Predicting ambient aerosol thermal–optical reflectance measurements from infrared spectra: elemental carbon

• Published in: Atmospheric measurement techniques 2015-10, Vol.8 (10), p.4013-4023
• Main Authors: Dillner, A M, Takahama, S
• Format: Article
• Language: English
• Subjects: Absorbance; Ammonium; Ammonium compounds; Analysis; Atmospheric aerosols; Atmospheric particulate matter; Atmospheric particulates; Atmospherics; Calibration; Carbon; Chemical analysis; Distribution; Environmental monitoring; Error analysis; Fourier analysis; Fourier transforms; Functional groups; High temperature; Infrared analysis; Infrared radiation; Infrared spectra; Infrared spectroscopy; Least squares method; Mass; Measurement methods; Mercury cadmium telluride detectors; Methods; Optical properties; Organic matter; Oxygen; Particulate matter; Performance measurement; Permissible error; Polytetrafluoroethylene; Polytetrafluoroethylenes; Predictions; Pretreatment; Ratios; Reflectance; Regression analysis; Robustness (mathematics); Solar radiation; Spectra; Spectrum analysis; Substrates; Suspended particulate matter; Test sets; Trends; Visibility; Visual stimuli
• ISSN: 1867-8548; 1867-1381; 1867-8548; 1867-1381
• DOI: 10.5194/amt-8-4013-2015

Elemental carbon (EC) is an important constituent of atmospheric particulate matter because it absorbs solar radiation influencing climate and visibility and it adversely affects human health. The EC measured by thermal methods such as thermal-optical reflectance (TOR) is operationally defined as the carbon that volatilizes from quartz filter samples at elevated temperatures in the presence of oxygen. Here, methods are presented to accurately predict TOR EC using Fourier transform infrared (FT-IR) absorbance spectra from atmospheric particulate matter collected on polytetrafluoroethylene (PTFE or Teflon) filters. This method is similar to the procedure developed for OC in prior work (Dillner and Takahama, 2015). Transmittance FT-IR analysis is rapid, inexpensive and nondestructive to the PTFE filter samples which are routinely collected for mass and elemental analysis in monitoring networks. FT-IR absorbance spectra are obtained from 794 filter samples from seven Interagency Monitoring of PROtected Visual Environment (IMPROVE) sites collected during 2011. Partial least squares regression is used to calibrate sample FT-IR absorbance spectra to collocated TOR EC measurements. The FT-IR spectra are divided into calibration and test sets. Two calibrations are developed: one developed from uniform distribution of samples across the EC mass range (Uniform EC) and one developed from a uniform distribution of Low EC mass samples (EC < 2.4 mu g, Low Uniform EC). A hybrid approach which applies the Low EC calibration to Low EC samples and the Uniform EC calibration to all other samples is used to produce predictions for Low EC samples that have mean error on par with parallel TOR EC samples in the same mass range and an estimate of the minimum detection limit (MDL) that is on par with TOR EC MDL. For all samples, this hybrid approach leads to precise and accurate TOR EC predictions by FT-IR as indicated by high coefficient of determination (R2; 0.96), no bias (0.00 mu g m-3, a concentration value based on the nominal IMPROVE sample volume of 32.8 m3), low error (0.03 mu g m-3) and reasonable normalized error (21 %). These performance metrics can be achieved with various degrees of spectral pretreatment (e.g., including or excluding substrate contributions to the absorbances) and are comparable in precision and accuracy to collocated TOR measurements. Only the normalized error is higher for the FT-IR EC measurements than for collocated TOR. FT-IR spectra are also div