AEROSOL MONITORING IN RURAL OKLAHOMA FOR APPLICATION IN UNDERSAMPLED REGIONS

Abstract

Aerosols are suspended solid or liquid particles in the atmosphere and, when inhaled, can affect human cardiorespiratory health and contribute to reduced quality of life. In the United States, two nationwide monitoring networks measure total aerosol mass concentrations and identify aerosol chemical composition: the Interagency Monitoring of Protected Visual Environments (IMPROVE) network and the Chemical Speciation Network (CSN). IMPROVE was established to maintain visual accessibility to natural environments and is present in more rural regions, particularly in national parks. The CSN was developed to help monitor aerosols with respect to human health implications and is therefore located in more urban communities. These networks provide valuable information to nearby communities, but station density in the central plains is sparse, leaving few stations near rural areas. The goal of this study was to expand access to total aerosol mass and speciated aerosol data by developing proxy measurements derived from nephelometer observations. While IMPROVE and CSN rely on filter-based sampling that requires laboratory analysis and may take weeks to months before data become available, nephelometers produce measurements in near real time. Similarly, EPA-certified instruments such as the Beta Attenuation Monitor (BAM) provide more rapidly accessible data, with averaging intervals that can be set to 1, 5, 10, 15, and 30 minutes. However, the standard operating procedure reports hourly averages. Unlike the BAM, nephelometers measure aerosol scattering rather than mass concentration. In this study, observations from a BAM were used as the reference aerosol mass concentration to calibrate nephelometer measurements and develop a nephelometer-derived PM2.5 mass concentration estimate. This calibration uses the optical measurements of blue-wavelength total scattering (Bs_B, Bs_B_1um, and Bs_B_10um) to estimate PM2.5 mass concentration. The resulting nephelometer-derived mass concentrations were compared with observations from a collocated IMPROVE station, PurpleAir sensor, and a SPARTAN site, as well as a five-year climatological site average derived from pre-campaign IMPROVE data, to evaluate the nephelometer-derived mass concentration performance. While shorter averaging periods (minutely, hourly, and daily) exhibit greater apparent discrepancies, longer-term averages (monthly, seasonal, and annual) demonstrate strong agreement. This consistency at longer timescales highlights the robustness of the nephelometer-derived approach for characterizing overall PM2.5 trends, despite variability at shorter temporal resolutions. The short-term measurement differences (maximum of only ± ~ 6 µg/m3) likely arise from either the averaging of instantaneous peaks by existing measurement systems or the underestimation of PM2.5 mass concentration in nephelometer-derived estimates under coarse-mode-dominated conditions. Despite these differences, short-term measurements from the BAM and the nephelometer proxy remain particularly valuable for identifying episodic pollution events and short-duration exposure peaks that may have important implications for human health. To further expand the available dataset, speciated PM2.5 mass concentrations were interpolated to fill gaps in the IMPROVE observational record. This approach used existing IMPROVE measurements to determine speciated mass fractions, which were then scaled by the nephelometer-derived total mass estimates to produce speciated PM2.5 mass concentration estimates. When compared with five-year averages, the interpolated speciation results for the available months in 2025 were generally consistent with the longer-term record. This work demonstrates that nephelometer-derived measurements can improve the temporal availability of aerosol mass and speciated aerosol data relative to existing monitoring approaches. Higher-temporal resolution information on aerosol composition has potential applications in public health, environmental monitoring, and industry. Moreover, the nephelometer-derived total mass relationship developed here may be broadly applicable to other locations that have access to nephelometer measurements, such as sites supported by the Atmospheric Radiation Measurement (ARM) Program’s mobile facilities.