Atmospheric Photooxidation Diminishes Light Absorption by Primary Brown Carbon Aerosol from Biomass Burning

Abstract

Light-absorbing organic aerosols, optically defined as brown carbon (BrC), have been shown to strongly absorb short visible solar wavelengths and significantly impact Earth's radiative energy balance. There currently exists a knowledge gap regarding the potential impacts of atmospheric processing on the absorptivity of such particles generated from biomass burning. Climate models and satellite retrieval algorithms parametrize the optical properties of BrC aerosols emitted from biomass burning events as unchanging throughout their atmospheric lifecycle. Here, using contact-free optical probing techniques, we investigate the effects of multiple-day photochemical oxidation on the spectral (375−532 nm) optical properties of primary BrC aerosols emitted from smoldering combustion of boreal peatlands. We find the largest effects of oxidation in the near-UV wavelengths, with the 375 nm imaginary refractive index and absorption coefficients of BrC particles decreasing by ∼36% and 46%, respectively, and an increase in their single scattering albedo from 0.85 to 0.90. Based on simultaneous chemical characterization of particles, we infer a transition from functionalization to fragmentation reactions with increasing photooxidation. Simple radiative forcing efficiency calculations show the effects of aging on atmospheric warming attributed to BrC aerosols, which could be significant over snow and other reflective surfaces.

Bulk and molecular-level characterization of laboratory-aged biomass burning organic aerosol from oak leaf and heartwood fuels

“…Photobleaching of BBOA, particularly at 254 nm, has been reported in previous literature (e.g., Sumlin et al, 2017;Wong et al, 2017;Zhao et al, 2015) and therefore should be considered when estimating oxidative aging. With the spreadsheet provided by Peng et al (2016), we estimate 254 and 185 nm exposure ratios (ratio of photon flux, photons cm −2 , to OH exp ; Peng et al, 2016) to be 1.2 × 10 5 and 8.1 × 10 2 cm s −1 , respectively, at a measured internally generated O 3 concentration of 1.7 ppm (at the highest PAM UV lamp intensity), a water mixing ratio of 1 % (RH = 30 %), and assuming a maximum OHR ext value of 1 .…”

“…This PMF method was performed on the compound and decomposition analytical windows separately for data obtained from both BBOA types. PMF output and solutions were evaluated using custom-built pre-and postprocessing analysis software in conjunction with the PMF Evaluation Tool (version 3.00A; Ulbrich et al, 2009) in Igor Pro version 6.38Beta01 (WaveMetrics, Inc.). Mass spectral identification of different factors was aided by the NIST MS Search Program version 2.0, available for download at http://chemdata.nist.gov/mass-spc/ ms-search/.…”

“…First, in a typical PMF analysis, the optimal number of factors in a solution is selected based on the objective function Q, which is the sum of weighed squared residuals (Paatero, 1997). The Q/Q exp value, or the ratio of the actual objective function to the expected objective function assuming normally distributed residuals, should ideally approach 1; too few factors may result in a large Q/Q exp , indicating that errors have been underestimated in PMF calculations (Ulbrich et al, 2009). Additionally, if too many factors are specified, the solution may feature split factors, where information from a compound or compound class is distributed across multiple factors.…”

“…The resistive heating technique applied in these experiments allows for the isolation of devolatilization (precombustion) and lowtemperature (≤ 300 • C) smoldering conditions, which is difficult to achieve in combustion chambers that require ignition of a flame. For example, Tian et al (2015) designed a chamber that allows the user to control the relative contributions of smoldering and flaming combustion, though smoldering combustion is only achieved in this chamber following the introduction of a flame to the biomass fuel (Tian et al, 2015). The devolatilization and combustion procedure presented here is thus advantageous for investigating aerosol from small masses of biomass fuel under tightly controlled conditions.…”

Apparatus for Dry Deposition of Aerosols on Snow

“…Details of this fuel -including its collection and preparation -have been given by Chakrabarty et al (2016). The optical, physical, and chemical properties of aerosol emissions from combustion of this fuel have been extensively studied to evaluate the impact of its combustion emissions on air quality and radiative forcing in the atmosphere through optical, physical, and chemical 5 characterization Samburova et al, 2016;Sengupta et al, 2018;Sumlin et al, 2017;Sumlin et al, 2018). Prior to combustion, the fuel samples were placed into a round, insulated container to mimic simple, real-world conditions in which there is little lateral heat flux due to largely homogeneous horizontal conditions.…”