IMPACTS OF NEW SOURCES BASED ON HIGH-ORDER SENSITIVITY ANALYSIS OF CAMX SIMULATIONS FOR SYDNEY

An efficient screening methodology is developed to evaluate ozone impacts of proposed new emission sources in the New South Wales Greater Metropolitan Region (GMR) which includes greater Sydney, Newcastle and Wollongong. Photochemical grid models, such as the Comprehensive Air quality Model with extensions (CAMx), are used to evaluate ozone impacts because they account for non-linear ozone chemistry and other important processes (transport, dispersion, etc.) However, applying CAMx for every proposed new source would impose an unnecessary burden that can be avoided by using an ozone screening method (OSM) to determine when more detailed evaluation is needed. We developed an OSM based on high-order sensitivity analysis of CAMx simulations for the GMR. CAMx simulations were performed for four summer months from 2003--2005. Prototypical new sources were introduced at five locations within the GMR and ozone impacts were analyzed within the plume downwind of each new source. Ozone sensitivity to new emissions of NOX and VOCs was computed at first and second-order by using the high-order decoupled direct method (HDDM) in CAMx. Ozone sensitivity coefficients were implemented in the screening tool (a spreadsheet) to calculate the incremental increase in ozone concentration that results from adding a new emission source in one of five sub-regions of the GMR. Thus, the OSM accounts for non-linear interaction between NOX and VOC emitted by the new source, differences in the reactivity of VOCs emitted by the new source, and sub-regional variation in ozone sensitivity to VOC and NOX within the GMR airshed. New sources that are estimated to cause ozone increases exceeding a screening threshold become candidates for more detailed evaluation using a model such as CAMx. Emissions performance standards for both nitrogen oxides (NOX) and volatile organic compounds (VOC) are contained in local clean air regulations. However, these performance standards are general and do not relate to the sensitivity of the receiving airshed. The Approved Methods for Modelling do not currently contain a procedure for predicting the impacts associated with ozone precursors. This study was conducted to address the potential need for additional, location-specific stationary source controls, combined with scientifically defensible methods to efficiently determine which sources will have a significant impact on ozone. A tiered approach providing for Level 1 and 2 screening methods was followed to be consistent with modelling approaches applied for other air pollutants. The tiered ozone impact assessment procedure is designed to provide direction about: (1) When an expedited Level 1 assessment will be adequate; (2) When a rigorous Level 2 ozone impact assessment needs to be conducted; (3) How to determine whether an ozone impact assessment is considered to be technically robust; (4) The extent to which additional ozone precursor controls are required at the proposal stage for a new source. Efficient methods for determining these outcomes are required to ensure that the development application process is conducted in a timely, transparent and scientifically rigorous manner. METHODOLOGY A literature review was conducted to identify regulatory requirements for new stationary sources in OECD member countries as they apply to ozone precursor emissions. The literature review also looked for precedents on what magnitude of ozone impact is considered permissible for a new stationary source and what modelling methods are used to evaluate ozone impacts of new stationary sources. Photochemical grid models (PGM) were found to be the primary tool for ozone impact assessments. No precedents were found for permissible ozone impacts and so screening thresholds were developed by analogy with levels used for other pollutants. Other regulatory requirements identified by the literature review are discussed in more detail below. Based on the literature review, the Level 2 assessment uses a PGM to calculate by brute force the ozone increment attributable to emissions from the proposed new source. For the more expedited Level 1 assessment, a screening tool was developed specifically for the Sydney GMR by analysing results of PGM simulations for prototypical new sources added at representative locations within the GMR. Criteria were developed to determine when a Level 1 assessment is adequate or when a Level 2 assessment is needed. These criteria consider the emissions from the new source, the ozone impact of the new source estimated using the Level 1 screening method, and the amount of ozone currently present in the receiving airshed.