Photophoretic spectroscopy in atmospheric chemistry – high-sensitivity measurements of light absorption by a single particle

Nir Bluvshtein,Ulrich K Krieger,Thomas Peter

doi:10.5194/amt-13-3191-2020

Abstract

Abstract. Light-absorbing organic atmospheric particles, termed brown carbon, undergo chemical and photochemical aging processes during their lifetime in the atmosphere. The role these particles play in the global radiative balance and in the climate system is still uncertain. To better quantify their radiative forcing due to aerosol–radiation interactions, we need to improve process-level understanding of aging processes, which lead to either “browning” or “bleaching” of organic aerosols. Currently available laboratory techniques aim to simulate atmospheric aerosol aging and measure the evolving light absorption, but they suffer from low sensitivity and precision. This study describes the use of electrodynamic balance photophoretic spectroscopy (EDB-PPS) for high-sensitivity and high-precision measurements of light absorption by a single particle. We demonstrate the retrieval of the time-evolving imaginary part of the refractive index for a single levitated particle in the range of 10−4 to 10−5 with uncertainties of less than 25 % and 60 %, respectively. The experimental system is housed within an environmental chamber, in which aging processes can be simulated in realistic atmospheric conditions and lifetimes of days to weeks. This high level of sensitivity enables future studies to explore the major processes responsible for formation and degradation of brown carbon aerosols.

Highlights

Most radiative transfer schemes in climate models treat organic aerosol, a major subset of atmospheric aerosols that comprise 20 %–90 % of the total particulate mass (Kanakidou et al, 2005; Zhang et al, 2007), as non-absorbing in the UV–vis wavelength range, attributing them with a negative radiative effect
It has been shown that including brown carbon (BrC) absorption properties in radiative transfer models leads to a stronger wavelength dependency of light absorption by aerosols and to significant changes in the overall effective radiative forcing from aerosol–radiation interactions (ERFari) (Feng et al, 2013; Lack and Cappa, 2010)
Reduce uncertainties, and resolve contradictions, this study aims to extend the approach of electrodynamic balance photophoretic spectroscopy (EDB-PPS) for high-sensitivity and high-precision measurements of UV–vis light absorption by a single particle, in an environmental chamber, exposed to realistic atmospheric aging processes

Summary

Introduction

Most radiative transfer schemes in climate models treat organic aerosol, a major subset of atmospheric aerosols that comprise 20 %–90 % of the total particulate mass (Kanakidou et al, 2005; Zhang et al, 2007), as non-absorbing in the UV–vis wavelength range, attributing them with a negative (cooling) radiative effect. It has been shown that including BrC absorption properties in radiative transfer models leads to a stronger wavelength dependency of light absorption by aerosols and to significant changes in the overall effective radiative forcing from aerosol–radiation interactions (ERFari) (Feng et al, 2013; Lack and Cappa, 2010). It is imperative to (i) better understand

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Conclusion