Standard and alternative procedures for projecting future ozone in the Houston area using relative reduction factors

Abstract

Abstract Air quality model results are used in a relative sense as part of the process to demonstrate attainment of the U.S. ozone standard. Ozone model predictions for a future year are divided by corresponding model results for a base year to obtain a Relative Reduction Factor (RRF), and the base-year measured ozone design value (DV) is multiplied by the RRF to estimate the future-year design value (DVF). We developed alternative RRFs and DVFs that focus on the increment of local ozone production above regional background (RB). An advantage of the alternative DVFs is that measurements can be used to determine RB ozone, thereby increasing the use of measurements in the DVF estimates. We simulated ozone formation in the Houston area for May–September 2012 and 2028 using the Comprehensive Air Quality Model with Extensions and compared the standard and alternative RRFs and DVFs. If model results are used for all the RB ozone concentrations in our new equation to calculate DVFs, the alternative and standard DVFs are very similar for Houston-Galveston-Brazoria (HGB). If measurements are used for the RB ozone concentrations we combine with the base-year DVs, the alternative DVFs are noticeably larger than the standard DVFs for about half the sites, with the increase being 7 ppb at the site (Deer Park) with the largest standard and alternative DVFs. The differences arise primarily from including different days in the averaging process for the RRFs versus the base-year DVs. We evaluated model performance for the days included in the RRFs and found that the model over-predicted RB ozone (normalized mean bias (NMB) = 26%) and under-predicted the anthropogenic ozone increment (NMB = -22%) from HGB emissions and therefore can be expected to under-estimate ozone response to HGB emission controls.