Exploration of uncertainty in the simulation of power plant plume chemistryEPA Grant Number: R826239
Title: Exploration of uncertainty in the simulation of power plant plume chemistry
Investigators: Gillani, Noor V.
Current Investigators: Gillani, Noor V. , Wu, Yuling
Institution: University of Alabama in Huntsville
EPA Project Officer: Shapiro, Paul
Project Period: January 3, 1998 through January 2, 2001 (Extended to June 30, 2002)
Project Amount: $450,000
RFA: Ambient Air Quality (1997) RFA Text | Recipients Lists
Research Category: Air , Air Quality and Air Toxics
The largest 200 fossil-fuel power plants emit nearly 2/3 and 1/3, respectively, of the US anthropogenic emissions of SO2 and NOx. Whilst these large and concentrated precursor emissions are well characterized, substantial, but unquantified, uncertainties remain concerning their contributions to the local/regional secondary pollution (sulfates, nitrates, ozone). The major sources of such uncertainties are related to parameterizations of chemistry, mixing, surface processes (emissions/depositions), and aerosol/cloud processes in power plant plume (PPP) simulations. This project will address all except the last of these sources of uncertainty.
Objectives: (A) Performance evaluation and process analysis of three leading chemical mechanisms (RADM2, CB-IV, SAPRC), and, investigation of the role of mixing, biogenic emissions and dry deposition, in the simulation of rural PPP chemistry ; (B) Integrated assessment of the research results, aimed at recommendation of improvements in the simulation of PPP chemistry in operational air quality simulation models (OAQMs).
A 3-year, 3-step study (somewhat overlapping phases), as follows:
- Diagnostic lagrangian reactive plume model (LRPM) study, in conjunction with the PPP data of the SOS-Nashville field study (1995), to address issues related to chemical mechanisms and dry deposition (particularly of HNO3 and H2O2). An upgraded version of the UAH-LRPM, with arbitrary horizontal and vertical resolution of the expanding plume in a reactive environment will be utilized. To our knowledge, none of the target mechanisms have ever been specifically and adequately tested for validity in the rural PPP scenario.
- Mixing of plume NOx and background species, including surface emissions of isoprene and species entrained from stable layers aloft, is of paramount importance in plume chemistry (generally very non-linear and, in the case of many reactions, very fast and diffusion-limited). In LRPMs and OAQMs, subgrid-scale (SGS) mixing is generally parameterized based on simplistic assumptions about CBL turbulence, resulting in well-mixed CBLs and Gaussian plumes of tracer species. High-resolution measurements and research modeling indicate CBL mixing to be vertically asymmetric and horizontally inhomogeneous, and the plumes to be often far from Gaussian, and their chemistry to be strongly sensitive to such complexities. Prelimi-nary results of high-resolution modeling involving large eddy simulations (LES) indicate significant SGS segregation of reactants (neglected in OAQMS which, therefore, overestimate reaction rates). We will perform pioneering LES modeling of the CBL and of the PPP, with detailed smog chemistry in condensed form, to explore the role of large eddies on chemistry.
- The findings of the LRPM and LES studies will be reviewed together to generate specific recommendations for improvement of parameterizations of chemistry, mixing, surface emissions and dry deposition for use in OAQMs.
(a) Evaluation of the three leading chemical mechanisms in the context of simulations of rural PPP chemistry, and guidance for best choice/improvements. (b) Quantitative understanding of the role of turbulence, at the detailed level of large eddies, in shaping the mixing of reactants in CBL updrafts and downdrafts, and in plume-background interactions, and guidance for improvement of mixing parameterizations in OAQMs. (c) Improved understanding of PPP chemistry, isoprene emissions and chemistry, and of dry depositions of nitrates and peroxides.