Biomass Burning Smoke Hygroscopicity Determined by Aerosol And/or Fuel Inorganic Content

Dubey, M. K., Los Alamos National Laboratory

Aerosol Properties

Aerosol Processes

Gomez S, C Carrico, C Allen, J Lam, S Dabli, A Sullivan, A Aiken, T Rahn, D Romonosky, P Chylek, S Sevanto, and M Dubey. 2018. "Southwestern U.S. Biomass Burning Smoke Hygroscopicity: The Role of Plant Phenology, Chemical Composition, and Combustion Properties." Journal of Geophysical Research: Atmospheres, 123(10), doi:10.1029/2017JD028162.


κneph versus smoke inorganic mass fraction and total cumulative ion concentration (mg/L) for selected fuels. Error bars represent an uncertainty analysis of κneph using error propagation of the uncertainties in light scattering and relative humidity measurements.


Comparison of κneph for ambient measurements during ambient smoke events with comparative laboratory measurements of alpine fuels: piñon pine, ponderosa pine, juniper, and aspen.


κneph versus smoke inorganic mass fraction and total cumulative ion concentration (mg/L) for selected fuels. Error bars represent an uncertainty analysis of κneph using error propagation of the uncertainties in light scattering and relative humidity measurements.

Comparison of κneph for ambient measurements during ambient smoke events with comparative laboratory measurements of alpine fuels: piñon pine, ponderosa pine, juniper, and aspen.

Science

Biomass burning emissions have substantially increased with warming and drying in the southwestern U.S. We measured the response of biomass burning aerosols to atmospheric humidity using optical and chemical measurements in the lab and field. We demonstrated that aerosol hygroscopicity is driven by the fuel’s inorganic ion content and developed empirical relationships to capture this in models to better predict their radiative forcing.

Impact

We found that biomass smoke hygroscopic response is variable, with variability driven by fuel chemical composition and ignition method. Coniferous evergreen species are weakly hygroscopic while salt-tolerant species containing inorganic ions are strongly hygroscopic. A framework for smoke hygroscopicity based on lab measurements shows consistent behavior when applied to ambient smoke measurements. Our results also identify the important role that inorganic ions play in determining fresh biomass burning smoke hygroscopicity, as demonstrated by the empirical linear relationships between κneph and inorganic composition, for both fuel and smoke. Our work will transform current prognostic aerosol models that do not resolve the ionic composition of the smoke and its effects on water uptake, which introduce large uncertainties in radiative forcing estimates.

Summary

To better quantify impacts of biomass burning aerosols, an extensive laboratory study of fresh smoke emissions was conducted at Los Alamos National Laboratory. Burn experiments with southwestern U.S. fuels were used to elucidate the roles of fuel type, chemical composition, and ignition method on the hygroscopicity of smoke. We focused on a custom controlled relative humidity (RH) nephelometry system using the direct measurement of aerosol light scattering with two nephelometers—one at dry conditions and one at a controlled high RH (RH ~ 85%). Aerosol hygroscopicity was highly variable with the enhancement in light-scattering coefficient in the range of 1.02 < f(RH = 85%) < 2.1 and corresponding to the kappa parameter (κneph) ranging from ~0 to 0.18. Hygroscopicity was determined primarily by the fuel’s inorganic ion content. For example, invasive halophytes with high inorganic salt content exhibit much greater water uptake than native coniferous species with low inorganic content. Combustion temperature and phase, flaming or smoldering, play a secondary role in the water uptake of smoke. High-temperature ignition methods create flaming conditions that enhance hygroscopicity while lower-temperature smoldering conditions diminish hygroscopicity. Our laboratory results constructed an empirical relation between κneph and the inorganic content of the fuel and smoke that was then tested on ambient fires by using mixing rules. Our framework should enable improved prediction of water uptake in future model simulations.