Quantifying organic matter and functional groups in particulate matter filter samples from the southeastern United States – Part 1: Methods

Abstract

Abstract. Comprehensive techniques to describe the organic composition of atmospheric aerosol are needed to elucidate pollution sources, gain insights into atmospheric chemistry, and evaluate changes in air quality. Fourier transform infrared absorption (FT-IR) spectrometry can be used to characterize atmospheric organic matter (OM) and its composition via functional groups of aerosol filter samples in air monitoring networks and research campaigns. We have built FT-IR spectrometry functional group calibration models that improve upon previous work, as demonstrated by the comparison of current model results with those of previous models and other OM analysis methods. Laboratory standards that simulated the breadth of the absorbing functional groups in atmospheric OM were made: particles of relevant chemicals were first generated, collected, and analyzed. Challenges of collecting atmospherically relevant particles and spectra were addressed by including interferences of particle water and other inorganic aerosol constituents and exploring the spectral effects of intermolecular interactions. Calibration models of functional groups were then constructed using partial least-squares (PLS) regression and the collected laboratory standard data. These models were used to quantify concentrations of five organic functional groups and OM in 8 years of ambient aerosol samples from the southeastern aerosol research and characterization (SEARCH) network. The results agreed with values estimated using other methods, including thermal optical reflectance (TOR) organic carbon (OC; R2=0.74) and OM calculated as a difference between total aerosol mass and inorganic species concentrations (R2=0.82). Comparisons with previous calibration models of the same type demonstrate that this new, more complete suite of chemicals has improved our ability to estimate oxygenated functional group and overall OM concentrations. Calculated characteristic and elemental ratios including OM∕OC, O∕C, and H∕C agree with those from previous work in the southeastern US, substantiating the aerosol composition described by FT-IR calibration. The median OM∕OC ratio over all sites and years was 2.1±0.2. Further results discussing temporal and spatial trends of functional group composition within the SEARCH network will be published in a forthcoming article.

Analysis of functional groups in atmospheric aerosols by infrared spectroscopy: method development for probabilistic modeling of organic carbon and organic matter concentrations

“…The typical downside of MCMC is the high computational cost, as a large number of samples is needed for convergence and to ensure that the parameters sampled non-independently can provide adequate characterization of the target density. Where possible, use of MCMC together with simpler methods to confirm results is recommended (Brooks et al, 2011).…”

Characterization of primary and aged wood burning and coal combustion organic aerosols in environmental chamber and its implications for atmospheric aerosols

“…The OM:OC ratio is an important parameter often reported for aerosols collected at monitoring networks (Boris et al, 2019;Ruthenburg et al, 2014;Bürki et al, 2020;Hand et al, 2019;Reggente et al, 2019a). We compared 445 OM:OC ratios from these chamber samples to burning samples identified by Bürki et al (2020).…”

Estimating mean molecular weight, carbon number, and OM/OC with mid-infrared spectroscopy in organic particulate matter samples from a monitoring network

“…The advantages of mid-infrared spectroscopy over other common techniques of quantifying OM are providing direct information on functional groups, minimizing sample alteration during the analysis, and having low sampling and analytical cost (Ruthenburg et al, 2014). In previous studies, different statistical methods were used to connect mid-infrared absorbances to molar abundance of different functional groups, from which OM, OC (organic carbon), and the OM/OC ratio were calculated with minimal assumptions (Coury and Dillner, 2008;Ruthenburg et al, 2014;Takahama et al, 2016;Boris et al, 2019). These studies showed good agreement between FT-IR measurements and other methods of OM characterization.…”