10th International Aerosol Conference September 2 - September 7, 2018 America's Center Convention Complex St. Louis, Missouri, USA
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The Hygroscopicity of Particles That Carry Differing Charges and Their Impact on Tandem Differential Mobility Analyzer Measurements of Biomass Burning Aerosol
CHRISTOPHER OXFORD, Rajan K. Chakrabarty, Brent Williams, Washington University in St. Louis
Abstract Number: 1456 Working Group: Aerosol Physics
Abstract Biomass burning aerosol (BBA) constitutes a significant fraction of atmospheric aerosol and impacts health, visibility, and radiative forcing. The nature and scale of these impacts are influenced by the size distribution of the aerosol, which is governed by hygroscopicity at elevated (90%) relative humidity. Characterization of BBA during the second Fire Lab At Missoula Experiment (FLAME-II) determined that BBA with high inorganic concentrations exhibited bimodality rather than a singular hygroscopicity1. Bimodal hygroscopicity could be caused by condensation of hydrophilic inorganics, release of hydrophobic soot aerosol, presence of non-spherical morphologies, and condensation of volatile organic compounds with low hygroscopicity. Conclusions from FLAME-II attribute the bimodality to externally mixed BBA at a given diameter2. Other authors, using different fuels, attribute differences in BBA hygroscopicity to non-spherical morphologies3.
Hygroscopicity can be determined with a Tandem Differential Mobility Analyzer (TDMA), which measures changes in mobility size due to the condensation of water on selected particles. TDMA inversion routines assume the selected particles are singly charged4,5. Investigating sizes smaller than the mean of the log-normal size distribution (μ) could select a population of multiply-charged particles, violating the single charge assumption. Therefore, TDMA users must choose a diameter larger than μ6 to reduce error in the singly-charged assumption. However, μ during chamber studies can often exceed 150 nm, and the upper measurement limit of many TDMAs prevent determination of the final diameter of high growth particles larger than a dry diameter of 200 nm. The measurement limits and assumption limits can create a narrow window of available mobility sizes.
We emitted primary organic aerosol from the flaming combustion of grasses sourced from Western Montana and employed a Centrifugal Particle Mass Analyzer (CPMA) in combination with the TDMA to determine separately the hygroscopicity of particles that carry single, double, and triple charges. To address the possibility of morphology as a contributing factor to bimodality, we also obtained Transmission Electron Microscopy (TEM) images of the aerosol. We show that the individual charges do not move equally in mobility space allowing the size distribution to influence the measured hygroscopicity.
[1] Carrico, C. M. et al. Water uptake and chemical composition of fresh aerosols generated in open burning of biomass. Atmos Chem Phys 10, 5165-5178, doi:10.5194/acp-10-5165-2010 (2010). [2] Petters, M. D. et al. Cloud condensation nucleation activity of biomass burning aerosol. J Geophys Res-Atmos 114, doi:Artn D22205 10.1029/2009jd012353 (2009). [3] Giordano, M., Espinoza, C. & Asa-Awuku, A. Experimentally measured morphology of biomass burning aerosol and its impacts on CCN ability. Atmos Chem Phys 15, 1807-1821, doi:10.5194/acp-15-1807-2015 (2015). [4] Gysel, M., McFiggans, G. B. & Coe, H. Inversion of tandem differential mobility analyser (TDMA) measurements. Journal of Aerosol Science 40, 134-151, doi:DOI 10.1016/j.jaerosci.2008.07.013 (2009). [5] Stolzenburg, M., McMurry, P.H. (University of Minnesota, Department of Mechanical Engineering, 1988). [6] Rader, D. J. & Mcmurry, P. H. Application of the Tandem Differential Mobility Analyzer to Studies of Droplet Growth or Evaporation. Journal of Aerosol Science 17, 771-787 (1986).