Grantee Research Project Results
1998 Progress Report: Orthogonal Background Suppression Technique for EPA's Field Infrared Data Processing
EPA Grant Number: R825366Title: Orthogonal Background Suppression Technique for EPA's Field Infrared Data Processing
Investigators: Blatherwick, R. D.
Institution: University of Denver
EPA Project Officer: Hahn, Intaek
Project Period: October 1, 1996 through September 30, 1999 (Extended to September 30, 2000)
Project Period Covered by this Report: October 1, 1997 through September 30, 1998
Project Amount: $248,743
RFA: Analytical and Monitoring Methods (1996) RFA Text | Recipients Lists
Research Category: Ecological Indicators/Assessment/Restoration , Environmental Statistics , Water , Air , Land and Waste Management
Objective:
The research project, specifically entitled 'Spectral Background Suppression (SBS)', is intended to develop a user-friendly tool which can be used to extract information on pollutant gases from FTIR spectra obtained at field sites having highly variable concentrations of water vapor.Progress Summary:
1. Water Vapor CharacterizationExtensive water vapor characterization studies have been performed using synthetic spectra generated with the LBLRTM computer code, which is a radiative transfer code used in the Department of Energy's ARM program. In these studies, singular value decomposition (SVD) was performed on H2O spectra calculated for a range of temperature, humidity, and pressure conditions. It was found that one can represent spectra calculated at 15 different temperatures by only five principal components. A least-squares polynomial was fit to each of the five components for each of the 15 spectra, thereby obtaining expressions which can be used to generate a water vapor spectrum at any temperature. Similarly, by generating spectra for a variety of water vapor concentrations, one can obtain equations fitting the components of the singular vectors as a function of humidity, and thus have the capacity to generate a spectrum for any value of humidity. This effort has been quite successful to date, resulting is the ability to generate H2O spectra over a wide range of temperatures which agree with the model to better than 0.1 percent.
2. Background Suppression on EPA-RTP spectra.
A number of studies have
been undertaken working with the closed path spectra provided to us by the EPA's
Office of Research and Development Research Triangle Park (RTP) facility. We
were provided with 430 spectra recorded during the early autumn of 1994 at the
EPA RTP site, and approximately 270 spectra recorded during the early summer of
the same year. Progress have been made in addressing the question of how large a
data base in needed to characterize a spectrum at this site recorded at an
arbitrary time of year. Singular value decomposition was performed on the fall
spectra. It was found that by using half of these spectra to build a database of
principal components, other members of the fall spectra set could be generated
from these the first 30 singular vectors to within a factor of 1.5 of the signal
to noise ratio. To assess the ability of the SVD code to effectively suppress
the atmospheric spectrum background, a synthetic transmission spectrum of
ammonia was multiplied into spectra from the fall data which was not used in
generating the database of principal components. When the background suppression
code was applied to this synthetically 'polluted' spectrum, the spectral
signature of the ammonia was clearly visible in the residual.
An investigation of the effects on the suppression technique of varying the spectral resolution has been undertaken. It appears that varying the resolution by as little as 25% has a very detrimental effect on the ability to suppress the background. If the reduced resolution spectrum is not used in building the principal component set, then the residuals increase by at least an order of magnitude in some parts of the spectrum. Hence, one must include spectra at varying resolution to generate a more general SVD.
The applicability of a set of SVD containing only spectra from one season of the year on data from another season has been investigated. A set of principal components was generated from the autumn spectra and applied to spectra from the June set. As in the case of differing spectral resolution, the residuals are more than an order of magnitude larger in some spectral regions than when the spectrum is regenerated from a data set generated during the same season. This is not unexpected, since the atmosphere contains much more water vapor in June than in the autumn.
Future Activities:
Work is in progress to find ways of incorporating the water vapor characterization techniques discussed above into the generation of the principal component matrix used to suppress backgrounds. In addition, we are investigating methods to suppress the instrument and site dependent components of field spectra from those due solely to atmospheric absorption. We are also using the data we have acquired with our solar radiometer from sites differing in altitude by thousands of feet to investigate background suppression under significantly different pressure conditions.We are also planning to make a significant number of laboratory measurements of water vapor under varying conditions with our long path White cell, and to supplement these measurements with synthetic spectra as needed.
Journal Articles:
No journal articles submitted with this report: View all 1 publications for this projectSupplemental Keywords:
Principal component factor analysis, target transformation factor analysis, radiative transfer model., RFA, Scientific Discipline, Ecosystem Protection/Environmental Exposure & Risk, Air, Toxics, Engineering, Chemistry, HAPS, Environmental Chemistry, tropospheric ozone, Monitoring/Modeling, water vapor, field portable monitoring, Fourier transform infrared, air quality data, atmospheric chemistry, environmental monitoring, spectroscopic, measurement methods , ambient particle properties, real-time monitoring, atmospheric monitoring, remote sensing, aerosol analyzers, analytical chemistry, ambient air, FTIR, orthogonal background suppression, spectroscopic studiesProgress 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.