CHEMSPEC

Cost-effective chemical speciation monitoring of particulate matter air pollution

Started
October 28, 2022
Status
Completed
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Abstract

Ambient particulate matter is one of the greatest environmental health risks to society, but cost-effective methods for chemical characterization and source apportionment necessary to inform robust regulatory strategies are lacking. Our team has demonstrated the possibility to use infrared spectroscopy as an inexpensive optical probe to obtain chemically informative spectra of particulate matter, and this analytical technique is increasingly being used in air quality monitoring networks and research campaigns worldwide. To obtain reliable predictions at such a prolific scale, advanced data-driven methods are necessary to fully take advantage of the rich but complex infrared spectra acquired from these samples.

CHEMSPEC brought together an unprecedented data set from multiple monitoring networks and laboratory-generated particles to learn the relations between chemical composition and spectroscopic signatures that can be used in quantitative modeling of chemical species. The results of this project have provided a step forward in the characterization of particulate matter, not only in existing monitoring networks, but in critical areas of the world where the particulate matter composition is virtually unknown.

People

Collaborators

SDSC Team:
Francois Kamper
Ekaterina Krymova
Guillaume Obozinski

PI | Partners:

EPFL, Laboratory of Atmospheric Processes and Their Impact:

  • Dr. Satoshi Takahama
  • Prof. Dr. Athanasios Nenes

More info

UC Davis, Air Quality Research Center:

  • Dr. Ann Dillner

More info

description

Motivation

Infrared spectroscopy has been shown to be a relatively inexpensive tool for generating chemically informative spectra of particulate matter and is becoming increasingly popular in air quality monitoring networks worldwide. To fully develop this tool, machine learning methods are required to remove interference from the spectra and decompose the resulting curves  into their chemical components so that accurate, interpretable and robust predictions can be made.

Proposed Approach

To remove interference from the spectra, we designed a probabilistic model by combining a loss function with a covariance model fitted to blank spectra, i.e. spectra free from any ambient particulate matter. In this correction procedure, all hyper-parameters are calibrated using the empirical Bayes principal. Once the background interference has been removed, the resulting spectra can be fed to an EM algorithm to learn the profiles of chemical species and perform predictions.

Impact

The anticipated impact of this project is the expansion in capabilities of monitoring networks worldwide – especially in areas where particle composition is virtually unknown – to cost-effectively provide chemical composition information required to identify the key sources and constituents contributing to PM2.5 at every location critical for shaping necessary emissions control policies, evaluating air quality action plans, supporting air quality modeling, and remote sensing efforts.

Note. Here PM2.5 is the mass concentration of ambient particulate matter less than 2.5 micrometers in diameter.

Figure 1. Illustration of filter sampling and scanning procedure by IR spectroscopy and example IR spectra. The sampled filter spectrum contains the contribution from the filter media (PTFE membrane) and collected particles. The wavenumber is a unit of frequency the reciprocal of the wavelength of electromagnetic radiation. Absorbance is a unitless quantity calculated from the reciprocal of the ratio of transmitted beam intensity through the sample (particles + filter) and transmitted intensity with no sample present, and is strictly additive when no interactions are present among constituents.

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Annexe

Additional resources

Bibliography

  1. Solomon, P. A. et al. U.S. National PM2.5 Chemical Speciation Monitoring Networks—CSN and IMPROVE: Description of networks. Journal of the Air & Waste Management Association 64, 1410–1438. eprint: https://doi.org/10.1080/10962247.2014.956904.
  2. Reggente, M., Dillner, A. M. & Takahama, S. Analysis of functional groups in atmospheric aerosols by infrared spectroscopy: systematic intercomparison of calibration methods for US measurement network samples. Atmospheric Measurement Techniques 12, 2287–2312.
  3. Bürki, C. et al. Analysis of functional groups in atmospheric aerosols by infrared spectroscopy: method development for probabilistic modeling of organic carbon and organic matter concentrations. Atmospheric Measurement Techniques 13, 1517–1538. AMT – Volume 13, 2020 1517/2020/ (2020).
  4. Boris, A. J. et al. Quantifying organic matter and functional groups in particulate matter filter samples from the southeastern United States – Part 2: Spatiotemporal trends. English. Atmospheric Measurement Techniques 14. Publisher: Copernicus GmbH, 4355–4374. issn: 1867-1381. https://amt.copernicus.org/articles/14/4355/2021/ (2021) (June 2021).

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