Airshed Model Evaluation of Reactivity Adjustment Factors Calculated with the Maximum Incremental Reactivity Scale for Transitional-Low Emission Vehicles

Abstract

The California Air Resources Board recently adopted regulations for light-and medium-duty vehicles that require reductions in the ozone-forming potential or "reactivity," rather than the mass, of nonmethane organic gas (NMOG) emissions. The regulations allow sale of all alternatively fueled vehicles (AFVs) that meet NMOG exhaust emission standards equivalent in reactivity to those set for vehicles fueled with conventional gasoline. Reactivity adjustment factors (RAFs), the ratio of the reactivity (per gram) of the AFV exhaust to that of the conventionally fueled vehicle (CFV), are used to correct the stringent exhaust emission standards. Complete chemical speciation of the exhaust and conversion of each NMOG species to an appropriate mass of ozone using the maximum incremental reactivity (MIR) scale of Carter determines the RAF. The MIR approach defines reactivity where NMOG control is the most effective strategy in reducing ozone concentrations, and assumes it is not important to define reactivity at other conditions, i.e., where NO X is the limiting precursor.This study used the Carnegie/California Institute of Technology airshed model to evaluate whether the RAF-adjusted AFV emissions result in ozone impacts equivalent to those of CFV emissions. A matrix of two ozone episodes in the South Coast Air Basin (SoCAB) of California, two base emission inventories, and exhaust emissions from three alternative fuels that meet the first level of the low emission vehicle standards bounds the expected range of conditions. Although very good agreement was found previously for individual NMOG species, 2 this study noted deviations of up to ±15 percent from the equal ozone impacts for any vehicle/fuel combination required by the California regulations. These deviations appear to be attributable to differences in spatial and temporal patterns of emissions between vehicle fleets, rather than a problem with the MIR approach. The first formally adopted RAF, a value of 0.41 for 85 percent methanol/15 percent gasoline-fueled vehicles, includes a 10 percent increase based on the airshed modeling. The correction to the RAF is different for other fuels and may be different for air basins other than the SoCAB. ImplicationsStringent "low emission vehicle" regulations recently adopted by the California Air Resources Board to reduce ozone precursor emissions are "fuel neutral" in the sense that reactivity adjustment factors {RAFs) are used to relax the standards for fuel/vehicle combinations with exhaust emissions having a lower ozone-forrning potential {per unit mass) than those fueled with conventional gasoline. This study uses the Carnegie/California Institute of Technology airshed model for a range ot ozone episodes and base emission inventories to evaluate whether the RAFs for alternative fuels computed with the maximum incremental reactivity (MIR) scale lead to ozone impacts equal to those of conventional gasoline for the first level of the low emission vehicle standards. The RAFs need corrections of \ip to ...15 percent, dependin...

Urban Ozone Control and Atmospheric Reactivity of Organic Gases

“…The two classes of photochetnical models that have been used most extensively are chetnically detailed but phystcally si~npl~fied zero-dimensional box models (8-1 3) and more comprehensive, phystcally detailed, three-dimensional (3D) airshed models 110,11,[14][15][16][17]. The method currentlv used for react~vity quantification in California was developed by Carter (8), who used a box model, and is based on the SAPRC90 chemical mechanism (18) to quantify how an incremental change in the etnissions of a specific VOC \vould affect ozone.…”

Atmospheric Process Evaluation of Mobile Source Emissions

“…Because of this, such models must have highly condensed chemical mechanisms. For example, the CB4 mechanism in the UAM model (the most widely used model in regulatory applications), uses only 11 model species to represent the hundreds of types of VOCs emitted, and the Lurmann, Carter, and Coyner (LCC) mechanism (Lurmann et al, 1987) in the CMU model (used in a number of alternative fuel assessments and other research applications [e.g., Russell et al, 1989Russell et al, , 1990McNair et al, 1992McNair et al, , 1994), is similar in its level of detail, though it uses somewhat different condensation approaches. Such mechanisms were developed and optimized to simulate current ambient mixtures, and were not designed for use in evaluating scenarios where the emissions speciation may change significantly.…”

“…LCC actually refers to two mechanisms, both developed by Lurmann et al (1987) as condensations of a more detailed mechanism developed earlier by Carter et al (1986a). The most condensed version is used in the CIT model by Russell and eo-workers in a number of assessments of alternative fuel use and VOC reactivity evaluations (McNair et al, 1992(McNair et al, , 1994Russell et al, 1989Russell et al, , 1990. The less condensed version was developed for use in single-cell models such as EKMA.…”

Effects of Uncertainty in SAPRC90 Rate Constants and Selected Product Yields on Reactivity Adjustment Factors for Alternative Fuel Vehicle Emissions

“…The box model on which CARB relies uses Carter's mechanism with time-varying photolysis conditions and temperatures that [1][2][3][4][5][6] control the chemical reaction rates. The box model formulation assumes instantaneous mixing of time-varying emissions throughout a single cell with a variable mixing height.…”

“…one fuel as another, and it is not clear how. best to account for these differences in [1][2][3][4] estimating the average composition of exhaust emissions. To assign the incremental reactivity values needed in Equation (l.l), CARB adopted the MIR scale.…”