Development and Validation of an Integrated Ambient Air Test Facility (AATF) for Multi-Instrument Aerosol Characterization

Johns, ﻿Paul; Scotto, Cathy; Hernandez, Landon; Hart, Matthew; Aduroja, Oyedoyin; Hammond, Mark; Giordano, Braden; Grabowski, Kenneth; Eversole, Jay; Sivaprakasam, Vasanthi

doi:10.5194/egusphere-2025-6043

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https://doi.org/10.5194/egusphere-2025-6043

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11 Jan 2026

| 11 Jan 2026

Status: this preprint is open for discussion and under review for Atmospheric Measurement Techniques (AMT).

Development and Validation of an Integrated Ambient Air Test Facility (AATF) for Multi-Instrument Aerosol Characterization

Paul Johns, Cathy Scotto, Landon Hernandez, Matthew Hart, Oyedoyin Aduroja, Mark Hammond, Braden Giordano, Kenneth Grabowski, Jay Eversole, and Vasanthi Sivaprakasam

Abstract. The U.S. Naval Research Laboratory has developed and validated an Ambient Air Test Facility (AATF) for controlled multi-instrument aerosol generation and characterization under realistic sampling conditions. The facility consists of a 14 meter flow tube system that provides turbulent (Re = 40,000) outdoor ambient air flow at 2 m/s for testing aerosol detection and measurement systems in a controlled indoor environment. The AATF integrates 13 diagnostic instruments across four measurement categories: individual particle measurement, aerosol loading, aerosol composition, and flow characterization. Multiple aerosol generation systems enable dispersion of both liquid solutions or suspensions and dry powders, producing particle concentrations from 50 to 3,000 μg/m³ and the ability to detect particles across a mean diameter range of 50 nm to 20 μm. Facility validation was conducted using multiple test chemicals including caffeine, oleic acid, phenanthrene, glycerol, tributyl phosphate, and Arizona test dust for three nominal concentration levels (low ∼100, medium ∼500, high >800 μg/m³). Aerosol concentration uniformity across the flow cross-section showed relative standard deviations below 3.5%. Multi-instrument comparisons between redundant particle sizing systems (dual APS units, UHSAS, and Promo) demonstrated good measurement consistency, with gravimetric validation confirming total aerosol mass concentrations with a 20% difference between the types of measurements. The Aerodyne Aerosol Mass Spectrometer correctly identified particle chemical signatures consistent with NIST fragmentation patterns for all test compounds. The facility employs shrouded probe sampling systems with isokinetic coupling to individual instruments to minimize particle losses and sampling biases across the particle size distribution. The AATF provides a repeatable and reliable aerosol generation testbed for detector development, evaluation, instrument intercomparison, and aerosol measurement validation under controlled yet realistic ambient air conditions with controlled size distributions and total mass concentrations for a wide range of chemical aerosols.

Received: 03 Dec 2025 – Discussion started: 11 Jan 2026

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Paul Johns, Cathy Scotto, Landon Hernandez, Matthew Hart, Oyedoyin Aduroja, Mark Hammond, Braden Giordano, Kenneth Grabowski, Jay Eversole, and Vasanthi Sivaprakasam

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RC1:
'Comment on egusphere-2025-6043', Anonymous Referee #1, 30 Jan 2026 reply
This manuscript describes the development and validation of the so-called Ambient Air Test Facility (AATF), a large-scale setup which enables intercomparison of aerosol instruments under controlled laboratory conditions. The setup comprises several aerosol generators, a 14-meter-long flow tube for aerosol homogenisation and isokinetic sampling ports. The scale of the facility is impressive, allowing parallel measurements with various devices-under-test.
The manuscript is well-structured and easy to read, and the figures are informative. I am puzzled, however, by the lack of reference instruments that would enable a more meaningful intercomparison of aerosol instruments. I am also sceptical about the lack of validation for particles larger than 5 micrometers.
Questions:
The authors refer in the abstract and main text to a "detection of particles across a mean diameter range of 50 nm to 20 μm". However, nowhere in the manuscript could I find any information about the homogenisation and measurement of particles larger than 10 μm. In fact, the validation measurements shown in Figure 3 refer to particle sizes up to 5 μm. Moreover, none of the diagnostic equipment at AATF (Table 1) can measure 20 μm particles. The APS (TSI Inc.), despite manufacturer's claims, can barely measure particles equal to or larger than 15 μm (see Vasilatou et al. https://doi.org/10.1080/02786826.2022.2139659).

In page 6, it is mentioned that "sampling takes place 4.5 m downstream of the aerosol injection". However, in Figure 1a) the test section is quite long. How is aerosol homogenisation along the horizontal test section ensured despite particle settling?

The authors compare instruments with one another without having any reference instruments available. The discussion of the results remains therefore descriptive without any in-depth analysis. Optical and aerodynamic particle sizers can be calibrated with respect to particle size and number concentration (see, for instance, Iida et al. https://doi.org/10.1063/1.4803302; Horender et al. https://doi.org/10.1063/1.5095853 and ISO standard 21501-1). By integrating calibrated instruments that provide reference measurements into the AATF facility, the authors could get more insight into the performance of the various devices-under-test.

Similarly, by generating size-certified particles (e.g. polystyrene spheres or silica particles) instead of droplets, it would be possible to check the accuracy of the size measurements.

To summarise, I appreciate the effort that went into developing the setup, but I would recommend a more thorough validation for large particle sizes. In addition, to make measurements more quantitative, I would recommend the use of calibrated instruments to provide reference data.

Reply
Citation: https://doi.org/10.5194/egusphere-2025-6043-RC1
- AC1:
  'Reply on RC1', Paul Johns, 09 Feb 2026 reply
  Your feedback has highlighted several areas where we can improve the clarity and detail of our manuscript. We have addressed your specific questions below, and we believe these clarifications will resolve the concerns you have raised.
  We acknowledge your concern regarding the validation of large particle sizes. Our claim of a 20 µm upper limit was based on the AATF's design capabilities. However, you are correct that our presented data does not fully support this range, especially given the Vasilatou et al reference. The validation data, combining the PSL experiments (up to 5 µm) and tests with other chemicals, robustly supports a range up to 15 µm. We agree that this is a more defensible claim based on the presented results. Therefore, we will revise the manuscript to claim a validated upper limit of 15 µm.
  
  Thank you for raising this important point regarding aerosol homogenization and the potential for particle settling, especially for larger particles. This is a critical consideration that we addressed through the engineering of the AATF's turbulent flow regime, and we appreciate the opportunity to clarify this in the manuscript. Additional explanation will be added to Section 2.2 “Aerosol Homogenization & Turbulence Characteristics,” and we will also add more explicit distances in our diagram of the facility. We will emphasize residence time in our analysis, as this is the more critical parameter for particle transport processes like settling and evaporation. Stokes’ Law gives the terminal settling velocity of a spherical particle in a quiescent environment with settling velocity increasing as the size of the particle increases. For a 10 µm diameter particle, this velocity is on the order of 0.3 cm/s. At 2 m/s flow through the tube, it takes approximately 2.5 s for air to travel from the aerosol input to past the collection tubes. In this short amount of time, the larger particles in the chamber would be expected to travel 0.75 cm downward in a still environment. However, the AATF is not quiescent, but rather has fully developed turbulent flow at Re = 40,000. With fully developed turbulent flow, turbulent fluctuation velocity is estimated as approximately 5–10% of the mean flow velocity, (in this case 10–20 cm/s.) With the turbulent fluctuation velocity being over 30 times greater than the particle's terminal settling velocity, turbulent fluctuation is the dominating factor in determining settling in the AATF. At these velocities, the particles are forcibly entrained in the chaotic flow, keeping them homogenized and suspended throughout their transit in the AATF.
  
  Using calibrated reference instruments is essential to the validation process. We appreciate you raising this point and apologize that this was not sufficiently clear in the manuscript. In fact, key instruments within the AATF are calibrated and serve as the reference instruments for our intercomparisons. Specifically, the two TSI APS units, the Promo, and the UHSAS were calibrated by their respective manufacturers immediately prior to this effort. Droplet Measurement Technologies (the manufacturers of the UHSAS) specifically states that their calibration exceeds ISO Standard 25101-1. We will revise the manuscript to explicitly state the calibration status of these instruments and to clarify their role as internal reference standards for the AATF. To ensure long-term data quality, we will add to our manuscript our plan to implement a routine verification protocol, such as periodically introducing PSL aerosols, to monitor instrument stability over time. We will also acknowledge that, as is fundamental to all aerosol sizing techniques, measurements of non-ideal aerosols carry inherent uncertainties related to material properties like density and refractive index.
  
  We would like to clarify that the validation measurements shown in Figure 3 were, in fact, performed using size-certified polystyrene latex (PSL) spheres. We apologize for omitting this critical detail from the figure caption and the main text. This method was chosen precisely to check the sizing accuracy of the instruments, as you correctly recommend, and to ensure sampling consistency throughout the cross-section of the AATF. We will amend the manuscript, specifically to Section 2.2 “Aerosol Homogenization & Turbulence Characteristics” and to the caption for Figure 3, to clearly state that size-certified PSL spheres were used in these validation experiments. This will reinforce the quantitative accuracy of our presented data.
  
  We believe these revisions will fully address your concerns and significantly strengthen the manuscript. We thank you again for your valuable feedback, which has helped us identify where our manuscript can be improved.
  
  Reply
  
  Citation: https://doi.org/10.5194/egusphere-2025-6043-AC1

Paul Johns, Cathy Scotto, Landon Hernandez, Matthew Hart, Oyedoyin Aduroja, Mark Hammond, Braden Giordano, Kenneth Grabowski, Jay Eversole, and Vasanthi Sivaprakasam

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https://doi.org/10.5194/egusphere-2025-6043-supplement

Paul Johns, Cathy Scotto, Landon Hernandez, Matthew Hart, Oyedoyin Aduroja, Mark Hammond, Braden Giordano, Kenneth Grabowski, Jay Eversole, and Vasanthi Sivaprakasam

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Short summary

The U.S. Naval Research Laboratory developed and tested a facility that allows for 13 air quality instruments and sizers to sample simultaneously from one controlled environment. The system creates uniform particulate aerosols from various chemicals, achieving excellent consistency across wide concentration ranges. This aerosol test facility enables researches to validate air monitoring equipment under realistic conditions indoors, supporting better atmospheric measurement tools.


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