Grantee Research Project Results
1999 Progress Report: Modeling the Influence of Boundary-Layer Processes in Biogenic Hydrocarbon Mixing and Chemistry
EPA Grant Number: R825379Title: Modeling the Influence of Boundary-Layer Processes in Biogenic Hydrocarbon Mixing and Chemistry
Investigators: Davis, Kenneth J. , Barth, Mary C. , Patton, Edward G. , Moeng, Chin-Hoh
Current Investigators: Davis, Kenneth J. , Nater, Edward , Barth, Mary C. , Patton, Edward G. , Moeng, Chin-Hoh
Institution: University of Minnesota , National Center for Atmospheric Research
Current Institution: University of Minnesota , National Center for Atmospheric Research , Pennsylvania State University
EPA Project Officer: Hahn, Intaek
Project Period: December 2, 1996 through December 1, 1999 (Extended to December 1, 2000)
Project Period Covered by this Report: December 2, 1998 through December 1, 1999
Project Amount: $399,040
RFA: Air Quality (1996) RFA Text | Recipients Lists
Research Category: Air Quality and Air Toxics , Air
Objective:
The oxidation of biogenic volatile organic carbon (BVOC) compounds such as isoprene influences the photochemical production of ozone. Air quality models commonly assume that BVOCs are well-mixed in the daytime atmospheric boundary layer, or convective boundary layer (CBL). This project employs large eddy simulation (LES) of CBL turbulence to determine the degree to which this well-mixed assumption is realistic. Several numerical simulations explore the impacts of forest canopies, heterogeneous surface conditions, and cumulus clouds on the mixing and chemistry of BVOCs.Progress Summary:
Large eddy simulations have been run covering the following scenarios:
1. Mixing and Chemistry of Idealized Chemical Species in a Highly Resolved Convective Boundary Layer with Homogeneous Surface and Entrainment of the Idealized Chemical Species. Idealized chemical scenarios include first- and second-order decay of surface emissions. Concentrations, fluxes, and reaction rates of chemical species were studied as a function of height above ground and chemical lifetime. Boundary layer turbulence was explicitly resolved down to a spatial scale of 50 x 50 x 20 meters. Flux and concentration profiles are significantly different than those for nonreactive tracers when their chemical lifetime is equal to or smaller than the time-scale for convective mixing in the CBL, about 15 minutes. Segregation between reactive chemicals, which will, for example, reduce isoprene oxidation rates below a well-mixed CBL assumption, also become important when approaching this chemical lifetime for chemicals with a homogeneous surface source or sink (e.g., isoprene). Reaction rates could be reduced by as much as 20 percent. If this is representative of isoprene oxidation, this could cause modest errors in ozone photochemistry models that rely on accurate isoprene oxidation rates. Note, however, that more realistic models of the photochemistry of ozone indicate that additional chemical reactions reduce the importance of segregation a great deal. The concentration and flux profile results, however, are significant results regarding the interpretation of field observations, especially ground based concentration measurements used to represent the entire CBL. A publication summarizing these results is in preparation.
2. Mixing and Chemistry of Idealized Chemical Species in a Forest Canopy. An LES of the forest canopy only was run as a prelude to nesting the forest canopy model within a full CBL model. The chemical species had sources that were homogeneous in the horizontal, and a function of leaf area density in the vertical direction. The forest canopy model extends upwards to about 60 m (3x typical deciduous forest height), while the CBL model covers a 2 km vertical domain. Detailed analyses of the concentration, variance, and budgets of reactive scalars were analyzed within this forest canopy simulation (Patton, et al., submitted for publication). Significant impacts on chemical profiles again are limited to cases where the chemical lifetime is similar to or shorter than the mixing time within the canopy, which is much shorter than the time-scale for the entire CBL because the canopy layer is much smaller than the whole CBL.
3. Mixing and Chemistry of Idealized Chemical Species Within a Nested Forest-CBL LES, Including Both Homogeneous and Heterogeneous Sources/Sinks of Chemical Species. This simulation captures what we expect are the most important impacts of forest canopy turbulence on isoprene chemistry in the CBL. First- and second-order decay was considered. Realistic isoprene chemistry is beyond current computing power given the great detail in the turbulence simulation. This work represents the first-ever coupled forest-CBL LES, with or without inclusion of chemical reactions. Preliminary analyses indicate that the forest canopy has a modest impact on chemical reaction rates for the case of homogeneous chemical sources and sinks (e.g., isoprene emission). The presence of the forest canopy also modifies the concentration and variance of the chemicals in the lower CBL, up to about 2/10 of the depth of the CBL (Patton, et al., 2000). The inclusion of heterogeneous surface emissions, however, causes only a small change in the vertical transport of chemical species, but a very large change in the segregation of reactants (Patton, et al., 1999). This large segregation of reactants, if realistic, would substantially reduce the rate of, for example, isoprene emissions from a heterogeneous forest canopy. The segregation is largest when the spatial scale of surface heterogeneity is similar to the depth of the CBL (about 1 km is typical). This may be an important effect that needs to be included in ozone photochemistry models. Results are being prepared for a peer-reviewed journal.
4. Realistic Hydrocarbon Chemistry with Homogeneous Surface Emissions Within a Highly Resolved, Clear-Air CBL. This is the first merger, to our knowledge, of realistic isoprene/hydrocarbon chemistry with LES of the CBL. Roughly 40 chemical species and associated reactions were simulated down to 50 x 50 x 20 m spatial resolution within a 5 x 5 x 2 km domain simulating mid-day turbulence. Moderately polluted and very clean rural environments were simulated. Most interesting is the result that the large isoprene-OH segregation predicted for idealized chemical species is greatly reduced for realistic chemistry. The source of this is not yet clear, but this result is consistent with similar recent investigations. Under very low NOx conditions, the segregation between isoprene and OH becomes significant. These results are being analyzed further and prepared for a peer-reviewed publication.
5. Cumulus-Capped CBL with Realistic Hydrocarbon Chemistry and Aqueous Chemistry. This scenario, also the first simulation of its kind to be simulated via LES, has been prepared but final simulations have not yet been run.
Future Activities:
The remaining portion of the project will focus almost entirely on the preparation of peer-reviewed publications. Kenneth Davis will complete one paper on the vertical distribution of chemically reactive scalars in the convective boundary layer. Edward Patton will author two papers, one on the influence of forest canopies on turbulence and mixing in the convective boundary layer, and one on the influence of heterogeneous surface emissions on chemistry in the convective boundary layer. Mary Barth will author a paper on the influence of convective layer turbulence on realistic hydrocarbon-ozone photochemistry. She also will complete large eddy simulations that include boundary layer cumulus clouds and aqueous photochemistry, and summarize the results of these simulations in preparation for publication.Journal Articles on this Report : 2 Displayed | Download in RIS Format
Other project views: | All 14 publications | 3 publications in selected types | All 2 journal articles |
---|
Type | Citation | ||
---|---|---|---|
|
Patton EG, Davis KJ, Barth MC, Sullivan PP. Decaying scalars emitted by a forest canopy: A numerical study. Boundary-Layer Meteorology 2001;100(1):91-129. |
R825379 (1999) R825379 (2000) |
Exit |
|
Patton EG, Sullivan PP, Davis KJ. The influence of a forest canopy on top-down and bottom-up diffusion in the planetary boundary layer. Quarterly Journal of the Royal Meteorological Society 2003;129(590 Pt A):1415-1434. |
R825379 (1999) R825379 (2000) |
Exit |
Supplemental Keywords:
ozone photochemistry, isoprene, biogenic hydrocarbons, VOC, boundary layer meteorology, environmental chemistry, environmental physics, large eddy simulation, ambient air quality, forest canopies, turbulence., RFA, Scientific Discipline, Air, Geographic Area, Chemistry, State, tropospheric ozone, Environmental Engineering, EPA Region, Minnesota, environmental monitoring, forest canopy venting, gas phase chemistry, boundry layer processes, air sampling, biogenic hydrocarbon mixing, atmospheric monitoring, oxidation by-products, Region 5, cumulus venting, airshed models, ozone productionProgress 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.