Grantee Research Project Results
1999 Progress Report: A Numerical Study of the Effects of Large Eddies on Trace Gas Measurements and Photochemistry in the Convective Boundary Layer
EPA Grant Number: R825262Title: A Numerical Study of the Effects of Large Eddies on Trace Gas Measurements and Photochemistry in the Convective Boundary Layer
Investigators: McNider, R. T. , Song, Aaron , Herwehe, Jerold A.
Current Investigators: McNider, R. T. , Song, Aaron , Herwehe, Jerold A. , Norris, W. B. , Biazar, Arastoo
Institution: The University of Alabama in Huntsville
EPA Project Officer: Hahn, Intaek
Project Period: February 3, 1997 through February 2, 2000
Project Period Covered by this Report: February 3, 1998 through February 2, 1999
Project Amount: $275,828
RFA: Air Quality (1996) RFA Text | Recipients Lists
Research Category: Air Quality and Air Toxics , Air
Objective:
Measurements of trace gases in the convective boundary layer show that strong gradients and structure can exist on short time scales (Andronache, et al., 1994). An assumption of photochemical models based on first-order closure is that flow in the convective boundary layer is ensemble-averaged. Since a single measured profile represents only one realization from an infinite ensemble of possible results for the same mean conditions, it may differ substantially from the ensemble average. The difference between incompletely averaged observations and fully averaged model results makes comparisons of the two difficult. Furthermore, by assuming reactants are well mixed within grid cells, photochemical models may not correctly simulate reactant and product concentrations. For reactions to occur, reactants must be mixed on the molecular scale, but observations and convective boundary layer theory show that air parcels can be maintained in isolation for length scales of the order of the boundary-layer depth. Under this activity we are coupling a large-eddy-simulation (LES) model with a photochemical model to study the effects of large eddies on photochemistry in the convective boundary layer.Progress Summary:
We have used the Regional Atmospheric Modeling System (RAMS, version 3b) to produce large-eddy simulations. Models runs have usually been made using a domain of 50 x 50 x 41 grid points with 200 m horizontal and 100 m vertical spacing and a time step of 3 seconds. Surface heat flux, an input to the model, is taken to be uniform over the model floor. A value of surface heat flux was experimentally determined such that a simulated convective boundary can be produced that has turbulence statistics well approximating those of observed convective boundary layers.A photochemistry model has been developed consisting of 45 trace gases and 77 kinetic reactions. The chemical mechanism is based on those of Biazar (1995) and Trainer, et al. (1987 and 1991). The chemical-reaction solver is SMVGEAR II. The photochemistry model is called during each time step of the LES model. The chemical solver adjusts the chemical time step as demanded by the stiffness of the equations. The first successful coupled model run was completed in this reporting period and was the first time that isoprene chemistry has been directly coupled to a turbulent boundary layer in a mesoscale meteorological model.
Figures 1 and 2 show for two cases the initial profiles and the horizontally averaged mixing-ratio profiles for nine trace gases after a 2-hour simulation. In Case 1, isoprene and NO2 were emitted continuously and uniformly from the model surface to simulate homogeneously mixed vegetation and active soil microbe emissions. In Case 2, isoprene alone was emitted uniformly and continuously from the surface into an NO-rich convective boundary layer to simulate a fresh, unreacted urban plume that has rapidly advected over a forested area. Figure 1 shows that for Case 1 there was a general loss of odd nitrogen compounds and a gain of isoprene + OH products. Figure 2 shows that for Case 2 the availability of more NOx from the original NO produced about 17 ppb of ozone in the boundary layer.
Work was begun on coupling an aerosol model with the LES/photochemistry model. In the convective boundary layer aerosols grow and become activiated in rising eddies as relative humidities increase because of decreasing pressure and associated reduction in temperature. These relatively short time scale effects on aerosol activation have not previously been investigated in an LES framework.
Initial work was carried out on the problem of trace-gas sampling of elevated, point-source plumes in the convective boundary layer. Because of high aircraft speeds, airborne sampling platforms measure single realizations of ensemble averages to a good approximation. Fast-response instruments are able to identify changing concentration gradients, present in part because of the coherent eddy structure of the boundary layer. Computer experiments were conducted in which an airborne measurement system having a specified response time and speed flew traverses through randomized Gaussian plumes. The purpose was to determine the difference in maximum and cross-plume integrated concentrations between the measured horizontal profile and the concentrations given by the unrandomized (ensemble-averaged) Gaussian plume.
Future Activities:
We will continue making simulations with the coupled LES/photochemical model. Work will be done to refine the isoprene chemical mechanism. Efforts will be made to better understand the non-linear processes in coupled dynamical-photochemical modeling. More work will be done on adding aerosol processes and heterogeneous aqueous chemistry. The model also needs to validated against observational data such the trace-gas data collected during the 1995 and 1999 field campaigns conducted by the Southern Oxidant Study.References:
Andronache C, Chameides WL, Rodgers MO, Martinez J, Zimmerman P, Greenberg J. Vertical distribution of isoprene in the lower boundary layer of the rural and urban southern United States. Journal of Geophysical Research 1994;99:16,989-16,999.
Biazar AP. The role of natural nitrogen oxides in ozone production in the southeastern environment. Ph.D. dissertation, Atmospheric Science Program, University of Alabama– Huntsville, 1995, 271 pp.
Trainer M, Buhr MP, Curran CM, Fehsenfeld FC, Hsie EY, Liu SC, Norton RB, Parrish DD, Williams EJ, Gandrud BW, Ridley BA, Shetter JD, Allwine EJ, Westberg HH. Observations and modeling of the reactive nitrogen photochemistry at a rural site. Journal of Geophysical Research 1991;96:3045-3063.
Trainer M, Hsie EY, McKeen SA, Tallamraju R, Parrish DD, Fehsenfeld FC, Liu SC. Impact of natural hydrocarbons on hydroxyl and peroxy radicals at a remote site. Journal of Geophysical Research 1987;92:11879-11894.
Journal Articles:
No journal articles submitted with this report: View all 4 publications for this projectSupplemental Keywords:
ambient air, atmosphere, tropospheric, chemical transport., RFA, Air, Scientific Discipline, Mathematics, tropospheric ozone, Atmospheric Sciences, Ecology and Ecosystems, photochemistry, trace gas measurement, air quality data, air sampling, convective boundary layer, environmental monitoring, atmospheric monitoring, air pollution models, phototchemical modeling, turbulent chemical interactions, ambient ozone data, chemical kinetics, isoprene emission algorithm, ambient aerosol particles, atmospheric chemical cyclesProgress and Final Reports:
Original AbstractThe perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.