Print Close

Extraction of aerosol size distribution and number concentration from multispectral light extinction measurements – a experimental study

Abstract No:

1703

Abstract Type:

Student Poster

Authors:

Y Shao¹, G Ramachandran²

Institutions:

¹Johns Hopkins University, Baltimore, MD, ²Johns Hopkins, Baltimore,, MD

Presenter:

Dr. Yuan Shao, MS, PhD, CIH
Johns Hopkins University

Faculty Advisor:

Gurumurthy Ramachandran, PhD, CIH
Johns Hopkins

Description:

Aerosol size distribution /concentration and its spatial variation is indispensable information for occupational exposure characterization studies. Conventionally, this information is estimated via a network of individual point measurements. But when the monitoring area is large or inaccessible, point measurement surveys can be infeasible, costly, time-consuming, or may require monitoring personnel to work under hazardous conditions. These disadvantages are the motivation to develop optical remote sensing methods for mapping spatial distributions of air pollutants. The overll goal of our current reserach is to build a optical romote sensoring combined with computed tomography (OS-CT) system and test its performance in the aerosol size and concentration measurement.

Situation/Problem:

The feasibility of extracting the aerosol size distribution and number concentration from multispectral light extinction measurements has been well demonstrated in the past through a series of numerical simulation studies. The goal of the present work was to design a chamber study to test this feasibility in a laboratory environment.

Methods:

We developed an optical system that comprised seven pairs of commercial laser sources and power meters. Each laser source had a unique wavelength (405nm, 488nm, 532nm, 685nm, 780nm, 880nm, and 980nm) and their transmitted intensities were measured in real-time by paired power meters. An airtight optically transparent acrylic chamber (0.151m³ in volume) was built and placed in between the laser sources and the power meters. In each experimental run, a monodisperse silica sphere solution (2% solids by volume, d=1.5µm, refractive index= 1.46) was aerosolized by a nebulizer and then injected into the chamber at a constant generation rate for 30 minutes. The optical system recorded the change in transmitted intensities over the entire trial period. The air in the chamber was well-mixed with two fans, and the chamber exhaust was passed through an impinger to recycle the silica spheres. After each experiment, the aerosol size and real-time concentration information were derived from the optical measurements via a least-square-based inversion algorithm in MATLAB. The above methodology was replicated for three different aerosol generation rates (2.8×10⁹-5.6×10⁹ particles/min).

Results / Conclusions:

A wide range of aerosol concentration profile were created in the chamber with the overall range of 0-2.2×10¹¹ particles/m³. When the concentration in the chamber was above 1.0×10¹¹ particles/m³, the predicted aerosol number concentration values had a good agreement with the true results (R² = 0.92). The predicted aerosol diameter values (geometric mean = 1.3 µm to 1.7 µm; geometric standard deviation = 1.2) were centered around the true value.
The aerosol size and concentration information for monodisperse silica spheres were successfully reconstructed from the multispectral light extinction measurements in our laboratory environment. This study finding was the initial step towards the commercialization of this technology. Further studies are needed to test the robustness of our system in a more realistic environment.

Primary Topic:

Aerosols

Secondary Topics:

Technology

Co-Authors

Please add your co-authors below. Co-authors are listed for professional courtesy and will not be communicated with regarding the decision notification or any on-site logistics, if accepted. Only the primary presenter listed is expected to attend and present the content on-site.

Gurumurthy Ramachandran, PhD
Department of Environmental Health and Engineering, Bloomberg School of Public Health, Johns Hopkins University, Baltimore, Maryland 21205, USA

Acknowledgements and References

List any additional people who worked on the project or provided guidance and support along with details on the role they played in the research. (Please include first name, last name, organization, city, state and country).

The current work is supported by a pilot project grant from the Johns Hopkins ERC center. The author would also like to thank the NIEHS for providing with a T32 postdoctroal traineeship.

Practical Application

How will this help advance the science of IH/OH?

Aerosol size distribution /concentration and its spatial variation is indispensable information for occupational exposure characterization studies. The current work is the inital step towards in productizing the OS-CT technique for aerosol measurment.