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Low-cost, time-resolved volatility and composition of atmospheric aerosols (Final Report)

Year: 2022
Type: report
Abstract: Significance of the problem. The composition of atmospheric particulate matter (“aerosols”) strongly influences its health and environmental impacts, but there exist few time-resolved, long-term records of particle-phase chemical composition. Currently, the composition is measured either by off-line analyses of time-integrated filter samples, or by in-situ instruments requiring near-constant operator oversight, resulting in sparse or limited data coverage. New measurement approaches are needed that are robust, easy-to-operate, and suitable for DOE ARM and other atmospheric monitoring facilities for operationally inexpensive measurements of particle composition. Technical approach. In the “ChemSpot™” instrument, airborne particles from 0.005 - 2.5 micron are captured as a concentrated “spot” deposit using a unique moderate-temperature, water-based condensation method that allows efficient sampling of small particles onto a passivated surface at near-atmospheric pressure. The collection surface is then heated to desorb samples for direct analysis by a suite of proven, robust, field-deployable detectors. A flame ionization detector (FID) coupled to a downstream CO2 detector provides organic carbon concentration as well as a novel measurement of oxygen content. In addition, sulfur-containing compounds are detected using a flame photometric detector (FPD). This approach is flexible, allowing easy expansion to include additional detectors to target other elements. Rapid and precise temperature stepping provides volatility-resolution by controlled desorption. Results. Validation of the measurement approach consisted of extensive laboratory testing. Particle collection efficiency exceeded 97% for particle sizes from 7 nm to the largest size tested of 900 nm, and transfer of samples to detectors was better than 94%. Limits of detection for sample mass were estimated as 100 ng/m3 for hourly measurements of both organic carbon and total sulfur; both organic and inorganic sulfur were measured. Calibration of detectors by gas-phase standards was shown to be equivalent to thermal desorption of known compounds. Measuring CO2 produced by an FID flame was shown to provide a new approach to measure oxygen-to-carbon ratios of individual components and mixtures to within 0.1 units across the range expected for atmospheric aerosol (0 to 1). Sample transfer was achieved in rapid temperature steps (20 °C/s) up to 800 °C, with volatility-resolution into 3-4 bins. A complete prototype instrument was demonstrated that interfaced ambient sample collection, thermal transfer to detectors, and analysis, all automated for continuous operation. Ambient samples were autonomously collected for several weeks with 3-hour time resolution. Measured carbon, oxygen, and sulfur content of ambient particulate matter agreed well with established instrumentation. Applications of this research. The novel measurement approaches and prototype instrument developed in this project offer a robust and straightforward method for the on-line measurement of particle composition, providing basic chemical and volatility information with hourly time-resolution and little operator effort. Autonomous measurements of volatility-resolved particle-phase carbon and oxygen-to-carbon ratio, as well as inorganic sulfur, will represent a substantial improvement to current routine monitoring capabilities. Furthermore, calibration can rely primarily on stable gas-phase standards, allowing simple automated calibration systems conducive to remote deployment and reliable operation. The simple data streams produced by these detectors enable automated data processing without the need for complex data analysis. This instrument will allow time-resolved measurement of the composition of atmospheric particulate matter, with capital and operational costs that will allow substantially increased spatial and temporal extent of available data. This material is based upon work supported by the U.S. Department of Energy, Small Business Technology Transfer program under Award Number DE-SC0018462. Any findings, opinions, or conclusions in this work are those of the authors and do not necessarily reflect the views of the United States Government or any agency thereof.
DOI: 10.2172/1889047
Language: English
Open Access status: green
OpenAlex ID: W4306151995
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