Humidified single-scattering albedometer (H-CAPS-PM_SSA): Design, data analysis, and validation

Abstract

We report the development and validation of a new humidified aerosol single-scattering albedometer to quantify the effects of water uptake on submicrometer particle optical properties. The instrument simultaneously measures in situ aerosol light extinction (σ_ep) and scattering (σ_sp) using a cavity-attenuated phase shift-single scattering albedo particulate matter (PM) monitor (CAPS-PM_SSA, Aerodyne Research, Inc., Billerica, MA, USA). It retrieves by difference aerosol light absorption (σ_ap) and directly quantifies aerosol single-scattering albedo (SSA), the aerosol “brightness.” We custom built a relative humidity (RH) control system using a water vapor-permeable membrane humidifier and coupled it to the CAPS-PM_SSA to enable humidified aerosol observations. Our humidified instrument (H-CAPS-PM_SSA) overcomes problems with noise caused by mirror purge-flow humidification, heating, and characterizing cell RH. Careful angular truncation corrections in scattering, particularly for larger particles, were combined with empirical observations. Results show that the optimal operational size to be D_p < 400 nm. The H-CAPS-PM_SSA was evaluated with several pure single-component aerosols including ammonium sulfate ((NH₄)₂SO₄), absorbing nigrosin, and levoglucosan, an organic biomass smoke tracer. The measured σ_ep, σ_sp, and the derived optical hygroscopicity parameter (κ) for size-selected ammonium sulfate are in good agreement with literature values. For dry size-selected nigrosin in the 100 < D_p < 400 nm range, SSA values increased from ∼0.3 to 0.65 with increasing D_p. The enhancement in nigrosin σ_ap at RH = 80% was a factor of 1.05–1.20 relative to dry conditions, with the larger particles showing greater enhancement. SSA increased with RH with the largest fractional enhancement measured for the smallest particles. For polydisperse levoglucosan, we measured an optical κ of 0.26 for both light extinction and scattering and negligible absorption. Our new instrument enables reliable observations of the effects of ambient humidity on mixed aerosol optical properties, particularly for light-absorbing aerosols whose climate forcing is uncertain due to measurement gaps.

Copyright © 2021 American Association for Aerosol Research

EDITOR:

• Nicole Riemer

Acknowledgment

Data are available by emailing the corresponding author ([email protected]).

Additional information

Funding

This material is a part based upon work supported by the National Science Foundation under Grant No.1832813 (CMC, SG). This work was supported in part by the U.S. Department of Energy, Office of Science, Office of Workforce Development for Teachers and Scientists (WDTS) under the Visiting Faculty Program (VFP) (CMC, JTL), and the DOE Student Undergraduate Laboratory Intern Program (JTL, JK). TJC was partially supported by a fellowship through a DOE Office of Science Graduate Student Research Program. The New Mexico Consortium is gratefully acknowledged for financial support in this research (CMC, JTL, SG, JK, CD). The acquisition of the CAPS-PM_SSA, some assembly parts, laboratory evaluation, and testing were primarily done at LANL and supported primarily by the U.S. Department of Energy's Atmospheric System Research, Office of Science, Office of Biological and Environmental Research’s Atmospheric System Research program under grant F265 (PI MKD) as well as LANL’s Laboratory Directed Research Exploratory Research (LDRD) project 20200035DR (PI MKD). Los Alamos National Laboratory is operated for the DOE by Los Alamos National Security under contract DE-AC52-06NA25396. We acknowledge LANL’s LDRD and the Center for Space and Earth Science and the support of Sanna Sevanto and Petr Chylek at LANL.