Final Report: A compact, low-cost, network accessible, optical particle counter for the real time measurement of submicron aerosol particle size distributions

EPA Grant Number: R835139
Title: A compact, low-cost, network accessible, optical particle counter for the real time measurement of submicron aerosol particle size distributions
Investigators: Bertram, Timothy H
Institution: University of California - San Diego
EPA Project Officer: Peterson, Todd
Project Period: February 1, 2012 through January 31, 2015
Project Amount: $250,000
RFA: Developing the Next Generation of Air Quality Measurement Technology (2011) RFA Text |  Recipients Lists
Research Category: Air Quality and Air Toxics , Air

Objective:

Atmospheric aerosol particles play a critical role in Earth’s radiation budget, act to limit visibility through the scattering and absorption of radiation, and represent a significant respiratory health hazard in urban environments. However, the existing network of aerosol particle measurements is significantly sparse, and unable to capture the strong heterogeneity in particles that exists in urban locations. In addition, current 24-hour air quality standards of particulate matter are based solely on the total mass of particles with diameters less than 2.5 µm, and do not account for variations in particle size or total number. As a result, air quality assessments and local and regional modeling efforts are: (1) limited by a paucity of data, and (2) unconstrained by routine observations of particle number and size, which are both critical metrics for assessing the impact of aerosol particles on visibility and human health. 

The primary objective of the original proposal was “the development of a miniature, wireless optical particle counter (OPC) capable of measuring and transmitting submicron aerosol particle number and size distributions to a remote server in real-time. The proposal aims to provide the framework for significant improvements in the spatial and temporal resolution of continuous aerosol particle measurements on the city scale, while dramatically improving the availability of these data in real time.” 

Summary/Accomplishments (Outputs/Outcomes):

As noted above, the primary objective of this award was the development of a miniature optical particle counter that is capable of measuring particulate size distributions in real-time, that extends the minimum detectable particle diameter down to 100 nm to capture the bulk of ambient particle mass as well as a larger fraction of ambient particle number. In this project, we successfully developed a blue diode based OPC proceeding through a series of prototype designs, before settling on cavity and laser design that permits capture of forward scattered light. In our design, a high power blue diode laser (80 mW) is focused on the particle scattering region using an optics train that includes a aspheric lens, variable aperture, cylindrical lens that yields a 100mm x 3mm beam for high spatial overlap with the particle beam. Scattered light is collected using an aspheric lens and imaged onto a Hamamatsu PMT. Following Clarke et al., single particle scattering pulses are analyzed using a logarithmic pulse height analyzer and a commercial A/D converter to save the scattering data into discrete voltage channels [Clarke et al., 2002]. In the early stages of this project, we focused our efforts in three critical areas that limit the precision and accuracy of the current generation of miniature OPCs. This included: (1) development of a stable laser diode driver capable of producing stable power output from inexpensive (< $100), air-cooled laser diodes, (2) development of an optics train capable of homogenizing the diode beam to achieve constant power over the spatial region that overlaps with the particle beam, and (3) refinement of an existing custom pulse height analyzer for accurate binning of scattered laser pulses for construction of particle size distributions.

The most critical performance metrics of the developed OPC include the minimum detectable size and the experimentally determined bin resolution. Analysis of the probability density function (PDF) of the PMT voltage at operational PMT gains, with no particles in the cavity, results in a FWHM of the PDF less than 1.5 mV, thus permitting detection of scattered light pulses as small as 3 mV above baseline. Mie scattering calculations based on diode power, photon collection efficiency, and the quantum yield for the PMT used, suggest that this would permit detection of particles smaller than 100 nm in diameter. Diode power stability is currently the limiting factor in the detection of small particles in these types of devices. We achieved sufficiently stable diode power through the use of a feedback photodiode circuit. PDFs of the pulse height distributions for polystyrene spheres of 100 nm, 210 nm, 530 nm, and 850 nm suggest that the resolution of the sensor is conservatively 100 nm. In the current design, the widths of the peak height distributions are currently limited by inconsistency in the overlap of the particle and laser beam. Based on the retrieve pulse height distributions, we determined the measureable the size range of the blueOPC to be 0.1 < dp < 1 µm. Continued advancement in laser beam shaping, focusing of the particle beam using particle free sheath air may permit the extension of this window particle of smaller and larger size. The size dependent detection efficiencies for the developed OPC, referenced to a condensation particle counter (CPC, TSI 3787), indicate that we maintain high detection efficiency (> 80%) down to particles of diameter 150 nm.

Following the development of the sensor we conducted a series of measurements to explore fine scale variability in the horizontal (2 km2 grid) and vertical (0-100 m) distribution of aerosol particle number and mass. Utilizing a combination of mobile platforms (e.g., quadrotor UAVs) and stationary measurements we were able to assess the future needs of high density distributed networks for capturing the spatial and temporal variability in particle number and mass on a city scale. 

Conclusions:

We have successfully developed a miniature optical particle counter for autonomous measurements of size resolved particle number concentrations from 0.1 – 1.0 mm (dp). The next stage of sensor development includes further automation and testing of the sensor for long term measurements in remote regions. Future deployment of the blue diode OPC will permit the study of: (1) particle number and mass concentrations on high spatial and temporal urban scales that is not captured by existing monitoring networks and (2) Vertically resolved measurements in the lowest 200 m of the atmosphere that can be used to calculate particle emission rates near large point sources. Data obtained from distributed networks, such as those that incorporate sensors such as the blue diode OPC, can be used by community organizations to guide decision making regarding exposure to atmospheric pollutants, and be used as a means to foster science – community collaborations, education, and frame future health studies. 

References:

Antony D. Clarke, Norman C. Ahlquist, Steven Howell, and Ken Moore. A miniature optical particle counter for in situ aircraft aerosol research*. J. Atmos. Oceanic Technol. 2002;19:1557–1566.


Journal Articles on this Report : 1 Displayed | Download in RIS Format

Other project views: All 1 publications 1 publications in selected types All 1 journal articles
Type Citation Project Document Sources
Journal Article Brady JM, Stokes MD, Bonnardel J, Bertram TH. Characterization of a quadrotor unmanned aircraft system for aerosol-particle-concentration measurements. Environmental Science & Technology 2016;50(3):1376-1383. R835139 (Final)
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  • Supplemental Keywords:

    Ambient air, mobile sources, exposure, health effects, environmental chemistry, engineering, monitoring, analytical

    Relevant Websites:

    Bertram Research Group | University of Wisconsin-Madison Department of Chemistry Exit

    Progress and Final Reports:

    Original Abstract
    2012 Progress Report
    2013 Progress Report