Dilution impacts on smoke aging: evidence in Biomass Burning Observation Project (BBOP) data

Abstract

Abstract. Biomass burning emits vapors and aerosols into the atmosphere that can rapidly evolve as smoke plumes travel downwind and dilute, affecting climate- and health-relevant properties of the smoke. To date, theory has been unable to explain observed variability in smoke evolution. Here, we use observational data from the Biomass Burning Observation Project (BBOP) field campaign and show that initial smoke organic aerosol mass concentrations can help predict changes in smoke aerosol aging markers, number concentration, and number mean diameter between 40–262 nm. Because initial field measurements of plumes are generally >10 min downwind, smaller plumes will have already undergone substantial dilution relative to larger plumes and have lower concentrations of smoke species at these observations closest to the fire. The extent to which dilution has occurred prior to the first observation is not a directly measurable quantity. We show that initial observed plume concentrations can serve as a rough indicator of the extent of dilution prior to the first measurement, which impacts photochemistry, aerosol evaporation, and coagulation. Cores of plumes have higher concentrations than edges. By segregating the observed plumes into cores and edges, we find evidence that particle aging, evaporation, and coagulation occurred before the first measurement. We further find that on the plume edges, the organic aerosol is more oxygenated, while a marker for primary biomass burning aerosol emissions has decreased in relative abundance compared to the plume cores. Finally, we attempt to decouple the roles of the initial concentrations and physical age since emission by performing multivariate linear regression of various aerosol properties (composition, size) on these two factors.

The characterization of long-range transported North American biomass burning plumes: what can a multi-wavelength Mie–Raman-polarization-fluorescence lidar provide?

“…Studies have demonstrated that the evaporation of condensed liquid water on the surface of BBA contributes strongly to the collapse of fractal-like aggregates into compact particles (Kütz and Schmidt-Ott, 1992;Ma et al, 2013). The concentration of BBA particles, which generally decrease during transport and aging, could also impact the evaporation and coagulation of particle aging, thus modifying the particles' morphology (Hodshire et al, 2021).…”

“…The fluorescence capacity varied in 2.0-2.8 × 10 −4 at 5300-8000 m, which is 26 %-47 % lower than that in the plume core, and comparable with that in the plume edge in Case 1. This decrease of fluorescence capacity may be linked to the evaporation of organic species when the BBA plumes are diluted during the transport (Hodshire et al, 2021).…”

Important role of stratospheric injection height for the distribution and radiative forcing of smoke aerosol from the 2019–2020 Australian wildfires

“…However, we cannot rule out the possibility that away from the core plume, more reflective secondary organic particles made a larger contribution to the stratospheric fire aerosol. In addition, dilution impacts on smoke aging with regard to particle size and mixing state could have influenced the smoke optical properties more strongly toward less absorption (Hodshire et al, 2021;Sellitto et al, 2022). The resulting effects of this spatial and temporal variability and evolution in the optical properties of the Australian wildfire plume would not or only insufficiently be represented by the model.…”

Dilution and photooxidation driven processes explain the evolution of organic aerosol in wildfire plumes

“…We also calculated a normalized excess mixing ratio (NEMR) for OA and VOCs by ratioing its background-corrected concentration (in μg m −3 for OA and ppbv for VOCs) with the background-corrected CO (in ppbv). The background-corrected OA O : C was calculated as a ratio of the background-corrected molar concentrations for O and C; equations can also be found in Hodshire et al 72 For this calculation, we assumed the OA to be only composed of C, H, and O such that the molar concentrations of C, H, and O were determined from the available OA, O : C, and H : C data. While wildfire OA is likely to be composed of nitrogen-containing organic compounds, the low N : C values measured during WE-CAN (∼0.02) 20 meant that accounting for nitrogen had a negligible impact on the reported OA mass concentrations and O : C and H : C values.…”

“…Fifth, Peng et al 74 and Hodshire et al 72 were recently able to study the distinct evolution of trace species resolved over the width of the wildfire plume. Both found evidence for increased photochemical activity near the edges and wings of the plume since these regions diluted much faster and were less optically dense compared to the core of the plume.…”