Atmospheric particulate matter characterization by Fourier transform infrared spectroscopy: a review of statistical calibration strategies for carbonaceous aerosol quantification in US measurement networks
Authors: Takahama Satoshi , Dillner Ann M. , Weakley Andrew T. , Reggente Matteo , Bürki Charlotte , Lbadaoui-Darvas Mária , Debus Bruno , Kuzmiakova Adele , Wexler Anthony S. 
Published: January 2019

Abstract. Atmospheric particulate matter (PM) is a complex mixture of many different
substances and requires a suite of instruments for chemical
characterization. Fourier transform infrared (FT-IR) spectroscopy is a
technique that can provide quantification of multiple species provided that
accurate calibration models can be constructed to interpret the acquired
spectra. In this capacity, FT-IR spectroscopy has enjoyed a long history in monitoring
gas-phase constituents in the atmosphere and in stack emissions. However,
application to PM poses a different set of challenges as the condensed-phase
spectrum has broad, overlapping absorption peaks and contributions of
scattering to the mid-infrared spectrum. Past approaches have used laboratory
standards to build calibration models for prediction of inorganic substances
or organic functional groups and predict their concentration in
atmospheric PM mixtures by extrapolation. In this work, we review recent studies pursuing an alternate strategy, which
is to build statistical calibration models for mid-IR spectra of PM using
collocated ambient measurements. Focusing on calibrations with organic carbon
(OC) and elemental carbon (EC) reported from thermal–optical reflectance
(TOR), this synthesis serves to consolidate our knowledge for extending FT-IR
spectroscopy
to provide TOR-equivalent OC and EC measurements to new PM samples when TOR
measurements are not available. We summarize methods for model specification,
calibration sample selection, and model evaluation for these substances at
several sites in two US national monitoring networks: seven sites in the
Interagency Monitoring of Protected Visual Environments (IMPROVE) network for
the year 2011 and 10 sites in the Chemical Speciation Network (CSN) for the
year 2013. We then describe application of the model in an operational
context for the IMPROVE network for samples collected in 2013 at six of the
same sites as in 2011 and 11 additional sites. In addition to extending the
evaluation to samples from a different year and different sites, we describe
strategies for error anticipation due to precision and biases from the
calibration model to assess model applicability for new spectra a priori. We
conclude with a discussion regarding past work and future strategies for
recalibration. In addition to targeting numerical accuracy, we encourage
model interpretation to facilitate understanding of the underlying structural
composition related to operationally defined quantities of TOR OC and EC from
the vibrational modes in mid-IR deemed most informative for calibration. The
paper is structured such that the life cycle of a statistical calibration
model for FT-IR spectroscopy can be envisioned for any substance with IR-active
vibrational modes, and more generally for instruments requiring ambient
calibrations.