Brown carbon in tar balls from smoldering biomass combustion

Abstract

Abstract. We report the direct observation of large-scale production of spherical, carbonaceous particles – "tar balls" – from smoldering combustion of two commonly occurring dry mid-latitude fuels. Real-time measurements indicate that brown carbon is an important component of tar balls. The spectrum of the imaginary parts of their complex refractive indices can be described with a Lorentzian-like model with an effective resonance wavelength in the ultraviolet (UV) spectral region. Sensitivity calculations for aerosols containing traditional organic carbon (no absorption at visible and UV wavelengths) and brown carbon suggest that accounting for UV absorption by brown carbon leads to a significant increase in aerosol radiative forcing efficiency and increased atmospheric warming. Since particles from smoldering combustion account for nearly three-fourths of the total carbonaceous aerosol mass emitted globally, inclusion of the optical properties of tar balls into radiative forcing models has significance for the Earth's radiation budget, optical remote sensing, and understanding of anomalous UV absorption in the troposphere.

Filter-based measurements of UV–vis mass absorption cross sections of organic carbon aerosol from residential biomass combustion: Preliminary findings and sources of uncertainty

“…However, this approach has been recently challenged by mounting observational evidence of carbonaceous aerosol exhibiting a strong absorption in the near ultra-violet wavelengths of the shortwave spectrum with an AǺE41 [2,[9][10][11][12]. This increased absorption at shorter visible wavelengths has been attributed to a class of near-UV light absorbing OC aerosols (hereafter referred to as light-absorbing OC or LAOC) [13].…”

T-matrix modeling of linear depolarization by morphologically complex soot and soot-containing aerosols

“…Recently Ueda et al [28] classified soot-containing particles into five major types: soot aggregates, dome-shaped particles, particles having satellite structures, single nearly spheroidal particles, and clusters of spheroidal units. R.K. Chakrabarty et al (private communication) have conducted a literature survey and identified three major biomass burning aerosol habits: fractal soot aggregates, individual and semi-externally mixed tar balls [25], and carbonaceous super-aggregates [30] (we use the definitions of external, semi-external, and internal mixtures of particles according to Ref. [44]).…”

“…They can also limit atmospheric visibility and have a highly negative effect on human health [12]. Owing to the notoriously complex morphology of such particles [13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30], theoretical modeling of their scattering and absorption properties is a highly nontrivial task and has often been based on approximate approaches with poorly defined accuracy and range of applicability. However, the growing need for much improved knowledge of BC and BC-containing aerosols and their climatic, ecological, and visibility effects imposes strict limitations on quantitative uncertainties in particle scattering and absorption properties entering optical characterization and remote sensing applications as well as atmospheric radiation budget computations [31][32][33][34][35][36][37][38][39][40][41][42].…”

Modification in light absorption cross section of laboratory-generated black carbon-brown carbon particles upon surface reaction and hydration

“…In our study, the effect of different sulfate coatings on absorption enhancement was investigated using this method. Briefly, the model utilized a volume equivalent diameter of 132.7 nm estimated from the effective density and OC fraction, a complex refractive index of 2.0 þ 0.2i for the BC-BrC core retrieved from Mie "closure" calculations using measured absorption and scattering coefficients (Chakrabarty et al, 2010), and the complex refractive indices of sulfates coating according to the volume weighted mixing rule (see more details in SI).…”

“…Condensable organic species produced from either biomass burning (Lewis et al, 2008) or diffusion flames (Schnaiter et al, 2006), characterized by absorption in the visible light range with an absorption Ångstr€ om exponent (AAE) greater than unity, has been linked to the source of BrC. The presence of both BC and BrC in particulate matters formed from biomass/biofuel burning at flaming and smoldering conditions complicates the estimation of their climate impacts (Chakrabarty et al, 2010;Kirchstetter et al, 2004;Lack et al, 2012).…”