TRACER-AOP

TRACER Filter Correction

1 June 2022 - 31 August 2022

Lead Scientist: Rajan Chakrabarty

Observatory: AMF

Aerosol light absorption and scattering of solar radiation play an important role in the Earth’s atmosphere in terms of direct and semi-direct radiative forcing. The U.S. Department of Energy Atmospheric Radiation Measurement (ARM) user facility has deployed Aerosol Observing Systems (AOS) containing several filter-based instruments to measure and constrain aerosol optical properties and related parameters at multiple sites worldwide. For measurement of aerosol light absorption, the AOS includes filter-based instruments (particle soot absorption photometer [PSAP] and tricolor absorption photometer [TAP]) that infer particle-phase aerosol absorption coefficients at nominal red, green, and blue wavelength bands from the attenuation of light passing through a particulate filter on which aerosols are deposited. Measurement of aerosol scattering is done in situ using a TSI nephelometer (model 3963). By combining inferred absorption coefficients from filter-based ATN measurements and in situ scattering coefficients, value-added products (VAPs) such as single scattering albedo, absorption Angstrom exponents, and asymmetry parameter are derived.

Filter-based measurements of aerosol light absorption are strongly influenced by filter type and specific filter characteristics that determine potential absorption enhancement due to multiple scattering by filter medium and deposited particles, location of particles in/on the filter medium, angular distribution of scattered light, and particle morphology changes upon deposition. Currently, two correction algorithms are implemented – Bond/Ogren and Virkkula – for inferring particle-phase optical properties from ARM filter-based measurements. Both correction algorithms were formulated for reference materials (and not real-world/ambient particles) and suffer from unquantified errors and artifacts.

This DOE ARM study is focused on deployment of our custom-built multiwavelength Integrated Photoacoustic Nephelometer spectrometers and other aerosol optical instruments to the TRACER campaign. The first ARM Mobile Facility (AMF1) is planned to be deployed for the entire campaign (15 April, 2021-15 April, 2022). We propose to participate in the intensive operational period (IOP) from June to August 2021 alongside the AMF1, which will house filter-based aerosol optical instruments among other others. By performing co-located measurements of in situ and filter-based aerosol optical properties, we will be able to address the following overarching research questions: (i) What are the error bounds and biases for filter-based measurements of spectral (UV-Vis-IR) optical properties for aerosol in urban environments? (ii) Are there identifiable dependencies for the errors/biases on aerosol size, morphology, and composition? (iii) How do the biases vary with filter media (e.g., E70, Emfab, Azumi) and aerosol loading?