Evolution of Aerosol Optical Properties from Wood Smoke in Real Atmosphere Influenced by Burning Phase and Solar Radiation

Abstract

Emissions of light-absorbing black carbon (BC) and organic aerosol (OA) from biomass burning are presented as complex mixtures, which introduce challenges in modeling their absorbing properties. In this study, we chose typical residential wood burning emission and used a novel designed chamber to investigate the early stage evolution of plumes from different burning phases under real ambient conditions. The detailed mixing state between BC and OA was evaluated, on the basis of which optical modeling was performed to achieve a closure of aerosol-absorbing properties. Intensive secondary OA (SOA) formation was observed under solar radiation. OA from flaming conditions showed a higher absorptivity than from smoldering conditions, as OA is mostly internally and externally mixed with BC, respectively. For flaming (smoldering), the imaginary refractive index of OA (k OA) was initially at 0.03 ± 0.01 (0.001) and 0.15 ± 0.02 (0.05 ± 0.02) at λ = 781 and 405 nm, respectively, with a half-decay time of 2–3 h in light but a <40% decrease under dark within 5 h. The production of less-absorbing SOA in the first 1–2 h and possible subsequent photobleaching of chromophores contributed to the decrease of k OA. The enhanced abundance but decreased absorptivity of coatings on BC resulted in a relatively maintainable absorptivity of BC-containing particles during evolution.

Light absorption by brown carbon over the South-East Atlantic Ocean

“…OA is assumed to be non-absorbing in the calculation. The average and standard deviation of calculated m BC-free is 1.52 ± 0.015, consistent with the values used in BrC studies in literature (Saleh et al, 2013;Liu et al, 2021Liu et al, , 2015. The particle effective refractive indices for the VM, MG, and BG models can then be obtained using the corresponding mixing rules (Sect.…”

“…The real part of m eBC is held constant at 1.95, the upper bound of the values commonly used for BC (Bond and Bergstrom, 2006;Saleh et al, 2013;Liu et al, 2015;Kahnert and Kanngießer, 2020). It only has a minor influence on absorption calculations (Liu et al, 2021): a sensitivity test of the calculated ab-sorption to the real part of m eBC when the latter is varied from 1.75 to 2.26 shows that, at the extreme case of RF10 with the thickest coating in this study, the absorption increased less than 5 % at 660 nm.…”

“…4) of 1.6. We calculated absorption coefficients for RF05_3 at 660 nm using the VM model, which usually yields a higher estimation of the measurements (Taylor et al, 2020) and a m BC of 1.95 + 0.79i, an upper bound of the commonly used BC refractive index (Liu et al, 2021). Uncertainties related to calculated absorption coefficients can be found in Sect.…”

“…8, σ mea abs,530 includes the contribution of BrC, which accounts for 4 % of the total absorption, and that of absorbers beyond BC and BrC, which accounts for 21 %. Note this attribution is based on the BC absorption coefficient calculated from the BG model with m BC = 1.95 + 0.79i, which yields an upper bound of BC absorption (Liu et al, 2021). All calculated parameters are subject to uncertainties as discussed in Sect.…”

Untitled

“…For BC-containing particles, f1 represents the volume fraction of BC, which can be calculated from BC 2-D size and mixing state distributions determined by the SP2. To support our assumption of negligible BrC absorption at 660 nm, we assumed the absorption not attributed to BC at 660 nm to be solely from OA, and calculated the imaginary part of the refractive index of OA, kOA, at 660 nm with different the VM, BG, MG, and CS models, following the method in Liu et al (2021). A refractive index of BC of 1.95+0.79i was used, which is an upper bound of commonly used BC refractive index and thereby yields an upper bound of BC absorption and a lower bound of BrC absorption.…”