Impact of combustion conditions on physical and morphological properties of biomass burning aerosol

Abstract

The study of biomass burning particle density provides information on aging, new particle formation, transport properties, and is an important parameter in aerosol impacts modeling. Density is used in mass closure techniques to estimate the temporal resolution of particulate mass concentrations. However, the study of BB particle density as a function of burning conditions is still limited. Laboratory measurement of six sub-Saharan African biomass fuels burned under a range of conditions, from pure smoldering to pure flaming conditions, is presented. Smoldering-dominated burning (modified combustion efficiency (MCE) < 0.9) particles has a very narrow range of effective density (q eff) 1.03 g cm À3 to 1.21 g cm À3 and a mass mobility exponent (D fm) of $3 (2.97 ± 0.05), indicating that they are spherical particles. For the flaming-dominated burning (MCE >0.95) particles, show a size dependent q eff for all six different fuels. In this case, the mean and standard deviation of the q eff decreased with increasing size, from (0.94 ± 0.21) g cm À3 at a mobility diameter of 80 nm to (0.31 ± 0.07) g cm À3 at a mobility diameter of 400 nm. The size-dependent q eff of flamingdominated aerosol suggests the fractal nature of freshly emitted particles. The relationship between D fm and the MCE shows three distinct morphology regimes, which we define as the spherical particle, the transition, and the fractal regime. Our proposed relationship of D fm with the MCE can be used as a tool to assess the applicability of Mie theory for optical closure calculations in the absence of particle morphological information.

Determination of Emission Factors of Pollutants From Biomass Burning of African Fuels in Laboratory Measurements

“…In this study, the SMPS sampled from 14‐720 nm and in‐software multiple charging correction was applied. We found that ρ eff was size independent for burns with MCE < 0.9 (Pokhrel et al., 2021), so for burns with MCE < 0.9, we used the average value of ρ eff calculated from 4–6 different sizes. For burns with MCE >0.9, ρ eff shows a size dependence, with smaller values of ρ eff for larger mobility sizes.…”

“…where C is a pre-factor and D fm is the mass mobility exponent (Park et al, 2003(Park et al, , 2004Pokhrel et al, 2021). On combining Equations 1 and 4, ρ eff can be expressed as:…”

Seasonal variations in fire conditions are important drivers in the trend of aerosol optical properties over the south-eastern Atlantic

“…During the BB season (from June to October), the physical and chemical properties of BB aerosols change with variations in combustion conditions, source fuel and meteorological conditions, resulting in variations in the optical characteristics of BB aerosols (Pokhrel et al, 2021). Although satellite and surface-based sun photometer observations are able to estimate long-term SSA variation, the presence of the extensive clouds makes it difficult to retrieve SSA in the marine boundary layer in the SEA (Haywood et al, 2004;Eswaran et al, 2019).…”

Diffusion Coefficients and Mixing Times of Organic Molecules in β-Caryophyllene Secondary Organic Aerosol (SOA) and Biomass Burning Organic Aerosol (BBOA)

“…Biomass burning organic aerosol (BBOA) is another abundant type of aerosol in the atmosphere. ,− In contrast to SOA, BBOA is emitted directly to the atmosphere during the combustion of vegetation, including forests and crop residues. ,− The chemical composition and properties of BBOA will depend on both the fuel type and the combustion conditions. − Like SOA, BBOA also consists of many different organic molecules. BBOA also includes light-absorbing organic molecules, referred to as brown carbon, that can have a large positive radiative effect on the Earth–Atmosphere system. − …”