Reactivity of Organic Gases

Tropospheric ozone, lot-med from nonlinear reactions between VOC5 and nitrogen oxides (NOr), is a primarr constituent of urban smog (1). Estimates of VOC control costs needed to attain the National Ambient Air Quality Standard (NAAQS) for ozone of 0.12 parts per million are on the order of billions of dollars per year, and in the most severely impacted region~, the necessary control technologies have not been identified completely (2, 3). Despite considerable resource investment since the promulgation of the NAAQS, most large cities d0 not meet this standard. A variety of new directions are being explored to find more effective control strategies. One path, controlling NO~emissions instead of VOC emissions, appears to be most effective for regional transport problems, in rural areas, and in urban areas with high hiogenic VOC emissions. However, in the largest urban areas with the worst ozone problems, reducing VOC emissions also appears to be effective (1, 2, 4). Currently, control strategies and air quality regulations are based on reducing the total mass of VOC5 emitted (excluding methane). There are a number of reasons to consider incorporating specific information about the individual VOC species emitted in designing more effective control strategies. Of the hundreds of different VOC compounds emitted, each has a different impact on ozone levels. The relative ozoneforming potentials of individual VOC5, or “reactivity,” can differ by more than an order of magnitude from one compound to another. For example, in a typical urban atmosphere, 1 kg of ethane will form about two orders of magnitude less ozone than I kg of formaldehyde. Ignoring the reactivity of emissions when regulations are developed may lead to ineffective, inefficient control strategies and possibly even lead to measures that worsen air quality. Consideration of reactivity focuses control efforts on those emissions with the greatest impacts on urban ozone. Other compelling reasons to consider reactivity-based strategies include providing strong incentives for accurate determination of emissions compositions, for pollution prevention through product redesign or reformulation, and the potential for large reductions in emissions control costs (5). We have examined the scientific basis for reactivity-based VOC regulations by quantifying the variability and uncertainties in reactivity estimates. We suggest that estimates of the relative impacts of individual VOCs on ozone can be incorporated into control strategies in order to refine ~ontrol efforts nati~nwide. Here we describe the analysis procedures used to quantify VOC reactivity, with particular attention to the reactivity scale used for automobile emission regulations in California (6). Although reactivity-based regulations are currently used in California, the potential environmental and economic advantages of this approach and the adoption of California vehicle regulations elsewhere (notably theNortheast) broaden the need to understand the scientific foundations, criticisms, benefits, and outstanding tesearch issues associated with reactivity weighting (7). We examine the dependence of reactivity measures on (i) environmental conditions, particularly meteorology and precursor ratios; (ii) the level of chemical and physical detail and uncertainty in the models used for quantifying reactivity; and (iii) the uncertainties in emissions compositions. Our analysis shows that the relative reactivity of emissions mixtures, such as exhaust from alternatively fueled vehicles normalized to emissions from a base case fuel, is hot very sensitive to any of these three factors. In conclusion, we present the results of an economic analysis which shows that strategies that use reactivity-based controls are noronly more effective than those relying on massbased controls but can also be less expensive. We used photochemical air quality models and a variety of analysis methods. The ti-o classes of phorochemical models that have been used most extensively are chemically detailed but physically simplified zero-dimensional box models (8—13) and more comprehensive, physically detailed, three-dimensional (3D) airshed models (10. II. 14—17). The method currently used for reactivity quantification in Calitornia was developed by Carter (8), who-used a box model, and is based on the SAPRC9O chemical mechanism (18) to quantify how an incremental change in the emissions of a specific VOC would affect ozone. In addition to examining results from Carter’s. studies, we have also developed and applied both a box model (10—12) and a chemically detailed 3D model (16, 17, 19) for studying reactivity issues. For both models, atmospheric chemistry is treated using a version of SAPRC9O with 91 species (27 detailed organics) and 203 reactions. The box model is used for statistical analysis of ~eactivity quantification and uncertainty estimation over a wide range of variabks. The more comprehensive 3D model is used to examine the dominant uncertainties identified through use of the box model while accounting for transport and rnultiday effects, and for estimation of pollutant exposure metrics. A linear optimization cost analysis model is developed to examine economic impactsof explicitly accounting for reactivity in control strategy design. To illustrate the use of reactivity in the development of control strategies and regulations, we consider the reactivity scale used in California’s Low Emission Vehicle (LEV) and Clean Fuels Regulations (20). Recently, Carter (9) used a chemically detailed, photochemical trajectory model to quantify the ozone formed from 180 differ.2