Photo-acoustic and Baseline Filter Absorption Inter-Comparison at SGP/ARM: Quantifying Biases in Long-Term Records

 
Poster PDF

Authors

Manvendra K. Dubey — Los Alamos National Laboratory
Allison C Aiken — Los Alamos National Laboratory
Luis Torres — LANL

Category

Absorbing aerosol

Description

Filter based PSAP absorption versus direct PASS absorption scaled to 405, 532 and 781 nm at SGP for 2013 and 2015 (green laser upgrade). Linear fit shows a consistent high bias of 1.8 to 3.8 in the PSAP data relative to PASS at all the wavelengths. The 1-1 line is shown in dash blue to illustrate the high bias.
The potential for large biases in filter-based light absorption observations in baseline instruments has been recognized, both mechanistically and operationally (Subramanium 2007, Lack 2008 & Cappa 2008). This calls into question the quality of long-term baseline absorption observations made by DOE-ARM and NOAA that are important to evaluate climate models. To resolve this problem we report long term direct filter free observations of optical properties with a 3-wavelength photo-acoustic spectrometer (PASS) of both light absorption and scattering at 781, 532 & 405 nm at SGP in 2013 and 2015. We compare our results with absorption measurements made by baseline filter-based measurements (PSAP at 660, 530 & 467 nm, and CLAP AT 653, 522 & 461 nm)) and the direct scattering observations made with a nephelometer (at 700, 550 & 450 nm) . All instruments were located on the same inlet and the optical observations were compared at the same wavelength using extrapolations with measured Angstrom exponents. One hour averaging was used for our comparison to gain sufficient signal/noise for the PASS. Our analysis shows that while the absorption measured by the PASS and PSAP are well correlated the PSAP is biased high by a factor of 2.2, 1.9 and 1.8 at 781, 532 and 405nm respectively in 2013 (Fig 1). The PASS was upgraded with a high power green laser and deployed at SGP in 2015. We find that the signal/noise of absorption at 532nm in 2015 improved significantly but the biases persisted at all wavelengths (Fig 1). The PASS scattering measurements are noisy but agree with those measured by the nephelometer. In order to understand the mechanisms responsible for PSAP high bias (factors of 2 to 4) we correlate them to the organic, nitrate and sulfate content measured by the Aerosol Chemical Species Monitor at SGP. Our objective is to develop a scientifically robust method to correct for the high bias in the baseline absorption data at SGP for more reliable climate model performance assessments. References: Subramanium et al. Aerosol Science and Technology 41:630–637, 2007 Lack et al. Aerosol Science and Technology, 42:1033–1041, 2008 Cappa et. al. Aerosol Science and Technology, 42:1022–1032, 2008