Photochemical Modeling of Industrial Flare Plumes Using SCICHEM

In this study, the Texas Commission on Environmental Quality (TCEQ) collaborated with Ramboll Environ (RE) to test whether reactive industrial flare plumes can be successfully simulated with the Second-order Closure Integrated puff model with CHEMistry (SCICHEM) to produce measured downwind ozone. The newly-revised Guideline on Air Quality Models (GAQM) (aka, Appendix W of 40 CFR Part 51), provides for Lagrangian photochemical models, such as SCICHEM, as an alternative model for single-source regulatory PSD applications for secondarily-created ozone and PM_2.5. This premise is tested in this paper. Plumes from industrial flares can promote active photochemistry occurring at spatial scales too fine to be resolved by grid models such as the GAQM-preferred Photochemical Grid Models (PGM), Comprehensive Air quality Model with extensions (CAMx) and the Community Multi-scale Air Quality (CMAQ) model. SCICHEM, is an open-source state-of-the-science puff model with gas, aerosol, and liquid phase chemistry modules comparable in detail to those in CAMx or CMAQ. The latest version, SCICHEM 3.1, includes the Carbon Bond 6 revision 2 (CB6r2) photochemical mechanism, which is used by the TCEQ for ozone SIP modeling with CAMx. Previous studies with SCICHEM have shown that SCICHEM can simulate the Highly Reactive Volatile Organic Compound (HRVOC)-NOx-ozone chemistry of the Houston ship channel plume, as confirmed by comparisons with aircraft data for ozone, NOy (total oxidized nitrogen compounds) and chemical tracers of reacted HRVOC. Thus, SCICHEM has been proven as a suitable tool for modeling flaring scenarios emitting HRVOC. The study described in this paper involved the selection of three industrial flaring events of HRVOC within Texas that could be linked to elevated levels of measured ozone, as well as base case and sensitivity modeling of these flaring events with SCICHEM. The TCEQ developed a hierarchical process to search and analyze the State databases for candidate flares that met the necessary criteria. For the SCICHEM modeling, RE developed emissions files from TCEQ files for the three flare scenarios and developed meteorological inputs using available surface and upper air measurements in the vicinity of the flares. We compared ozone formation in the flares with ozone events measured at downwind monitors to determine the flare contributions to these events. For flares with relatively low HRVOC emission rates, these contributions were small (less than 5 ppb). For a wintertime flare with high ethylene emission rates during the night, significant ozone formation (nearly 30 ppb) was predicted during the following day. The study shows that the model is an efficient tool to quantify flare event contributions to ozone formation downwind.