Multi Seasonal Functional Group Analysis by FT-IR Spectroscopy of Atmospheric Aerosol in Zurich

Atmospheric particulate matter (PM) has been associated with increased morbidity and mortality, reduced visibility, and is one of the least understood components of the climate system. Organic matter (OM) constitutes a substantial fraction (up to 80%) of PM (Lim and Turpin 2002), and its quantification and characterization can help in understanding the impact of atmospheric aerosol on health and climate. OM is a complex mixture of thousands of different organic molecules that vary in structure and physicochemical properties (Hamilton et al. 2004). The functional group (FG) representation simplifies the chemical analysis of OM by providing information about properties like volatility and hygroscopicity and can be used for overall organic matter and organic carbon (OC) quantifications, and its apportionment by source class (Russell et al., 2011). We use the Fourier transform infrared (FT-IR) absorbance spectra of atmospheric aerosol (PM2.5) collected on Teflon filters to characterize the chemical composition of OM using the FG representation. Teflon filters are collected daily at the National Air Pollution Monitoring Network (NABEL) station in Zurich (Switzerland) from the 1st of April 2016 (until the 31st of March 2017). We processed the spectrum of each sample to correct the drift of the baseline and substrate interference using the method proposed by Kuzmiakova et al. (2016). We quantify the functional group composition of the ambient samples by fitting individual Gaussian line shapes (top panel in Fig. 1). We quantified alcohol COH, carboxylic COH, alkane CH, carbonyl CO, and amine NH functional groups, as described in Takahama et al. (2013). We use the FT-IR spectra to apportion OM and FGs associated with traffic emission and wood burning using collocated measurements of black carbon (BC – light absorption of PM2.5 measured at multiple wavelengths). An example of FG study is shown in Fig. 1 for a sample collected in November 2016. The FG distribution shows that alkane CH accounts the 39% of the total OM. The significant contributions of alcohol and carboxylic acid (16% and 29 respectively) exemplify the influence of processed aerosol from surrounding regions affecting the PM2.5 in Zurich. Moreover, the high OM/OC ratio (2.01) with substantial contributions from alcohol, and carbonyl FGs are consistent with those found in biogenic or wood-burning samples (Russell et al., 2011). These sources have also been reported in previous studies of Zurich PM by carbon isotope analysis Szidat et al. (2004) and aerosol mass spectrometry (Canonaco et al., 2015)

Estimation of Organic and Elemental Carbon using FT-IR absorbance spectra from PTFE Filters

Organic carbon and elemental carbon are major components of atmospheric PM. Typically they are measured using destructive and relatively expensive methods (e.g., TOR). We aim to reduce the operating costs of large air quality monitoring networks using FT-IR spectra of ambient PTFE filters and PLS regression. We achieve accurate predictions for models (calibrated in 2011) that use samples collected at the same or different sites of the calibration data set and in a different year (2013)

Analysis of functional groups in atmospheric aerosols by infrared spectroscopy: ElnetPLS model for statistical selection of relevant absorption bands for OC predictions

Organic carbon (OC) is a major component of atmospheric particulate matter (PM). Typically OC concentrations are measured using thermal methods such as thermal-optical reflectance (TOR) from samples collected on quartz filters. However, TOR measurements are destructive and expensive. We estimate TOR OC concentrations using Fourier transform infrared (FT-IR) spectra of ambient samples collected on Teflon filter. We have developed a sparse statistical calibration model (ElnetPLS), which excludes unnecessary wavenumbers from infrared spectra during the model building process, permitting the identification and evaluation of the most relevant vibrational modes of molecules in complex aerosol mixtures associated with reported TOR OC concentrations. The sparsest ElnetPLS model has similar model performances of the full (2784) wavenumber models while requiring only ten wavenumbers associated with carbonyl groups

Atmospheric particulate matter characterization by Fourier transform infrared spectroscopy: a review of statistical calibration strategies for carbonaceous aerosol quantification in US measurement networks

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

Statistical gas distribution modelling for mobile robot applications

In this dissertation, we present and evaluate algorithms for statistical gas distribution modelling in mobile robot applications. We derive a representation of the gas distribution in natural environments using gas measurements collected with mobile robots. The algorithms fuse different sensors readings (gas, wind and location) to create 2D or 3D maps. Throughout this thesis, the Kernel DM+V algorithm plays a central role in modelling the gas distribution. The key idea is the spatial extrapolation of the gas measurement using a Gaussian kernel. The algorithm produces four maps: the weight map shows the density of the measurements; the confidence map shows areas in which the model is considered being trustful; the mean map represents the modelled gas distribution; the variance map represents the spatial structure of the variance of the mean estimate. The Kernel DM+V/W algorithm incorporates wind measurements in the computation of the models by modifying the shape of the Gaussian kernel according to the local wind direction and magnitude. The Kernel 3D-DM+V/W algorithm extends the previous algorithm to the third dimension using a tri-variate Gaussian kernel. Ground-truth evaluation is a critical issue for gas distribution modelling with mobile platforms. We propose two methods to evaluate gas distribution models. Firstly, we create a ground-truth gas distribution using a simulation environment, and we compare the models with this ground-truth gas distribution. Secondly, considering that a good model should explain the measurements and accurately predicts new ones, we evaluate the models according to their ability in inferring unseen gas concentrations. We evaluate the algorithms carrying out experiments in different environments. We start with a simulated environment and we end in urban applications, in which we integrated gas sensors on robots designed for urban hygiene. We found that typically the models that comprise wind information outperform the models that do not include the wind data