Comment on amt-2021-382

doi:10.5194/amt-2021-382-rc1

Abstract

Abstract. To enable chemical speciation, monitoring networks collect particulate matter (PM) on different filter media, each subjected to one or more analytical techniques to quantify PM composition present in the atmosphere. In this work, we propose an alternate approach that uses one filter type (teflon or polytetrafluoroethylene, PTFE, commonly used for aerosol sampling) and one analytical method, Fourier transform infrared (FT-IR) spectroscopy to measure almost all of the major constituents in the aerosol. In the proposed method, measurements using the typical multi-filter, multi-analytical techniques are retained at a limited number of sites and used as calibration standards. At all remaining sites, only sampling on PTFE and analysis by FT-IR is performed. This method takes advantage of the sensitivity of the mid-IR domain to various organic and inorganic functional groups and offers a fast and inexpensive way of exploring sample composition. As a proof of concept, multiple years of samples collected within the Interagency Monitoring of PROtected Visual Environment network (IMPROVE) are explored with the aim of retaining high quality predictions for a broad range of atmospheric compounds including mass, organic (OC), elemental (EC), and total (TC) carbon, sulfate, nitrate, and crustal elements. Findings suggest that models based on only 21 sites, covering spatial and seasonal trends in atmospheric composition, are stable over a 3 year period within the IMPROVE network with acceptable prediction accuracy (R2â>â0.9, median bias less than 3â%) for most constituents. The major limitation is measuring nitrate as it is known to volatilize off of PTFE filters. Incorporating additional sites at low cost, partially replacing existing, more time- and cost-intensive techniques, or using the FT-IR data for quality control or substitute for missing data, are among the potential benefits of the one-filter, one-method approach.

Highlights

We explore the use of Fourier transform-infrared spectroscopy (FT-IR) to reproduce most of the existing speciation data because the most components exhibit optical activity in the mid-IR
We investigate the feasibility of an Fourier Transform Infrared (FT-IR) method that uses ambient samples as calibration standards and is adapted for use by a large monitoring network
Re-calibration is especially important as atmospheric changes occur and as conditions in the network evolve over time, for example new reference instruments, new or significantly modified FT-IR instruments, changes to sampling protocol or possibly change in filter material

Summary

Introduction

We explore the use of Fourier transform-infrared spectroscopy (FT-IR) to reproduce most of the existing speciation data because the most components exhibit optical activity in the mid-IR. The number and bands of organic compounds are numerous, but generally group frequencies can be found above 1500 cm-1 and compound-specific spectral patterns (“fingerprint region”) below this frequency; down to approximately 700 cm-1 (for example,(Weakley et al., 2016; Mayo et al, 2004). Graphitic carbon displays peaks near 1600 cm-1 due to lattice defects (Tuinstra and Koenig, 1970; Friedel and Carlson, 1971), displacement vibrations near 868 cm-1. Inorganic substances containing polyatomic ions such as sulfate, nitrate, and ammonium have strong vibrational modes above 600 cm-1 (Mayo, 2004). FT-IR spectra with partial least squares (PLS) calibrations have been shown to reproduce OC and EC concentrations using organic and graphitic carbon absorption bands, respectively, at a limited number of sites in the IMPROVE network (Dillner and Takahama, 2015a, b; Reggente et al, 2016), CSN (Weakley et al, 2016, 2018a) and FRM (Weakley et al, 2018b).

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