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Biofuel combustion is an important source of particulate light absorbing organic carbon (OC), also known as brown carbon (BrC). We applied spectrophotometry to characterize methanol-extracted BrC from emission tests of 'real-world' biofuel combustion in India and Malawi, including wood stoves ('traditional', 'improved' and 'chimney') and artisanal charcoal kilns. Average mass absorption coefficient (MACbulk,λ) of extracted BrC was highest for 'traditional', followed by 'improved', 'chimney', and 'charcoal', at near-ultraviolet to blue wavelengths, with this order reversed for BrC absorption Angstrom exponent (AAE). MACbulk,λ in UV wavelengths was positively correlated with the elemental carbon to organic aerosol ratio (EC/OA), though the correlation was weaker than that observed in laboratory cookstove samples. BrC imaginary refractive indices (k) were anti-correlated with wavelength dependence (w), thus less wavelength dependent BrC had higher light absorptivity. MACbulk,λ correlated with the fraction of OC evolving at higher temperature steps in thermo-optical analysis, consistent with a link between BrC absorptivity and OC volatility, and suggesting that BrC absorption may be parameterized using existing OC data. Modeling analyses showed that BrC makes a strong contribution to overall absorption (average of 48% to 80% at 365 nm), and a strong negative correlation between EC/OA and the relative contribution of BrC to total aerosol (BrC + BC) light absorption; the latter trend is dominated by the quantity versus optical properties of BrC. The estimated direct radiative effect of BrC is approximately equal to that of BC for biofuel combustion emissions in India, highlighting the importance of BrC in the climate energy budget. Copyright © 2022 American Association for Aerosol Research [ABSTRACT FROM AUTHOR]

Nanoscale dimers have application in studies of aerosol physicochemical properties such as aerosol viscosity. These particle dimers can be synthesized using the dual tandem differential mobility analyzer (DTDMA) technique, wherein oppositely charged particle streams coagulate to form dimers that can be isolated using electrostatic filtration. Although some characterization of the technique has been published, a detailed thesis on the modes and theory of operation has remained outside the scope of prior work. Here, we present new experimental data characterizing the output DTDMA size distribution and the physical processes underlying its apparent modes. Key experimental limitations for both general applications and for viscosity measurements are identified and quantified in six distinct types of DTDMA experiments. The primary consideration is the production of an adequate number of dimers, which typically requires high mobility-selected number concentration in the range 25,000–100,000 cm−3. The requisite concentration threshold depends upon the rate of spontaneous monomer decharging, which arises predominately from interactions of the aerosol with ionizing radiation within the coagulation chamber and is instrument location dependent. Lead shielding of the coagulation chamber reduced the first-order decharging constant from ∼2.0 × 10−5 s−1 to ∼0.8 × 10−5 s−1 in our laboratory. Dimer production at monomer diameters less than 40 nm is hindered by low bipolar charging efficiency. Results from the characterization experiments shed light on design considerations for general applications and for characterization of viscous aerosol phase transitions. Copyright © 2019 American Association for Aerosol Research [ABSTRACT FROM AUTHOR]

Thermodenuders (TD) are a tool widely used for measuring aerosol volatility in the laboratory and field. Extracting the parameters that dictate organic aerosol volatility from TD data is challenging because gas-particle partitioning rarely reaches equilibrium inside a TD operating under atmospheric conditions, thus a wide variety of parameter sets can explain observed evaporation. Component volatilities (as represented by saturation vapor pressure,Csat), cannot be directly extracted due to uncertainties in potential limitations to mass transfer (represented by mass accommodation coefficient, α) and components’ enthalpies of evaporation (ΔHvap). To address these limitations, we have developed a “dual TD” experimental approach in which one line uses a temperature-stepping TD (TS-TD) with a relatively long residence time (RT) and the other operates isothermally at variable residence time (VRT-TD). Data from this approach are used in tandem with an optimizing evaporation kinetics model to extract the values of parameters dictating volatility (Csat, and associated values of ΔHvapand α). The system was evaluated using laboratory generated dicarboxylic acid aerosols (adipic acid and succinic acid). Excellent agreement with previously published evaporation data collected with other TD systems was observed. Parameter values reported in the literature for the tested acids vary widely, but our results are generally consistent with those from studies that allow for nonunity values of α. For example, our results suggest that α for these aerosols are of order 0.1, in agreement with results determined by Saleh et al. (2009, 2012). Modeling results suggest that the addition of VRT-TD data provides tighter constraint on feasible ΔHvapand α values. The dual TD approach presented here does not rely on equilibration in the TD and thus can be directly applied to extract volatility parameters for more complex laboratory and ambient organic aerosol systems. Copyright 2015 American Association for Aerosol Research [ABSTRACT FROM PUBLISHER]

Thermodenuders (TD) have been used to quantify the volatil-ity of aerosol species, frequently with the aid of modeling. Here we present a two-dimensional model of flow, heat transfer, and aerosol dynamics that is fast, yet includes spatial resolution of the complete aerosol size distribution. We first demonstrate the utility of the model by interpreting nonequilibrium TD measure-ment data previously reported in the literature. It is shown that the thermogram (temperature vs. mass fraction remaining) curve is re-markably insensitive to radial variations in temperature and vapor concentration under typical conditions. Therefore, the discrepan-cies among vapor pressure estimates determined in TD studies are unlikely to be due to oversimplified flow models, but are instead likely due to faulty assumptions concerning evaporation kinetics. We then show that the best-fit range for the parameters that dictate equilibrium partitioning (saturation vapor pressure at a reference temperature and enthalpy of vaporization) can also be obtained by fitting nonequilibrium TD data using a three-parameter model that accounts for mass transfer limitations (by also fitting the evap-oration coefficient). The degree of agreement between experiments and model simulations are examined for two dicarboxylic acids using the model developed in this study. The best-fit parameters were within the uncertainty range previously found using an "equi-librated" TD approach for butanedioic acid, whereas significantly better model-experiment agreement was obtained for a much lower value of enthalpy of vaporization than previously reported for hex-anedioic acid. [ABSTRACT FROM AUTHOR]