Abstract

Integrating nephelometers are key instruments for quantifying aerosol light scattering, a fundamental parameter governing the direct radiative effect of aerosols on climate and visibility, and for interpreting satellite observations of aerosol optical depth. The AirPhoton integrating nephelometer quantifies aerosol light scattering by measuring total scattering from the sample air and subtracting a clean-air reference that represents Rayleigh scattering, instrument wall scattering, and detector background noise. Measurement accuracy depends on rigorous calibration and regular performance evaluation to account for instrumental drift and environmental influences. This study presents a field performance evaluation of the AirPhoton integrating nephelometer within the global Surface Particulate Matter Network (SPARTAN), focusing on sensor stability and long-term clean-air reference baseline drift. Calibration experiments were performed at the SPARTAN laboratory at Washington University in St. Louis. Field evaluations were performed using blue (463 nm) scattering measurements from three SPARTAN sites: Rehovot (Israel), Sherbrooke (Canada), and Melbourne (Australia), each representing distinct climatic regimes and aerosol loadings. Blue (463 nm) scattering data were analyzed in detail, given its higher sensitivity to fine aerosol particles. Across all sites, the forward and backward blue (463 nm) reference sensor signals demonstrated stable long-term values, with a coefficient of variation consistently below 5%. A small seasonal variability observed was primarily driven by temperature-dependent sensor response. Clean-air reference baseline drift exhibited strong dependence on both aerosol loading and precipitation intensity and frequency. At the Rehovot site, characterized by high dust concentrations and a distinct wet season, the baseline remained stable during the dry months (May-October) but increased rapidly from October to January with the onset of rainfall. This increase is attributed to condensation of water vapor inside the nephelometer and enhanced particle deposition on interior surfaces, which becomes more pronounced when aerosol concentrations are high and humidity promotes particle adhesion. Frequent short-duration spikes in daily blue (463 nm) clear air reference scattering were observed during the rainy season, reflecting episodic condensation that subsequently evaporated. At Sherbrooke, where aerosol levels are lowest, the clean-air baseline remained nearly constant year-round, with only a modest rise during winter months (November-March), likely related to snowfall events. Spikes occurred throughout the year but were smaller in magnitude during winter due to lower rainfall and lower absolute humidity. Melbourne exhibited intermediate behavior, with baseline drift following the annual rainfall cycle and spike magnitudes reflecting its moderate aerosol burden and higher absolute humidity. This study recommends recalibrating the nephelometer when the moving-median clean-air baseline exceeds 20% of the 30-day moving average of ambient scattering and flagging days as invalid when the average scattering during the clean-air reference period exceeds 10% of ambient levels. This study provides new insights into environmental influences on nephelometer clean-air reference baseline drift and establishes site-specific patterns that can inform recalibration intervals. These findings improve the reliability and quality assurance of nephelometer scattering measurements within SPARTAN, supporting more accurate aerosol optical characterization for interpretation of satellite observations and aerosol modeling.

Committee Chair

Randall Martin

Committee Members

Jay Turner, Jenna Ditto

Degree

Master of Science (MS)

Author's Department

Energy, Environmental & Chemical Engineering

Author's School

McKelvey School of Engineering

Document Type

Thesis

Date of Award

Fall 12-17-2025

Language

English (en)

Author's ORCID

https://orcid.org/0009-0001-1366-3036

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