Grantee Research Project Results
Final Report: Improving Emission Inventories Using Direct Flux Measurements and Modeling
EPA Grant Number: R834556Title: Improving Emission Inventories Using Direct Flux Measurements and Modeling
Investigators: Schade, Gunnar W. , Collins, Don , Ying, Qi
Institution: Texas A & M University
EPA Project Officer: Chung, Serena
Project Period: April 1, 2010 through March 31, 2014
Project Amount: $499,992
RFA: Novel Approaches to Improving Air Pollution Emissions Information (2009) RFA Text | Recipients Lists
Research Category: Air Quality and Air Toxics , Air
Objective:
This project used a novel approach to measure real-world pollutant fluxes on an extended spatial and temporal scale. Our goals were to infer from our measurements the source-specific pollutant emissions needed for a comparison to and an improvement of current emissions inventories, addressing both EPA criteria pollutants (CO, NOx, PM) and volatile organics that are either air toxics (e.g., benzene) and/or photochemical ozone formation precursors (e.g., isoprene). We (i) commenced and extended previous micrometeorological measurements of pollutant fluxes in urban Houston, Texas, for 2 years, and (ii) compared measured to modeled fluxes in a top-down versus bottom-up approach.
Summary/Accomplishments (Outputs/Outcomes):
Our experimental approach used well-established micrometeorological methods to measure fluxes of CO and NOx via a gradient method, and PM and volatile organic compounds (VOCs) via relaxed eddy accumulation (REA) methods. We used novel means, such as a flux footprint analysis in conjunction with multivariate data analysis to evaluate the measured top-down fluxes from the urban area against modeled, bottom-up emissions derived from MOBILE6 and MOVES informed by multiple rubber-tube traffic surveys in the neighborhood the flux tower is located. In addition, we used our long-term measurements in combination with ground surveys (land use) to inform an analysis on whether emissions unidentified by current emissions modeling existed. Incorporation of identified emissions model shortcomings into air quality models, to determine the effects of improved inventories on air quality hind- and forecasts, is ongoing.
Conclusions:
Long-term measurement of VOC fluxes (instead of concentrations only) confirmed that BTEX emissions are dropping over time. Comparing data from winter 2008/09 to data from winter 2012/13, both obtained at our flux tower, we find that BTEX emissions are dropping, on average between 7% and 14% per year. The results are slightly affected by a local industrial source emitting xylenes and ethylbenzene, which we were able to quantify.
Multivariate data analysis of VOC concentration and flux measurements using the Positive Matrix Factorization (PMF) model v3.0 developed by the U.S. EPA reveals that car traffic exhaust emissions are likely estimated without a major bias by the MOVES model, considering that the sub-county scale vehicle composition sampled at our site may represent a different average model year. At the same time, the analysis revealed that the model-based estimate of evaporative emissions likely is underestimating real-world emissions by up to a factor of two. MOVES model revisions should thus focus on improving the evaporative emissions estimates.
The combination of a tracer release study with a focus on an industrial source with permitted VOC emissions showed that combining tower-based flux measurements with bulk footprint modeling is capable of quantifying point source emissions, used for source identification or verification. In our case, the tracer release study verified and characterized the usefulness of the bulk footprint model, while the permitted source further demonstrated its said characteristics and abilities for a distant source, which then enabled the quantification of a third, intermediate distance point source.
Flux measurements of CO and NOx at the site were dominated by car traffic, as expected, but are seemingly much lower (factor 2-6) than estimated by current MOVES modeling. A modified Community Multiscale Air Quality Model (CMAQ) capable of tracking emissions from vehicles separately from other emissions sources was developed. Predicted CO and NOx concentrations at ambient air quality monitoring sites in Houston during the hours when vehicle emissions dominated indicated that vehicle CO and NOx emissions were overestimated by approximately 60-70% in the 2005 NEI with MOVES-based vehicle emissions. This is consistent with previous works by various authors and thus highlights the need to adjust tailpipe CO and NOx emissions in the emissions model.
In addition, the flux measurements showed that welding processes could be significant CO point sources, while soil and plant CO emissions can become a significant CO area source during hot and dry summers. The latter was probably the case during the 2011 Texas drought, a situation repeated in the central United States in 2012, and expected to become more common as global warming progresses.
Summertime isoprene emissions in the Houston area predicted by the Model of Emissions of Gases and Aerosol from Nature (MEGAN) were evaluated using a source-oriented CMAQ Model that separately tracked biogenic isoprene emissions and isoprene emissions from other sources. Daytime isoprene concentrations at nine surface sites in Houston were significantly over-predicted. Upper-air isoprene and its first generation oxidation products of methacrolein and methyl-vinyl-ketone also were significantly higher. Over-prediction of isoprene and its oxidation products both at the surface and the upper air strongly suggests that biogenic isoprene emissions are significantly over-predicted. Comparison of gridded leaf area index (LAI), plant functional type (PFT), and gridded isoprene emission factor (EF) used in MEGAN modeling with estimates of the same factors from a field survey surrounding our field site showed that the isoprene over-prediction likely is caused by the combined effects of a large overestimation of the gridded EF of the temperate broadleaf deciduous trees and an underestimation of LAI in urban areas.
Journal Articles on this Report : 4 Displayed | Download in RIS Format
Other project views: | All 10 publications | 4 publications in selected types | All 4 journal articles |
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Kota SH, Ying Q, Zhang Y. Simulating near-road reactive dispersion of gaseous air pollutants using a three-dimensional Eulerian model. Science of the Total Environment 2013;454-455:348-357. |
R834556 (Final) |
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Kota SH, Park C, Hale MC, Werner ND, Schade GW, Ying Q. Estimation of VOC emission factors from flux measurements using a receptor model and footprint analysis. Atmospheric Environment 2014;82:24-35. |
R834556 (Final) |
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Kota SH, Zhang H, Chen G, Schade GW, Ying Q. Evaluation of on-road vehicle CO and NOx National Emission Inventories using an urban-scale source-oriented air quality model. Atmospheric Environment 2014;85:99-108. |
R834556 (Final) |
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Kota SH, Schade G, Estes M, Boyer D, Ying Q. Evaluation of MEGAN predicted biogenic isoprene emissions at urban locations in Southeast Texas. Atmospheric Environment 2015;110:54-64. |
R834556 (Final) |
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Supplemental Keywords:
criteria air pollutants, VOC, flux, footprint modeling, emission modeling, MOVES, CMAQRelevant Websites:
Energy and trace gas flux measurements from a tall lattice tower near downtown Houston, Texas Exit
Progress 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.