Photochemical Modeling in California with Two Chemical Mechanisms: Model Intercomparison and Response to Emission Reductions

Abstract

An updated version of the Statewide Air Pollution Research Center (SAPRC) chemical mechanism (SAPRC07C) was implemented into the Community Multiscale Air Quality (CMAQ) version 4.6. CMAQ simulations using SAPRC07C and the previously released version, SAPRC99, were performed and compared for an episode during JulyAugust, 2000. Ozone (O 3 ) predictions of the SAPRC07C simulation are generally lower than those of the SAPRC99 simulation in the key areas of central and southern California, especially in areas where modeled concentrations are greater than the federal 8-hr O 3 standard of 75 parts per billion (ppb) and/or when the volatile organic compound (VOC)/nitrogen oxides (NO x ) ratio is less than 13. The relative changes of ozone production efficiency (OPE) against the VOC/NO x ratio at 46 sites indicate that the OPE is reduced in SAPRC07C compared with SAPRC99 at most sites by as much as approximately 22%. The SAPRC99 and SAPRC07C mechanisms respond similarly to 20% reductions in anthropogenic VOC emissions. The response of the mechanisms to 20% NO x emissions reductions can be grouped into three cases. In case 1, in which both mechanisms show a decrease in daily maximum 8-hr O 3 concentration with decreasing NO x emissions, the O 3 decrease in SAPRC07C is smaller. In case 2, in which both mechanisms show an increase in O 3 with decreasing NO x emissions, the O 3 increase is larger in SAPRC07C. In case 3, SAPRC07C simulates an increase in O 3 in response to reduced NO x emissions whereas SAPRC99 simulates a decrease in O 3 for the same region. As a result, the areas where NO x controls would be disbeneficial are spatially expanded in SAPRC07C. Although the results presented here are valuable for understanding differences in predictions and model response for SAPRC99 and SAPRC07C, the study did not evaluate the impact of mechanism differences in the context of the U.S. Environmental Protection Agency's guidance for using numerical models in demonstrating air quality attainment. Therefore, additional study is required to evaluate the full regulatory implications of upgrading air quality models to SAPRC07. INTRODUCTIONRegional chemical transport models (CTMs) are widely used in air quality applications for forecasting ozone (O 3 ) and particulate matter (PM) pollution events 1,2 and for developing emission control strategies for reducing pollution to protect human health. 3 CTM simulations are the only reliable method for estimating the response of pollutant concentrations to changes in emissions and meteorology over regional domains. Therefore air quality management agencies use CTM-derived estimates of concentration responses to emission reductions in developing State Implementation Plans for reducing O 3 and PM pollution to comply with the National Ambient Air Quality Standards (NAAQS). 4,5 The U.S. Environmental Protection Agency's (EPA) Community Multiscale Air Quality (CMAQ) model 6 is routinely used for regulatory applications in California.A major component of a CTM is the gas-phase chemical mecha...

Role of meteorological processes in ozone responses to emission controls in California's San Joaquin Valley

“…Cai et al . [] also indicated that the NO x disbenefit areas are spatially expanded using the condensed version of SAPRC07. Using observable indicators [ Liang et al ., ] and ozone production efficiencies calculated by ambient concentration measurements may provide another independent measure of ozone sensitivities to precursors. Forest fire emissions were found to be important ozone precursors and can interact with urban emissions [ Pfister et al ., ; Bein et al ., ; Jaffe and Wigder , ; Strada et al ., ; Singh et al ., ].…”

“…Cai et al . [] also indicated that the NO x disbenefit areas are spatially expanded using the condensed version of SAPRC07. Using observable indicators [ Liang et al ., ] and ozone production efficiencies calculated by ambient concentration measurements may provide another independent measure of ozone sensitivities to precursors.…”

“…A recent study indicated greater ozone sensitivities to NO x simulated by the newly released Statewide Air Pollution Research Center '07 SAPRC07 mechanism [Shearer et al, 2012] than by the Statewide Air Pollution Research Center '99 SAPRC99 mechanism that is used by this study. Cai et al [2011] also indicated that the NO x disbenefit areas are spatially expanded using the condensed version of SAPRC07. Using observable indicators [Liang et al, 2006] and ozone production efficiencies calculated by ambient concentration measurements may provide another independent measure of ozone sensitivities to precursors.…”

Assessing Temporal and Spatial Patterns of Observed and Predicted Ozone in Multiple Urban Areas

“…Given the complex nature of O 3 chemistry and emissions source mixes in a given urban area, photochemical grid models provide a credible tool for evaluating responsiveness to emissions changes and have been used extensively for O 3 planning purposes in the past (Cai et al 2011; Hogrefe et al 2011; Kumar and Russell 1996; Simon et al 2012). In the present study, we used a photochemical grid model applied in three different urban areas to illustrate temporal and spatial patterns of O 3 and how those patterns may change as a result of reduced precursor emissions.…”

Representing Organic Compound Oxidation in Chemical Mechanisms for Policy-Relevant Air Quality Models under Background Troposphere Conditions

“…Chemical mechanism selection has been a difficult issue for regulatory policy development since the early days [2]. These concerns still exist [3,4]. These difficulties are compounded by the often limited choice of chemical mechanisms available within the widely used air quality modelling systems because chemical mechanisms are not offered on a plug-and-play basis as with meteorological parameterisations, for example.…”